# How do enzymes not change the overall energy change of the reaction they're catalysing if they lower the activation energy?

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Based on the Induced-Fit model of enzyme action, enzymes catalyse a reaction by lowering the activation energy of a single forward reaction over and over. But I read that enzymes don't change the overall energy change of the reaction $$(Delta H_{solution})$$ they just speed up the reaction.

How can enzymes reduce the activation energy of every forward reaction but still not affect the $$Delta H$$? $$Delta H$$ is sometimes called change in heat because it is measured in Joules and it's either endothermic or exothermic (this bit is chemistry).

I'm a highschool student and I'm just trying to find out how the required activation energy doesn't affect the energy change for the whole reaction.

You're on a valley on one side of a mountain (representing some fairly stable state of your reactants), and you'd like to go to a town on the other side at a lower elevation (representing another stable state for your products, at lower energy than your current state: this is an energetically favorable reaction).

You can go over the mountain: high activation energy to climb up to the top, then you go down the slope to eventually get down to the town.

You can go through a pass between mountains instead: much lower activation energy to get over the pass, then you eventually get down to the town.

Whichever path you choose, you're at the same elevation (energy, $$Delta H$$ compared to where you started), with the same products (your location in space), but if you took the pass you've gotten there via a different chemical route that was easier to achieve energetically at the peak.

Enzymes let you take the path through the pass rather than over the top of the mountain. They don't change where you started from or where you're going, so they don't change the overall energy of the reaction.

## Catalytic Efficiency of Enzymes

Enzymes exist in all biological systems in abundant numbers, but not all of their functions are fully understood. Enzymes are important for a variety of reasons, most significantly because they are involved in many vital biochemical reactions. Increasing the reaction rate of a chemical reaction allows the reaction to become more efficient, and hence more products are generated at a faster rate. These products then become involved in some other biological pathway that initiates certain functions of the human body. This is known as the catalytic efficiency of enzymes, which, by increasing the rates, results in a more efficient chemical reaction within a biological system.

## Contents

### The mechanism of Enzyme Action:

Enzymes increase the rate of reaction between 2 reactants in various possible ways:

• They improve the proximity of the substrate being that they increase the local concentration of the substrate. This increases the likelihood of substrate collision, resulting in an increases reaction rate.
• They affect the orientation and hold the atoms in positions that favour the reaction
• They produce strain distortion they put strains on the bonds that are associated with the reaction
• In acid-base catalysis they aid in the exchange of H + or generation of –OH [2]
• They provide an alternative route of reaction with a lower activation energy so a higher proportion of substrate molecules collide with an energy above the activation energy and are able to move into the transition state.

2 models used to explain how enzymes work are stated as follows:

• the lock and key model - this is very simple and basically suggests that a substate with a shape complementary to an enzyme's active site will fit into it and the reaction will take place to give products.
• Induced fit - this model suggests that as the active site of a given enzyme come into contact with its subtrate it changes shape slightly to fit around the substate.

### Specificity

Enzymes are very specific that they are able to distinguish between optical isomers.

The amino acids forming the active site mainly determine the specificity of the enzyme a change in only a few amino acids in this region can result in a large change in the shape of the active site and this could then vastly change to affinity for the substrate or even change the substrate the enzyme is specific for.

The specificity of enzymes is exhibited in the ‘Lock and Key’ mechanism:

This illustrates the Lock and Key mechanisms and how the shape of the substrate is exactly complementary to the shape of the active site. The lock represents the enzyme and the key represents the substrate. Only a correct key (substrate) can bind to its corresponding lock (enzyme). However the lock and key model doesn't fully explain enzymatic activity. The model indicates that the enzyme and substrate are unable to change shape.

A modification to the Lock and Key Model of enzymes is the Induced Fit Hypothesis, also known as the "Hand-shake Model" [4] . This hypothesis states that the structure of both the enzyme and substrate can change on binding. In essence, the enzyme can wrap itself around the substrate molecule, until the substrate is completely bound. This produces an enzyme-substrate complex, which places a strain on a particular bond, therefore weakening said bond to a point where it can interact with the enzyme amino acid groups, further non-organic groups or further bound substrates.

The change in the shape of the enzyme is known as a conformational change the purpose of which is two-fold:

1. As mentioned above, the conformational change places strain on the desired bond, allowing for a more efficient reaction to take place,
2. The new conformation brings amino acid groups essential to the enzyme reaction, which in the unbound conformation may distant from the active site, into the active site. These groups ensure the catalytic reaction will be optimal [5] . The most common groups to be brought into the Active Site of the Enzyme are those relating to Acid/Base chemistry - therefore promoting the reaction and ensuring optimal conditions [6] .

### Substrates and the Active Site

Whether an enzymatic reaction will occur is dependant on the substrate colliding and binding to the active site. Once a substrate binds to the active site, it is held there by a variety of interactions. These interactions take place between charged residual groups of the amino acids in the confirmed active site. Hydrogen Bonds and Ionic bonds generally occur - however they are very weak. These weak interactions are of the order of 3 - 12 kcal mol -1 (12.5 - 50.2 kJ mol -1 ) [7] this is of the order of 1/10 th the strength of an, on average, single covalent bond [8] .

This ensures that enzyme-substrate formation is a reversible process.

### Allostery

An allosteric enzyme couples the effector levels to enzyme activity it couples the signal to functionality. Allosteric enzymes have multiple binding sites (allosteric sites) and show cooperative binding [9] .

Allosteric control of enzymes can be positive or negative and can have effects such as up regulate or down regulate activity.

Types of Allosteric control:

1. Homotropic - The modulator is a substrate for the target enzyme as well as the regulator e.g. Oxygen acting on Haemoglobin.
2. Heterotropic - The modulator is the regulatory molecule but is not also the substrate of the enzyme.

### Enzyme Types

There are many enzymes used in labs, each has it's own unique active site and so will catalyse a specific reaction. Restriction enzymes are one type of enzymes that are frequently used.

Restriction endonucleases are used naturally in a wide range of prokaryotes as a self-defence mechanism against foreign DNA molecules. The prokaryotes own DNA is methylated so it will not be cut by the enzyme.They recognise a specific 4-8 base pair palindromic sequence and by carrying out a hydrolysis reaction cut at that specific point. They may cut to form a blunt end or a sticky end. A blunt end is when the enzyme cut the DNA symmetrically. Asymmetrical cleavage leaves sticky end, these are unpaired bases. These sticky end can anneal to complementary bases on another strand [10] .

### Kinetics

Two important enzyme parameters in a simple enzyme catalysed reaction are the Michaelis-Menten constant (Km) and the maximum reaction velocity (Vmax)

• Km is the approximate measure of the enzyme affinity for the substrate. This can be calculated from the graph as ½ Vmax. Generally, a lower Km value signifies a higher affinity for the substrate.
• Kd is the dissociation constant for substrate binding to enzyme
• Kcat is the turnover number for the enzyme
• Vmax is the maximal activity of the enzyme when all of the active sites are saturated.

The Michaelis-Menten equation:

To obtain Vmax and Km the enzyme activity must be recorded and then plotted on a double reciprocal plot, a Lineweaver-Burk plot, and the Michaelis Menten equation is then rearranged to look like this: 1/V = (Km/Vmax)(1/S)+1/Vmax [11] .

Taken from www.search.com/reference/Lineweaver-Burk_plot [12] A Lineweaver-Burk plot showing all the necessary parameters.

### Inhibition

Enzymes can be inhibited by denaturing which is when a protein is changed in structure to form a randomly coiled peptide which exhibits none of its usual functions. Denaturing can result from extreme temperatures and pHs, as these alter the bonding in the molecule.

Inhibition can also be initiated by the binding of specific molecules called inhibitors. These can be split into categories:

1. Irreversible Inhibitors are molecules that permanently bind to the enzyme's active site or specific side chain, commonly to the serine (CH2OH) or cysteine (CH2SH) by covalent bonds. This inactivates the enzyme so the substrate cannot bind.
2. Competitive Inhibitors are competing for molecules that will have a very similar structure to that of the natural substrate and thus will be complementary to the enzyme active site. Vmax stays the same, but Km increases. This type of inhibition can be overcome by an increase in substrate concentration. They are therefore useful therapeutic agents and unlike irreversible inhibitors (like aspirin) their effect isn't long lasting.
3. Non-competitive inhibitors bind to the allosteric site on the enzyme other than the active site, causing changes to enzyme shape resulting in disruption of the active site. This decreases the turnover number of the enzyme rather than preventing substrate binding- Vmax decreases but Km stays the same. This cannot be overcome with an increase in substrate concentration.
4. Uncompetitive inhibitors only bind to an enzyme-substrate complex, so both Km and Vmax decrease as it takes longer for the substrate to leave the active site. This inhibition works best when the concention of enzyme-substrate complex is high.We are able to distinguish the types of inhibition occurring by looking at the graph of enzyme activity [13] .

Competitive inhibitors show the same Vmax value however we see an increased Km value and non-competitive inhibitors show a decreased Vmax but the same Km.

## How do enzymes not change the overall energy change of the reaction they're catalysing if they lower the activation energy? - Biology

Introduction

There is an obesity epidemic in the United States that is paralleled by an increase in high blood pressure, or hypertension. This is extremely relevant to medical students because hypertension increases the risk of stroke, heart failure, and kidney failure.

Each year, physicians encourage millions of Americans to improve their diets, add exercise to their daily regimens, or even take prescription drugs to control their hypertension. Many of these anti-hypertensive medications are called ACE (angiotensin-converting enzyme) inhibitors. In healthy patients, ACE catalyzes a reaction that converts a peptide called angiotensin I to angiotensin II. The angiotensin II peptide then not only directly causes constriction of the blood vessels to raise blood pressure, but also stimulates the release of the hormone aldosterone, which activates the kidneys to reabsorb more water back into the bloodstream. The increase in blood volume also increases blood pressure. Physicians take advantage of this complicated pathway with a straightforward solution: stop the pathway early by inhibiting ACE, and blood pressure will decrease.

Enzymes are crucial proteins that dramatically increase the rate of biological reactions. They're used to regulate homeostatic mechanisms in every organ system and are highly regulated themselves by environmental conditions, activators, and inhibitors. These regulators may be naturally occurring or may be given as a drug, such as the ACE inhibitors used to treat hypertension. Some enzymes are kept in an inactivated form called a zymogen and are only activated as needed. In this chapter, we'll learn about how enzymes work and how different conditions influence their activity. We'll also see how enzymes are regulated, which will help us tie together concepts about every organ system and metabolic process we learn about for the MCAT.

### 2.1 Enzymes as Biological Catalysts

Enzymes are incredibly important as biological catalysts. Catalysts do not impact the thermodynamics of a biological reaction that is, the &DeltaHrxn and equilibrium position do not change. Instead, they help the reaction proceed at a much faster rate. As a catalyst, the enzyme is not changed during the course of the reaction. Enzymes increase the reaction rate of a process by a factor of 100, 1000 or even 1,000,000,000,000 (10 12 ) times when compared to the uncatalyzed reaction. Without this increase, we wouldn't be alive. Table 2.1 summarizes the key points to remember about enzymes.

Lower the activation energy

Increase the rate of the reaction

Do not alter the equilibrium constant

Are not changed or consumed in the reaction (which means that they will appear in both the reactants and products.)

Are pH- and temperature-sensitive, with optimal activity at specific pH ranges and temperatures

Do not affect the overall &DeltaG of the reaction

Are specific for a particular reaction or class of reactions

Table 2.1. Key Features of Enzymes

Enzymes are picky. The molecules upon which an enzyme acts are called substrates a given enzyme will only catalyze a single reaction or class of reactions with these substrates, a property known as enzyme specificity. For example, urease only catalyzes the breakdown of urea.Chymotrypsin, on the other hand, can cleave peptide bonds around the amino acids phenylalanine, tryptophan, and tyrosine in a variety of polypeptides. Although those amino acids aren't identical, they all contain an aromatic ring, which makes chymotrypsin specific for a class of molecules.

Enzymes can be classified into six categories, based on their function or mechanism. We'll review each type of enzyme and give examples of those that you are most likely to see on Test Day. If you encounter an unfamiliar enzyme on the MCAT, keep in mind that most enzymes have descriptive names ending in the suffix – ase: lactase, for example, breaks down lactose.

Oxidoreductases

Oxidoreductases catalyze oxidation–reduction reactions, that is, the transfer of electrons between biological molecules. They often have a cofactor that acts as an electron carrier, such as NAD + or NADP + . In reactions catalyzed by oxidoreductases, the electron donor is known as thereductant, and the electron acceptor is known as the oxidant. Enzymes with dehydrogenase or reductase in their names are usually oxidoreductases. Enzymes in which oxygen is the final electron acceptor often include oxidase in their names.

The convention for naming reductants and oxidants of oxidoreductases is the same as the convention for naming reducing agents and oxidizing agents in general and organic chemistry. This is a good time to brush up on oxidation–reduction reactions if you haven't seen them in a while&mdashthey're covered in Chapter 11 of MCAT General Chemistry Review and Chapter 4 of MCAT Organic Chemistry Review.

Transferases

Transferases catalyze the movement of a functional group from one molecule to another. For example, in protein metabolism, an aminotransferase can convert aspartate and &alpha-ketoglutarate, as a pair, to glutamate and oxaloacetate by moving the amino group from aspartate to &alpha-ketoglutarate. Most transferases will be straightforwardly named, but remember that kinases are also a member of this class. Kinases catalyze the transfer of a phosphate group, generally from ATP, to another molecule.

Hydrolases catalyze the breaking of a compound into two molecules using the addition of water. In common usage, many hydrolases are named only for their substrate. For example, one of the most common hydrolases you will encounter on the MCAT is a phosphatase, which cleaves a phosphate group from another molecule. Other hydrolases include peptidases, nucleases, and lipases, which break down proteins, nucleic acids, and lipids, respectively.

Lyases catalyze the cleavage of a single molecule into two products. They do not require water as a substrate and do not act as oxidoreductases. Because most enzymes can also catalyze the reverse of their specific reactions, the synthesis of two molecules into a single molecule may also be catalyzed by a lyase. When fulfilling this function, it is common for them to be referred to as synthases.

Isomerases catalyze the rearrangement of bonds within a molecule. Some isomerases can be can also be classified as oxidoreductases, transferases, or lyases, depending on the mechanism of the enzyme. Keep in mind that isomerases catalyze reactions between stereoisomers as well as constitutional isomers.

Ligases catalyze addition or synthesis reactions, generally between large similar molecules, and often require ATP. Synthesis reactions with smaller molecules are generally accomplished by lyases. Ligases are most likely to be encountered in nucleic acid synthesis and repair on Test Day.

Major Enzyme Classifications: LI'L HOT

·&emspOxidoreductase

·&emspTransferase

IMPACT ON ACTIVATION ENERGY

Recall that thermodynamics relates the relative energy states of a reaction in terms of its products and reactants. An endergonic reaction is one that requires energy input (&DeltaG > 0), whereas an exergonic reaction is one in which energy is given off (&DeltaG

## How Do Enzymes Work?

Enzymes are biological molecules (typically proteins) that significantly speed up the rate of virtually all of the chemical reactions that take place within cells.

They are vital for life and serve a wide range of important functions in the body, such as aiding in digestion and metabolism.

Some enzymes help break large molecules into smaller pieces that are more easily absorbed by the body. Other enzymes help bind two molecules together to produce a new molecule. Enzymes are highly selective catalysts, meaning that each enzyme only speeds up a specific reaction. [What Is Chemistry?]

The molecules that an enzyme works with are called substrates. The substrates bind to a region on the enzyme called the active site.

There are two theories explaining the enzyme-substrate interaction.

In the lock-and-key model, the active site of an enzyme is precisely shaped to hold specific substrates. In the induced-fit model, the active site and substrate don't fit perfectly together instead, they both alter their shape to connect.

Whatever the case, the reactions that occur accelerate greatly &mdash over a millionfold &mdash once the substrates bind to the active site of the enzyme. The chemical reactions result in a new product or molecule that then separates from the enzyme, which goes on to catalyze other reactions.

Here's an example: When the salivary enzyme amylase binds to a starch, it catalyzes hydrolysis (the breakdown of a compound due to a reaction with water), resulting in maltose, or malt sugar.

## Lesson 1: Activation Energy

### Overview/Objectives

After completing this lesson, you should be able to:

1. Demonstrate an understanding of the role of catalysts in chemical reactions.
2. Define activation energy and discuss the role of catalysts in activation energy.
3. Define transition state.
4. Demonstrate an understanding of the reversibility of reactions and how the energy difference of the products and reactants determine how a reaction will proceed.

1. Read &ldquoIntroduction&rdquo and &ldquoActivation Energy&rdquo on page 82 of the textbook.
2. You can also watch the three video lectures on Catalytic Mechanism:

(Links to these are also provided on page 82 of the textbook.)

### Commentary

The general definition of a catalyst is a substance that alters the rate of a reaction, but is not changed in the process. As we learned in Unit 2, the Gibbs Free Energy (G) of a reaction is the thermodynamic potential of a reaction, meaning the amount of maximum or reversible work that may occur in a reaction at a constant temperature and pressure.

Looking at the graph on page 82 of the textbook, it can be seen that as a reaction proceeds, the total Gibbs energy or change in energy (&DeltaGTOTAL) for the substrates to products is the same whether the reaction involves a catalyst or not. However, if you look at the two slopes of the graph above the line denoting the free energy of the substrates, you will see that the slope of a catalyzed reaction is smaller compared to the slope of the reaction if it was uncatalyzed. This means that the catalyst does not change the overall energy of the reaction, but allows that reaction to proceed faster (&DeltaG°). What the catalyst does, however, is it lowers the activation energy of the reaction, meaning that it enhances the ability of the substrate molecules to get the energy needed to enable the reaction to occur.

### Study Questions

1. Define the terms &ldquoactivation energy&rdquo and &ldquotransition state.&rdquo
2. How does a catalyst influence the overall energy of a reaction? How does a catalyst affect the activation energy of a reaction?

If you wish to discuss any of these questions or need assistance with the material, please contact your Academic Expert (AE) by emailing the Student Success Centre at [email protected]

## How do enzymes not change the overall energy change of the reaction they're catalysing if they lower the activation energy? - Biology

on a mission to make biochemistry fun and accessible to all

Km on &ndash let&rsquos applaud Maud! You can&rsquot get through a biochemistry class without hearing her work Menten-ed but the scientist behind the famous enzyme kinetics equation herself deserves more attention! Enzymes are a special type of (usually) proteins that speed up chemical reactions, and the Michaelis-Menten equation (which I&rsquoll explain in detail later in the post) helps describe the rate at which these reactions occur. And, thanks to women being largely excluded from science in the early days of biochemistry, it&rsquos one of the only biochemistry equations I know of that&rsquos named after a woman. Maud published this formula with Leonor Michaelis in 1913, and it is still in wide use because it&rsquos incredibly useful. As a biochemist, I know it well, but until a few years ago I didn&rsquot realize there was an underlying awesome female scientist story to tell!

Acc. 90-105 &ndash Science Service, Records, 1920s-1970s, Smithsonian Institution Archives

Born in Canada in 1879, Maud became one of that country&rsquos first female medical doctors, but there were limited opportunities in Canada for a woman to pursue a research career, so she moved to the US, where she worked at the Rockefeller Institute, researching the effects of radium on tumors, before emigrating to Germany to work with Michaelis. After her groundbreaking work there, she moved back to the US to obtain a PhD at the University of Chicago and went on to work as a professor and pathologist at the University of Pittsburgh and a research fellow at the British Columbia Medical Research Institute. Menten also had a full life outside of the lab &ndash in addition being a brilliant biochemist and histologist, she was also a skilled musician and artist and she spoke 6 languages! Menten passed away in 1960, but her name will forever be synonymous with enzyme kinetics.

So let&rsquos look at enzyme kinetics more closely.

Enzymes are biochemical reaction mediators/speed-uppers (typically proteins, sometimes protein/RNA combos or just RNA). Different enzymes speed up (catalyze) different reactions and they do so at really different rates. A couple extreme examples are the sluggish lysozyme, which takes a whole 2 seconds to cut a protein, to the mind-blowing-ly fast carbonic anhydrase, which can combine CO₂ with water to make carbonic acid &ndash or do the reverse &ndash 1 MILLION TIMES PER SECOND.

No matter what job an enzyme is specialized to help out with, how fast they can do it that job (convert the starting materials (substrate) to the final product) depends on things like how much they like the substrate (reflected in Km) and how good they are at changing it (reflected in kcat). In the early 1900s, Maud Menten & Leonor Michaelis figured out a mathematical formula that connects these various fundamental properties of an enzyme to observable things (like the disappearance of reactants and/or appearance of products over time). This allows scientists to do enzyme activity experiments to do things like see which substrates an enzyme prefers and which enzymes are most efficient under different conditions.

The &rdquo1 million times per second&rdquo rate of carbonic anhydrase is 10 million times faster than that carbonic acid making reaction would happen on its own, which brings me to an important point. All of the reactions that enzymes catalyze could happen on their own- everything from splitting up molecules (e.g. breaking down sugar into smaller parts for energy) to stitching together molecules (e.g. DNA ligase sealing breaks in DNA strands) to just &ldquoshifting molecules&rdquo (e.g. chromatin remodeling complexes shifting the histone proteins DNA is coiled around (for space-saving) in order to reveal regions to be read).

Although these processes can be super complex and require a lot of coordination, they all *could* happen without enzymes &ndash because they have the biochemical &ldquodrive&rdquo to do so. They would just be super unlikely to happen because you&rsquod have to have freely roaming molecules collide at just the right orientation with just the right conditions, etc.

This biochemical &ldquodrive&rdquo is a negative change in Gibbs free energy (G). I like to think of G as molecular &ldquocomfiness&rdquo &ndash imagine you&rsquore getting a picture taken &ndash it takes energy to hold an awkward pose (uncomfortable molecules have a high G) whereas you can relax if they just want a candid pic (comfy molecules have a low G). I don&rsquot know about you, but I hate those &ldquosay cheese&rdquo-y pics &ndash I would much rather be comfortable.

And molecules would too. So they react and interact in ways that get them to a more comfy position (lower G)-> reactions are energetically favorable if the product(s) P have LESS free energy (lower G) than the reactants (R), and we can call this difference in free energy between the products and the reactants &DeltaG (&Delta is pronounced &ldquodelta&rdquo and it means &ldquochange in&rdquo). With enzyme-catalyzed reactions, we usually call the reactants SUBSTRATES (S) &ndash as in, &ldquothing&rdquo is a substrate for &ldquocool enzyme.&rdquo So we can write the overall reaction as

Not all reactions have a negative &DeltaG, but you can couple unfavorable ones to favorable ones to get the job done. This is one reason some enzymes use ATP &ndash splitting ATP releases a lot of energy &ndash and if the ATP splitting & the unfavorable reaction-doing are both happening together in the same enzyme, that energy released from ATP-splitting can be used to power the unfavorable one.

Even when a reaction is really favorable &ndash the products are way comfier than the reactants, so there&rsquos a highly negative &DeltaG, it often has to go a really uncomfy state to get there &ndash we call this most uncomfy (highest energy) state the transition state (⧧). Imagine you&rsquore trying to snap a stick in half (for the sake of the analogy pretend it&rsquos perforated in the center so there&rsquos only one possible breakpoint). So you start bending the stick and it gets harder and harder & the transition point would be that really tense moment right before the stick breaks (if you think your arms are sore, imagine how the stick feels!). It is at this transition state (TS) that the enzyme hugs the substrate tightest &ndash thus it kinda lures the substrate into adopting that awkward position &ndash the uncomfiness as the substrate &ldquobends&rdquo is offset by positive interactions with the enzyme that bending allows for.

We can figure-ize the changes in energy over the course of a reaction with a REACTION COORDINATE DIAGRAM. It reminds me of a candy-cane shaped rainbow with a pot of gold at the bottom &ndash you have to get over the rainbow before you can get to the gold. The top of the rainbow is the energy of the transition state and the energy required to get there is the activation energy. Enzymes provide an alternative transition state with a &ldquoshorter rainbow&rdquo &ndash a lower transition state energy means a lower activation energy, so there&rsquos less of a barrier for the reaction to occur. But since the starting and ending points are the same, the overall &DeltaG for the reaction is the same regardless of whether it&rsquos enzyme-catalyzed or not.

When you have a quantity like this where you only care about the start and finish &ldquostates and not the route taken, we call it a &ldquostate property.&rdquo And it&rsquos this overall &DeltaG of the reaction that provides the &ldquodrive&rdquo for the reaction &ndash so the enzyme doesn&rsquot change that drive &ndash it doesn&rsquot choose whether or not the reaction will happen &ndash and when you lower the barrier you make it easier to &ldquogo backwards&rdquo too, so enzymes don&rsquot change the equilibrium constants &ndash if you start with the same amount of reactant you&rsquoll eventually reach the same amount of product formed (even if you have to wait billions of years) but they do affect reaction rates &ndash so you don&rsquot have to wait billions of years &ndash but how long do you have to wait? To figure out this we go from the thermodynamic stuff we&rsquove been talking which deals with reaction favorability to kinetics, which deals with speeds (reaction velocity, v).

A reaction&rsquos only as fast as its slowest step, and you can split a reaction up into a few steps: bind (E+S ⇌ ES), change (ES⇌EP), & release (EP⇌E+P). For simplicity&rsquos sake, let&rsquos say that the reaction&rsquos really favorable so it&rsquos really unlikely to go in the reverse direction (you&rsquore not gonna change product into substrate). (Even if the reaction does go backwards, if you measure kinetics at the very beginning, when there isn&rsquot any product to get turned back into substrate you can ignore the reverse). Then we can remove a couple of those backwards arrows so we have

bind (E+S ⇌ ES), change (ES->EP), & release (EP->E+P)

You can see that I didn&rsquot take away that first backwards arrow because the binding step is still reversible &ndash and which way the equilibrium lies (is E + S or ES more favored) depends on the affinity (stickiness) of the 2 for one another. And even though we&rsquore assuming you *can* only go ES->EP (and not EP->ES) &ndash that doesn&rsquot mean you *will* make that change. If you think of things chance-wise (probabilistically), each time a substrate collides with an enzyme, it has a chance of sticking and each time it sticks it has a chance of converting to product. How fast the reaction will occur depends on:

• how likely E & S are to collide (depends on their concentrations & how much energy they have)
• how likely E & S are to stick (depends on their stickiness &ldquoaffinity&rdquo for one another)
• how likely S is to convert (depends on how high the activation barrier is & how long the substrate&rsquos stuck there so it can &ldquokeep trying&rdquo)

You also have the releasing step, but, except for some weird cases, that&rsquos usually not a hold-up, so we&rsquoll combine the change and release steps into ES -> E + P. So now we have 2 steps, bind (E+S ⇌ ES) & change/release (ES -> E + S) and each of these can happen at different speeds

We can give each step a rate constant, k. It&rsquos &ldquoconstant&rdquo in terms of it not being affected by concentrations because it&rsquos an inherent property of the molecules, but it *is* affected by differences in the environment (pH, etc.) so it&rsquos specific for a specific reaction under specific conditions.

So, how many sticks could a stick-snapper snap if a stick-snapper could snap sticks? To &ldquoanswer&rdquo this let&rsquos turn to Michaelis-Menten kinetics.

Say you want to figure out how fast a snapper can snap sticks &ndash if you took a single stick-snapper, give her some sticks, set a timer for 60 seconds, then counted how many sticks she snapped, you could divide that by the time to get sticks snapped per second per snapper. This is equivalent to a kinetic constant called kcat (aka &ldquoturnover number&rdquo) which tells you how many substrate molecules a single enzyme can convert to product per unit time (usually seconds) &ndash as long as there were enough sticks!

If you have a huge excess of sticks, (S >>>> E), you don&rsquot have to worry about running out, so the speed you observe isn&rsquot affected by [S] (concentration of substrate) and each enzyme molecule can work at kcat (each time the snapper snaps a stick there&rsquos another stick there to snap). But if you don&rsquot have a big excess, the substrate all gets used up, so it can only work its fastest in the very beginning of the reaction, right after you mix E & S.

Well, not quite *right* at the beginning. At the very very beginning (we&rsquore usually talking the first couple milliseconds) you have to fill up the enzymes &ndash the snapper has to grab their first stick, so you have a burst of ES formation, but they haven&rsquot had time to snap the sticks yet so product formation starts slow thanks to this lag as the snappers snatch sticks. We call this brief start-up phase the PRE-STEADY STATE.

As the name implies, it&rsquos followed by the STEADY STATE &ndash and this is where kinetic measurements are usually taken. In fact, in Michaelis-Menten kinetics we make a &ldquosteady state assumption&rdquo &ndash basically we assume that ES is at equilibrium &ndash you know how I said E+S⇌ES was reversible &ndash well, equilibrium is where the *rates* of the forward (E+S -> ES) & reverse (ES -> E+S) reactions are the same &ndash this doesn&rsquot mean that you have the same amount of S bound as unbound, it just means that the amount that&rsquos bound vs. unbound isn&rsquot changing (for every snapper that drops a stick another snapper picks one up).

Initially none was bound and in the pre-steady state the ES vs. E+S was changing as S now had the option of binding. But eventually you reach a dynamic equilibrium where, for every S that gets released or converted, another one binds. This is usually reached within microseconds & is what you measure when you measure the &ldquoinitial velocity&rdquo (vo)

But it can only bind if there&rsquos still S left to bind! And S isn&rsquot just binding &ndash it&rsquos also &ldquodisappearing&rdquo because it gets converted into products (and you can&rsquot snap an already snapped stick!). So you enter the POST-STEADY STATE where the substrate starts getting used up, fewer ES complexes can form (poor snappers left stickless) and less product can form

kcat was looking at a a single snapper &ndash but it&rsquos not easy to measure a single enzyme molecule because these guys are really tiny and you usually have a ton of them &ndash so instead of dealing with single molecules you&rsquore dealing with mols of molecules. The mol is the biochemist&rsquos &ldquodozen&rdquo &ndash it just means 6.02×10^23 of something. http://bit.ly/2OfeHQg

So now we can imagine a ton of identical snappers, and the amount of sticks snapped depends on how well each snapper can snap sticks (kcat) AND how many snappers there are (enzyme concentration, [E]).

Instead of finding the maximum rate per snapper, you first find the maximum velocity of product formation (Vmax) for a group of snappers, and then you take the # of snappers into account.

Vmax = kcat[E]o, where [E]o is enzyme concentration

But &ldquoturnover rate&rdquo isn&rsquot all you care about. How good of a grip does the snapper have on the stick? Is it really a good match? When you have way more substrate than enzyme, the enzyme&rsquos constantly getting bombarded with substrate, so even if it doesn&rsquot like the substrate that much it&rsquos still bound most of the time because every time it lets go there&rsquos another substrate molecule waiting to jam itself in. So you can&rsquot tell if the substrate is actually a really good match for the enzyme.

But if you have lower substrate concentrations, if an enzyme releases a substrate molecule it&rsquos less likely to quickly find another molecule to collide with. So, when those rarer collisions do occur stickiness will matter more &ndash if they don&rsquot stick they&rsquoll have to wait for another one &ndash and you&rsquoll have to wait for product.

How do we take this into account when &ldquoscoring&rdquo enzymes? &ndash enter the MICHAELIS-MENTEN CONSTANT (Km). I derive this in the pics for you so you can see how it comes from the rate constants and concentrations (and I encourage you to carry out the derivation yourself in the process of studying), but here&rsquos the spoiler alert: Km is the substrate concentration at which product formation is at 1/2 it&rsquos maximal speed (so 1/2 Vmax).

You usually find it by measuring Vo (that initial rate of product formation) at multiple concentrations of the substrate (another chance to use those serial dilutions!).Then you plot substrate concentration on the horizontal (x) axis & reaction rate on the vertical (y) axis. Do this and (often) you&rsquoll get a ,- like curve, a parabola that starts steep and then plateaus. Enzymes that give curves like this are said to obey Michaelis-Menten kinetics.

The curve starts steep (but slow) because at the beginning there&rsquos more than enough enzyme to quickly change all the substrate into product but then substrate runs out (too many snappers, too few sticks) But as the substrate concentration increases, the enzyme gets saturated &ndash each enzyme is working its hardest, but it can only go so fast &ndash the enzyme becomes limiting (too many sticks, too few snappers). The height at which the curve plateaus is called the maximum velocity (Vmax) and the substrate concentration at which the curve gets halfway to Vmax is called the Km.

This Vmax depends on enzyme concentration (which is why we adjust it to get kcat), but Km doesn&rsquot depend on enzyme concentration, just on how well the enzyme likes the substrate. It&rsquos not quite this simple, but, in general terms & typical cases

lower Km -> better binder (higher affinity for substrate)

higher Km -> worse binder (lower affinity for substrate)

higher kcat -> better changer

lower kcat -> worse changer

If you want a good idea about how &ldquogood&rdquo an enzyme is overall for a particular substrate, you need to know how well it binds substrate & how well it changes it. So you combine those 2 measures to get another value, the &ldquospecificity constant&rdquo (aka &ldquocatalytic efficiency&rdquo) which = kcat/Km

You know how I said &ldquosometimes&rdquo you&rsquoll get a parabola? Well, other times you&rsquoll have enzymes where when you make that graph you&rsquoll get an S-shaped curve. This usually implies that you have an &ldquoallosteric enzyme&rdquo &ndash allostery is where something happens somewhere on the molecule that changes something elsewhere on the molecule. Allosteric enzymes usually have multiple active sites and when substrate binds one of them it makes it easier for the other active sites to bind substrate, so you get cooperativity and see a switch-like transition. We saw this when we looked at hemoglobin &ndash which has 4 oxygen-binding subunits and biding of oxygen to one site makes it much easier for oxygen to bind the other sites (positive cooperativity) http://bit.ly/39p5RqW

This post was posted from the lab &ndash which finally has power! This post is part of my weekly &ldquobroadcasts from the bench&rdquo for The International Union of Biochemistry and Molecular Biology. Be sure to follow the IUBMB if you&rsquore interested in biochemistry (@the_iubmb)! They&rsquore a really great international organization for biochemistry.⠀

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0

• enzyme
• free energy

## What causes lactose intolerance?

Both children and adults can get lactose intolerance. Here are some common causes of this condition:

Lactose intolerance often runs in families (hereditary). In these cases, over time a person&rsquos body may make less of the lactase enzyme. Symptoms may occur during the teen or adult years.

In some cases, the small intestine stops making lactase after an injury or after a disease or infection.

Some babies born too early (premature babies) may not be able to make enough lactase. This is often a short-term problem that goes away.

In very rare cases some newborns can&rsquot make any lactase from birth.

## How Do Enzymes Catalyze Chemical Reactions?

Enzymes catalyze chemical reactions by first binding to molecules and then lining them up in ways that increase the probability of the molecules exchanging atoms when they collide. Enzymes therefore allow scientists to control the exchange of atoms mechanically, as explained by Science Daily.

One way researchers have done this is by attaching a controllable molecular spring composed of DNA to an enzyme, which is simply a large protein. They then turned the enzyme on and off mechanically, which in turn controlled how fast the chemical reactions, or atom exchanges, occurred.

Different steps within the reactions were influenced in accordance with where on the enzyme the molecular spring was attached, as reported by UCLA Physics professor Giovanni Zocchi and his colleagues.

In general, enzymes make chemical bonds easier to manipulate by stretching or twisting molecules so the amount of activation energy normally required for reactions to occur, usually in the form of heat, stirring or shaking, is reduced or eliminated.

Different types of enzymes have different types of reactions they activate, and there are some enzymes that must be in particular environments or operate under certain sets of conditions to work well, if at all. Enzymes may also fail to catalyze chemical reactions if they suffer damage.

## How do enzymes not change the overall energy change of the reaction they're catalysing if they lower the activation energy? - Biology

This page is an introduction to how proteins can work as enzymes - biological catalysts. You should realise that this is written to cover the needs of a number of UK-based chemistry syllabuses for 16 - 18 year olds. If you want detailed knowledge about enzymes for a biology or biochemistry course, you are probably in the wrong place! This is just an introduction.

Note: This page follows on from a page about protein structure. If you don't have a reasonable knowledge of the structure of proteins and the sorts of attractions that can be found in them, you may not understand bits of the present page. Read the protein structure page first and come back here later.

Important: The specific examples of the enzymes that you will find on this page are only intended to give you a feel for the way that enzymes work. Unless your syllabus specifically asks for a particular enzyme, there is no need for you to remember the details.

Enzymes are mainly globular proteins - protein molecules where the tertiary structure has given the molecule a generally rounded, ball shape (although perhaps a very squashed ball in some cases). The other type of proteins (fibrous proteins) have long thin structures and are found in tissues like muscle and hair. We aren't interested in those in this topic.

These globular proteins can be amazingly active catalysts. You are probably familiar with the use of catalysts like manganese(IV) oxide in decomposing hydrogen peroxide to give oxygen and water. The enzyme catalase will also do this - but at a spectacular rate compared with inorganic catalysts.

One molecule of catalase can decompose almost a hundred thousand molecules of hydrogen peroxide every second. That's very impressive!

This is a model of catalase, showing the globular structure - a bit like a tangled mass of string:

Note: This diagram was obtained from the RCSB Protein Data Bank.

You should be able to identify the alpha-helices and beta-pleated sheets. If you can't, you obviously didn't read the page about protein structure mentioned above!

If you look very carefully, you might also spot two pink, non-protein structures hidden in the main structure. More about these in a while . . .

An important point about enzymes is that they are very specific about what they can catalyse. Even small changes in the reactant molecule can stop the enzyme from catalysing its reaction. The reason for this lies in the active site present in the enzyme . . .

Active sites are cracks or hollows on the surface of the enzyme caused by the way the protein folds itself up into its tertiary structure. Molecules of just the right shape, and with just the right arrangement of attractive groups (see later) can fit into these active sites. Other molecules won't fit or won't have the right groups to bind to the surface of the active site.

The usual analogy for this is a key fitting into a lock. For the key to work properly it has to fit exactly into the lock.

In chemistry, we would describe the molecule which is actually going to react (the purple one in the diagram) as the reactant. In biology and biochemistry, the reactant in an enzyme reaction is known instead as the substrate.

You mustn't take this picture of the way a substrate fits into its enzyme too literally. What is just as important as the physical shape of the substrate are the bonds which it can form with the enzyme.

Enzymes are protein molecules - long chains of amino acid residues. Remember that sticking out all along those chains are the side groups of the amino acids - the "R" groups that we talked about on the page about protein structure.

Active sites, of course, have these "R" groups lining them as well - typically from about 3 to 12 in an active site. The next diagram shows an imaginary active site:

Remember that these "R" groups contain the sort of features which are responsible for the tertiary structure in proteins. For example, they may contain ionic groups like -NH3 + or -COO - , or -OH groups which can hydrogen bond, or hydrocarbon chains or rings which can contribute to van der Waals forces.

Groups like these help a substrate to attach to the active site - but only if the substrate molecule has an arrangement of groups in the right places to interact with those on the enzyme.

The diagram shows a possible set of interactions involving two ionic bonds and a hydrogen bond.

The groups shown with + or - signs are obvious. The ones with the "H"s in them are groups capable of hydrogen bonding. It is possible that one or more of the unused "R" groups in the active site could also be helping with van der Waals attractions between them and the substrate.

If the arrangement of the groups on the active site or the substrate was even slightly different, the bonding almost certainly wouldn't be as good - and in that sense, a different substrate wouldn't fit the active site on the enzyme.

This process of the catalyst reacting with the substrate and eventually forming products is often summarised as:

. . . where E is the enzyme, S the substrate and P the products.

The formation of the complex is reversible - the substrate could obviously just break away again before it converted into products. The second stage is shown as one-way, but might be reversible in some cases. It would depend on the energetics of the reaction.

So why does attaching itself to an enzyme increase the rate at which the substrate converts into products?

It isn't at all obvious why that should be - and most sources providing information at this introductory level just gloss over it or talk about it in vague general terms (which is what I am going to be forced to do, because I can't find a simple example to talk about!).

Catalysts in general (and enzymes are no exception) work by providing the reaction with a route with a lower activation energy. Attaching the substrate to the active site must allow electron movements which end up in bonds breaking much more easily than if the enzyme wasn't there.

Strangely, it is much easier to see what might be happening in other cases where the situation is a bit more complicated . . .

What we have said so far is a major over-simplification for most enzymes. Most enzymes aren't in fact just pure protein molecules. Other non-protein bits and pieces are needed to make them work. These are known as cofactors.

In the absence of the right cofactor, the enzyme doesn't work. For those of you who like collecting obscure words, the inactive protein molecule is known as an apoenzyme. When the cofactor is in place so that it becomes an active enzyme, it is called a holoenzyme.

Note: If you don't collect obscure words, don't worry about these! Neither of them occurs in the syllabuses I am trying to cover with this material. I have spent a lifetime in chemistry education without having come across either of them until researching this!

There are two basically different sorts of cofactors. Some are bound tightly to the protein molecule so that they become a part of the enzyme - these are called prosthetic groups.

Some are entirely free of the enzyme and attach themselves to the active site alongside the substrate - these are called coenzymes.

Prosthetic groups

Prosthetic groups can be as simple as a single metal ion bound into the enzyme's structure, or may be a more complicated organic molecule (which might also contain a metal ion). The enzymes carbonic anhydrase and catalase are simple examples of the two types.

Zinc ions in carbonic anhydrase

Carbonic anhydrase is an enzyme which catalyses the conversion of carbon dioxide into hydrogencarbonate ions (or the reverse) in the cell. (If you look this up elsewhere, you will find that biochemists tend to persist in calling hydrogencarbonate by its old name, bicarbonate!)

In fact, there are a whole family of carbonic anhydrases all based around different proteins, but all of them have a zinc ion bound up in the active site. In this case, the mechanism is well understood and simple. We'll look at this in some detail, because it is a good illustration of how enzymes work.

Important: This is just an example! If you are doing a UK-based chemistry syllabus for 16 - 18 year olds, there is almost certainly no need to learn this. If in doubt, check your syllabus. If it doesn't explicitly ask for this reaction in detail, you don't need to learn it. To repeat - I'm just using it to illustrate how enzymes carry out a simple reaction.

The zinc ion is bound to the protein chain via three links to separate histidine residues in the chain - shown in pink in the picture of one version of carbonic anhydrase. The zinc is also attached to an -OH group - shown in the picture using red for the oxygen and white for the hydrogen.

Note: This diagram comes from Wikipedia. I have no reason to doubt its accuracy, but I can't guarantee it.

As far as I have been able to find out, all the various forms of carbonic anhydrase have the zinc ion bound to three histidine residues in this way - irrespective of what is happening in the rest of the protein molecule. If I am wrong about this generalisation, could you please let me know via the address on the about this site page.

The structure of the amino acid histidine is . . .

. . . and when it is a part of a protein chain, it is joined up like this:

If you look at the model of the arrangement around the zinc ion in the picture above, you should at least be able to pick out the ring part of the three molecules.

The zinc ion is bound to these histidine rings via dative covalent (co-ordinate covalent) bonds from lone pairs on the nitrogen atoms. Simplifying the structure around the zinc . . .

The arrangement of the four groups around the zinc is approximately tetrahedral. Notice that I have distorted the usual roughly tetrahedral arrangement of electron pairs around the oxygen - that's just to keep the diagram as clear as possible.

So that's the structure around the zinc. How does this catalyse the reaction between carbon dioxide and water?

A carbon dioxide molecule is held by a nearby part of the active site so that one of the lone pairs on the oxygen is pointing straight at the carbon atom in the middle of the carbon dioxide molecule. Attaching it to the enzyme also increases the existing polarity of the carbon-oxygen bonds.

If you have done any work on organic reaction mechanisms at all, then it is pretty obvious what is going to happen. The lone pair forms a bond with the carbon atom and part of one of the carbon-oxygen bonds breaks and leaves the oxygen atom with a negative charge on it.

What you now have is a hydrogencarbonate ion attached to the zinc.

The next diagram shows this broken away and replaced with a water molecule from the cell solution.

All that now needs to happen to get the catalyst back to where it started is for the water to lose a hydrogen ion. This is transferred by another water molecule to a nearby amino acid residue with a nitrogen in the "R" group - and eventually, by a series of similar transfers, out of the active site completely.

. . . and the carbonic anhydrase enzyme can do this sequence of reactions about a million times a second. This is a wonderful piece of molecular machinery!

Let me repeat yet again: If you are doing a UK-based chemistry exam for 16 - 18 year olds, you are unlikely to need details of this reaction. I've talked it through in some detail to show that although enzymes are complicated molecules, all they do is some basic chemistry. It is just that this particular example is a lot easier to understand than most!

The haem (US: heme) group in catalase

Remember the model of catalase from further up the page . . .

At the time, I mentioned the non-protein groups which this contains, shown in pink in the picture. These are haem (US: heme) groups bound to the protein molecule, and an essential part of the working of the catalase. The haem group is a good example of a prosthetic group. If it wasn't there, the protein molecule wouldn't work as a catalyst.

The haem groups contain an iron(III) ion bound into a ring molecule - one of a number of related molecules called porphyrins. The iron is locked into the centre of the porphyrin molecule via dative covalent bonds from four nitrogen atoms in the ring structure.

There are various types of porphyrin, so there are various different haem groups. The one we are interested in is called haem B, and a model of the haem B group (with the iron(III) ion in grey at the centre) looks like this:

Note: This diagram comes from Wikipedia, and you will also find a proper structure for the group on the page you will get to by following this link if you are interested (and a lot more information that you probably won't want to know about!).

You may have come across haemoglobin in the transport of oxygen around the blood. This is the same haem group that is at the heart of that - with one small difference. In haemoglobin, the iron is present as iron(II) rather than iron(III).

The reaction that catalase carries out is the decomposition of hydrogen peroxide into water and oxygen.

A lot of work has been done on the mechanism for this reaction, but I am only going to give you a simplified version rather than describe it in full. Although it looks fairly simple on the surface, there are a lot of hidden things going on to complicate it.

Essentially the reaction happens in two stages and involves the iron changing its oxidation state. An easy change of oxidation state is one of the main characteristics of transition metals. In the lab, iron commonly has two oxidation states (as well as zero in the metal itself), +2 and +3, and changes readily from one to the other.

In catalase, the change is from +3 to the far less common +4 and back again.

In the first stage there is a reaction between a hydrogen peroxide molecule and the active site to give:

The "Enzyme" in the equation refers to everything (haem group and protein) apart from the iron ion. The "(III)" and "(IV)" are the oxidation states of the iron in both cases. This equation (and the next one) are NOT proper chemical equations. They are just summaries of the most obvious things which have happened.

The new arrangement around the iron then reacts with a second hydrogen peroxide to regenerate the original structure and produce oxygen and a second molecule of water.

What is hidden away in this simplification are the other things that are happening at the same time - for example, the rest of the haem group and some of the amino acid residues around the active site are also changed during each stage of the reaction.

And if you think about what has to happen to the hydrogen peroxide molecule in both reactions, it has to be more complicated than this suggests. Hydrogen peroxide is joined up as H-O-O-H, and yet both hydrogens end up attached to the same oxygen. That is quite a complicated thing to arrange in small steps in a mechanism, and involves hydrogen ions being transferred via amino acids residues in the active site.

So do you need to remember all this for chemistry purposes at this level? No - not unless your syllabus specifically asks you for it. It is basically just an illustration of the term "prosthetic group".

It also shows that even in a biochemical situation, transition metals behave in the same sort of way as they do in inorganic chemistry - they form complexes, and they change their oxidation state.

And if you want to follow this up to look in detail at what is happening, you will find the same sort of interactions around the active site that we looked at in the simpler case of carbonic anydrase. (But please don't waste time on this unless you have to - it is seriously complicated!)

Coenzymes are another form of cofactor. They are different from prosthetic groups in that they aren't permanently attached to the protein molecule. Instead, coenzymes attach themselves to the active site alongside the substrate, and the reaction involves both of them. Once they have reacted, they both leave the active site - both changed in some way.

A simple diagram showing a substrate and coenzyme together in the active site might look like this:

It is much easier to understand this with a (relatively) simple example.

NAD+ as coenzyme with alcohol dehydrogenase

Alcohol dehydrogenase is an enzyme which starts the process by which alcohol (ethanol) in the blood is oxidised to harmless products. The name "dehydrogenase" suggests that it is oxidising the ethanol by removing hydrogens from it.

The reaction is actually between ethanol and the coenzyme NAD+ attached side-by-side to the active site of the protein molecule. NAD+ is a commonly used coenzyme in all sorts of redox reactions in the cell.

NAD+ stands for nicotinamide adenine dinucleotide. The plus sign which is a part of its name is because it carries a positive charge on a nitrogen atom in the structure.

The "nicotinamide" part of the structure comes from the vitamin variously called vitamin B3, niacin or nicotinic acid. Several important coenzymes are derived from vitamins.

Note: I'm not going to confuse you with the structures of NAD+ or even nicotinic acid - this page is already long enough! If you are interested, they are easy to find via a Google search. In common with what I have done on the rest of this page, this is just an example to illustrate how a coenzyme works which is reasonably easy to understand. You are unlikely to need details for any chemistry exam at this level.

Ethanol is oxidised by a reaction with NAD+ helped by the active site of the enzyme. At the end of the reaction, ethanal (acetaldehyde) is formed, and the NAD+ has been converted into another compound known as NADH.

As far as the NAD+ is concerned, it has picked up a hydrogen atom together with an extra electron which has neutralised the charge.

Both major products - ethanal and NADH - leave the active site and are processed further in other cell reactions.

The very poisonous ethanal is oxidised at once to ethanoic acid using a different enzyme, but again using NAD+ as the coenzyme. And the ethanoic acid from that reacts on through a whole set of further enzyme-controlled reactions to eventually end up as carbon dioxide and water.

What about the NADH? This is a coenzyme in its own right, and takes part in reactions where something needs reducing. The hydrogen atom and the extra electron that it picked up from the ethanol are given to something else. In the process, of course, the NADH gets oxidised back to NAD+ again.

In general terms, for a substrate S which needs reducing:

And one final time - do you need to remember any of this? No, unless this particular example is on your syllabus.

Note: Because this page is getting so long, and because there is still quite a bit of enzyme chemistry to talk about, it continues on another two pages. You will find the link to the first of these below.

What follows is a page about the effect of substrate concentration, temperature and pH on enzymes, and then a further page about enzyme inhibitors.