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Cellular respiration, why double membrane in mitochondria and not bacteria?

Cellular respiration, why double membrane in mitochondria and not bacteria?


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Bacteria perform cellular respiration across a single membrane, their plasma membrane. What are the benefits of having double membranes in eukaryotes (in the mitochondria), and, how do bacteria achieve the proton gradient?


4.5: Cellular Respiration

  • Contributed by John W. Kimball
  • Professor (retired) at Tufts University & Harvard

Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water.

[C_6H_<12>O_ <6>+ 6O_2 + 6H_2O &rarr 12H_2O + 6 CO_2 ]

The energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell. The process occurs in two phases:

  • glycolysis, the breakdown of glucose to pyruvic acid
  • the complete oxidation of pyruvic acid to carbon dioxide and water

In eukaryotes, glycolysis occurs in the cytosol and the remaining processes take place in mitochondria.


What are Mitochondria?

Mitochondria are known as the &lsquoPowerhouse of the cell&rsquo. Their immediate function is to convert glucose into ATP (Adenosine Triphosphate). ATP can be considered the &lsquocurrency&rsquo of the cell. It is the basic unit of energy that is required to power the chemical reactions in our body. The process of this conversion is known as aerobic respiration and it is the reason why humans need to breathe oxygen.

Animal mitochondrion diagram(Photo Credit : Mariana Ruiz Villarreal LadyofHats / Wikimedia Commons)

Structure

Mitochondria have no constant shape and size, but the overall structure remains same, which is rod-shaped. Their size varies between 1 and 10 micrometers in length. Their number in each cell varies according to the metabolic activity of each specific cell. The structure is very simple, with four different compartments. It has a smooth outer membrane and a highly convoluted inner membrane. These convolutions give rise to cristae. Then it has an inter-membrane space, and finally, we have the matrix in the interior.


Double membrane

double membrane -- In mitochondria and plastids, there is a two-layered membrane which surrounds the organelle. This is believed to be the result of endosymbiosis, with the outer membrane coming from the eukaryotic cell, and the inner membrane belonging to the original prokaryote which was "swallowed".

A double membrane across the midline of a dividing plant cell, between which the new cell wall forms during cytokinesis.
cell theory
All living things are composed of cells cells arise only from other cells. No exception has been found to these two principles since they were first proposed well over a century ago.

Nucleus
Double membrane surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm. The nucleolus is a site for synthesis of RNA making up the ribosome.

enclosed organelle that contains genetic material DNA and processes messenger RNA. Lecture - Cell Nucleus
nucleolus .

across the equator of a dividing cell that develops from the phragmoplast it marks where the new cell walls will form.
cell wall The rigid outermost layer of the cells found in plants, some protists, and most bacteria. Found in plants composed principally of cellulose.

. This is not a valid comparison-the inner mitochondria membrane is used to run proton pumps and carry out oxidative phosphorylation across to generate ATP energy.

enclosed cell organelles found in the cell cytoplasm. It has many functions but by far the most important role that it possesses is as the cells power plant. It provides energy in the form of ATP from the breaking down of sugar molecules.

DNA
circular (usually)
linear molecules (chromosomes) with histone proteins
RNA/protein synthesis
coupled in the cytoplasm .

that forms at the equator of dividing plant cells during mitotic telophase. A new cell wall forms between the two membranes.
cell sap The liquid inside a vacuole.

A cell nucleus is surrounded by a

, known as the nuclear envelope. This membrane covers and protects the DNA from physical and chemical damage. In doing so, the membrane creates a separate environment to process the DNA in.

The ellipsoid-shaped chloroplast is enclosed in a

is much more permeable than the inner layer, which features a number of embedded membrane transport proteins.

that separates the contents of the nucleus from the cytoplasm. Nuclear pores Gaps in the nuclear envelope that allow substances to move in and out of the nucleus. Nucleic acidA polymer of nucleotides found in all living things.

which has pores in it for molecules to come in and out. The outer membrane of the nucleus is continuous with the endoplasmic reticulum, a network of membranes in the cytoplasm.

Sorry that organelle there called the chloroplast and you can see it has its own

around it just like the mitochondria has two membranes in and out of it.

The nucleus is surrounded by a nuclear envelope, which is a

shields the nucleus and its contents from unwanted guests. Some proteins need to be in the nucleus to assist in processes such as replication and transcription.

A relatively large structure that can occupy a significant amount of the volume of a cell, the mitochondria is a

-bound organelle found in nearly all eukaryotic cells.

that encloses the cell nucleus. It serves to separate the chromosomes from the rest of the cell. The nuclear membrane includes an array of small holes or pores that permit the passage of certain materials, such as nucleic acids and proteins, between the nucleus and cytoplasm.

A cup-shaped structure with a thin

surrounding the glomerulus of each nephron of the vertebrate kidney. It serves as a filter to remove organic wastes, excess inorganic salts, and water.

Inner, fluid-filled sac composed of a thin

that surrounds the embryo in reptiles, birds, and mammals.
Source: Curtis, Helena. 1968. Biology. New York, NY. Worth Publishers
.

A specialized organelle in plant cells that is surrounded by a

and contains internal chlorophyll-containing membranes (thylakoids) where the light-absorbing reactions of photosynthesis occur. (Figure 16-34)
Full glossary .

Mitochondria are membrane-bound organelles, and like the nucleus have a

. The outer membrane is fairly smooth. But the inner membrane is highly convoluted, forming folds (cristae) when viewed in cross-section. The cristae greatly increase the inner membrane's surface area.

chloroplasts Disk-like organelles with a

found in eukaryotic plant cells contain thylakoids and are the site of photosynthesis. ATP is generated during photosynthesis by chemiosmosis. PICTURE .

The nucleus is separated from the surrounding cytoplasm by the

around it, the nuclear envelope. This regulates the flow of substances into and out of the nucleus.
Other organelles .

Contains chromosomes (genes made of DNA which control cell activities)
Separated from the cytoplasm by a nuclear envelope
The envelope is made of a

containing small holes
These small holes are called nuclear pores (100nm)
Nuclear pores allow the transport of proteins into the nucleus .

Green algae have chloroplasts containing chlorophylls a and b, bound by

s, and come in a variety of forms: flagellate, colonial, filamentous, and even primitively multicellular.

And both organelles use their DNA to produce many proteins and enzymes required for their function. A

surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive host.

The nucleus averages about 5 microns in diameter.
The nucleus is separated from the cytoplasm by a

called the nuclear envelope.

the differences in their structures, the bacterial 70S ribosomes are vulnerable to these antibiotics while the eukaryotic 80S ribosomes are not. Even though mitochondria possess ribosomes similar to the bacterial ones, mitochondria are not affected by these antibiotics because they are surrounded by a


Mitochondria

Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. The formation of ATP from the breakdown of glucose is known as cellular respiration. Mitochondria are oval-shaped, double-membrane organelles (Figure 1) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae, which increase the surface area of the inner membrane. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria because muscle cells need a lot of energy to contract.

Figure 1This transmission electron micrograph shows a mitochondrion as viewed with an electron microscope. Notice the inner and outer membranes, the cristae, and the mitochondrial matrix. (credit: modification of work by Matthew Britton scale-bar data from Matt Russell)

Like mitochondria, chloroplasts also have their own DNA and ribosomes. Chloroplasts function in photosynthesis and can be found in eukaryotic cells such as plants and algae. Carbon dioxide (CO2), water, and light energy are used to make glucose and oxygen in photosynthesis. This is the major difference between plants and animals: Plants (autotrophs) are able to make their own food, like glucose, whereas animals (heterotrophs) must rely on other organisms for their organic compounds or food source.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked, fluid-filled membrane sacs called thylakoids (Figure 2). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane and surrounding the grana is called the stroma.

Figure 2This simplified diagram of a chloroplast shows the outer membrane, inner membrane, thylakoids, grana, and stroma.

The chloroplasts contain a green pigment called chlorophyll, which captures the energy of sunlight for photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria also perform photosynthesis, but they do not have chloroplasts. Their photosynthetic pigments are located in the thylakoid membrane within the cell itself.

Theory of Endosymbiosis

We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species live in close association and typically exhibit specific adaptations to each other. Endosymbiosis (endo-= within) is a relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. Microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and are provided a stable habitat and abundant food by living within the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that mitochondria and chloroplasts have DNA and ribosomes, just as bacteria do. Scientists believe that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them. Through evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the photosynthetic bacteria becoming chloroplasts.


Respiration in Bacteria | Microbiology

Like other living things bacteria respire. They oxidize food materials present in the cytoplasm to obtain energy. Most bacteria make use of the free oxygen of the atmosphere or oxygen dissolved in the liquid environment.

They are called the aerobes or aerobic bacteria. They are so called because they can live only in the presence of free oxygen. Free oxygen is necessary for their respiration.

The free oxygen diffuses in through the bacterial cell wall and oxidizes the food materials present in the cytoplasm. The reaction takes place in two steps. The, first step includes the oxidation of food materials with the removal of pairs of hydrogen atoms.

The second step comprises the oxidation of hydrogen atoms by oxygen with the liberation of energy. The released energy is captured in ATP (Adenosine triphosphate) formed from Adenosine diphosphate (ADP) and phosphoric acid.

The oxidation of food thus results in the production of CO2, H2O and formation of ATP. Car bon dioxide diffuses to the exterior through the body surface. The important feature of respiration is not so much the gaseous exchange but the formation of ATP.

It is a compound in the body of a living organism which captures and stores the energy released in respiration which otherwise would simply have produced heat.

The energy entrapped in ATP is used gradually as the need arises to run many reactions in the bacterial cell.

Aerpbic respiration is represented by the following equation:

Anaerobes or Anaerobic bacteria:

There are a considerable number of bacteria which are able to live and multiply in the absence of free oxygen. In fact they perish in the presence of free oxygen. These peculiar bacteria obtain oxygen for their respiration from organic compounds such as sugar.

They are called the anaerobes or anaerobic bacteria. A good example of this type are the bacteria which decompose glucose to form alcohol and carbon dioxide.

Anaerobic respiration is accomplished by the secretion of certain oxidizing enzymes. The latter bring about breakdown of foods. It is followed by the rearrangement of atoms within the organic molecule.

Certain molecular groups take up the contained oxygen from the others. The amount of energy available from this type of respiration is much less than when free oxygen is used.

This is because glucose molecules are not completely oxidised. Much of the energy stored in glucose remains in alcohol. Some of the anaerobic bacteria do not require free oxygen. They are, actually poisoned or killed by its presence.

They can live and multiply only in the absence of free oxygen. Examples are syphilis and tetanus bacteria. Such bacteria are called obligate anaerobes.

There are other anaerobic bacteria which can live and grow whether oxygen is present or not. They are called the facultative anaerobes.


Mitochondria and cell death

In many models of cell injury or disease, the irreversibility of cell injury is primarily determined by aspects of mitochondrial biology. Cell death is broadly classified as apoptotic or necrotic – programmed or accidental – although the boundaries between forms of cell death are not always so clearly defined. Apoptotic cell death plays a crucial role in early development and later in life, in removing cells that are damaged without the energy loss associated with necrotic cell death. Apoptosis is an energy dependent, active and coordinated process while necrosis is typically the result of a metabolic failure leading to energetic collapse, breakdown of ion gradients, cell swelling and structural disorganization.

A major mechanism driving necrotic cell death is opening of the mPTP. Pore opening is implicated in an ever increasing array of disease states in many different tissues, although the strongest experimental case probably lies in cell death during ischaemia and reperfusion injury in the heart. This is important and exciting as the pore is a viable therapeutic target and so identification of its involvement carries with it implications of therapeutic opportunities.

First described by Hunter and Hapworth, the abrupt loss of the mitochondrial permeability barrier following additions of Ca 2+ or pro oxidants was later shown to result from the opening of a large conductance pore in the inner mitochondrial membrane large enough to entrap deoxyglucose. 59 Pore opening causes collapse of the mitochondrial membrane potential, ATP depletion and the rapid progression to cell death. It has been suggested that the pore is generated by a transformation of membrane proteins with other ‘normal’ functions into a pore forming configuration – a favoured candidate has been the adenine nucleotide translocase (ANT), as this protein can undergo a Ca 2+ dependent switch to a pore forming conformation, and pore opening is modulated by drugs which bind to the ANT. Recent experiments on tissues from an ANT knockout mouse have thrown a question mark over this model leaving the molecular identity of the pore uncertain. It is clear, however, that pore opening is regulated by the matrix protein cyclophilin D (CypD), which binds to cyclosporine A (CsA), preventing pore opening. Protection by CsA has now become the benchmark for pore opening and is now being used in clinical trials for mPTP involvement in various pathologies. The role of the mPTP in cell death during ischaemia and reperfusion in the heart is clear and unambiguous, and infarct size is clearly reduced in the CypD knockout. 60,61 Protection against a variety of pathologies has now been shown in the CypD knockout, including a reduction in stroke damage, and protection from experimental allergic encephalopathy. 62 Thus identification of cell death as necrotic does not necessarily mean that the injury is untreatable.

Programmed cell death or apoptosis occurs via two signalling pathways: (i) the extrinsic pathway which involves cell surface receptors culminating in caspase 8 activation and (ii) the intrinsic pathway that requires mitochondrial outer membrane permeabilization. 63 The complex role of mitochondria in mammalian cell death was highlighted when several studies elucidated resident mitochondrial proteins were able to stimulate cell death directly. 2,63,64 Under normal cellular conditions these proteins reside in the intermembrane space, and in response to death stimuli are released into the cytosol. They promote cell death by activating caspases and/or inactivating cytosolic inhibitors of this process. The intrinsic pathway is therefore a delicate balance between mitochondria and various cytosolic factors and it is this equilibrium that governs cellular integrity.

Apoptogenic proteins and mitochondria

Cytochrome c, an essential component of the electron transport chain initiates apoptosis when released from mitochondria. 65 Once released, cytochrome c binds to Apaf-1. Further stabilisation and binding of ATP to the Apaf-1/cytochrome c complex results in the oligomerisation and formation of the apoptosome ( Fig.ਃ ). This multimeric complex exposes the CARD domains of Apaf-1, resulting in an open conformation. This complex is able to recruit procaspase-9, and form the active apoptosome. 66 It is only caspase-9 that can cleave and activate the downstream executioner caspase-3. Loss of function studies in mice show that knockout of cytochrome c is embryonic lethal, however, at the level of a whole organism, it is difficult to distinguish whether this is largely due to its role in oxidative phosphorylation or cell death. 63 Studies of embryonic stem cells and fibroblasts from these mice show the importance of cytochrome c in terms of death stimuli. In response to UV, γ-irradiation and treatment with chemotherapeutic drugs, cells failed to show caspase activity and are essentially resistant to apoptosis. 64

Apoptotic activation via the intrinstic pathway. Apoptotic stimuli activates the BH3-only proteins, concurrently inactivating Bcl-2 and activating Bax translocation to mitochondria. Bak is held in check by Mcl-1, VDAC2 and Bcl-xL. Bax/Bakoligomerisation results in cytochrome c release and MOMP. apaf-1 is activated by cytochrome c binding, displacing the CARD domain. The apoptosome forms with caspase-9, activating caspase-3 and triggering apoptosis.

Bcl-2 was the first example of an oncogene that inhibits cell death rather than promotes proliferation. 67 The Bcl-2 family of proteins is classified into two groups, pro-survival (Bcl-xL, Bcl-w, A1 and Mcl-1) and pro-apoptotic (Bax, Bak, Bok, Bid, Bim, Bad, Noxa and Puma). 68,69 The apoptogenic proteins can further be classified by the amount of Bcl-2 homology domains they contain. The BH3-only class of proteins contain a BH3 domain and amphipathic helix responsible for the interaction with the Bcl-2 family members. 70 The majority of the BH3-only proteins translocate to the mitochondrial outer membrane upon death stimuli. The relocation to mitochondria is a critical and essential stage in cell death as it is the interaction of the BH3-only proteins with the pro-apoptotic Bcl-2 family members (Bax and Bak) that promote cell death. 70 This translocation of the BH3-only proteins occurs simultaneously with the conformational changes and subsequent oligomerisation of Bax and Bak at the mitochondrial surface. 71

In viable mammalian cells Bax is located in the cytosol with small amounts loosely associated with themitochondrial surface. 72 Bax cycles on and off the outer membrane where it is retrotranslocated to the cytosol by Bcl-xL. 73 This may be a regulatory checkpoint to ensure Bax levels on mitochondria do not accumulate to levels that result in auto-activation. Conversely, upon apoptotic stimuli Bax undergoes a two-step conformational change where the hydrophobic C-terminal region once concealed within the hydrophobic pocket is exposed causing the protein to translocate to mitochondria. 74 A second conformational change occurs when the 㬕 and 㬖 helices insert directly into the outer membrane, culminating in mitochondrial outer membrane permeabilization (MOMP) and cytochrome c release. 75 The mechanism that triggers Bax association with the mitochondrial surface in healthy cells is unclear, however, experiments performed with liposomes suggest that contact with the lipid bilayer may be sufficient. 76 In addition to the Bcl-xL checkpoint that prevents lethal levels of Bax accumulation on mitochondria, the composition of the outer membrane itself, namely cholesterol content may hamper the complete conformational change required to activate apoptosis. 77 The regulation of Bax is a complex process that requires many additional proteins including the pro-apoptotic Bak. Bak is a resident protein of the mitochondrial outer membrane and is held in an inactive state by VDAC2, Mcl-1 and Bcl-xL. 78,79 As with Bax, it requires BH3-only proteins to oligomerise and cause MOMP. 80 Early in the activation process, the BH3 domain of Bak is exposed and subsequently interacts with the hydrophobic groove of another Bak molecule. 81 It is proposed that the newly oligomerised Bax and Bak form a transitional pore, allowing apoptogenic proteins, such as cytochrome c to pass through, form the active apoptosome and trigger downstream executioner caspases to complete the apoptotic process. 82,83


Structure of the Mitochondria

Mitochondria are membrane-bound organelles enclosed by a double membrane.

They have a smooth outer membrane enclosing the organelle and a folded inner membrane. The folds of the inner membrane are called cristae, the singular of which is crista, and the folds are where the reactions creating mitochondrial energy take place.

The inner membrane contains a fluid called the matrix while the intermembrane space located between the two membranes is also filled with fluid.

Because of this relatively simple cell structure, mitochondria have only two separate operating volumes: the matrix inside the inner membrane and the intermembrane space. They rely on transfers between the two volumes for energy generation.

To increase efficiency and maximize energy creation potential, the inner membrane folds penetrate deep into the matrix.

As a result, the inner membrane has a large surface area, and no part of the matrix is far from an inner membrane fold. The folds and large surface area help with the mitochondrial function, increasing the potential rate of transfer between the matrix and the intermembrane space across the inner membrane.


Cellular respiration, why double membrane in mitochondria and not bacteria? - Biology

All cells, whether they are prokaryotic or eukaryotic, have some common features. These common features are:

DNA, the genetic material contained in one or more chromosomes and located in a nonmembrane bound nucleoid region in prokaryotes and a membrane-bound nucleus in eukaryotes

Plasma membrane, a phospholipid bilayer with proteins that separates the cell from the surrounding environment and functions as a selective barrier for the import and export of materials

Cytoplasm, the rest of the material of the cell within the plasma membrane, excluding the nucleoid region or nucleus, that consists of a fluid portion called the cytosol and the organelles and other particulates suspended in it

1. The genetic material (DNA) is localized to a region called the nucleoid which has no surrounding membrane.

2. The cell contains large numbers of ribosomes that are used for protein synthesis.

3. At the periphery of the cell is the plasma membrane. In some prokaryotes the plasma membrane folds in to form structures called mesosomes, the function of which is not clearly understood.

4. Outside the plasma membrane of most prokaryotes is a fairly rigid wall which gives the organism its shape. The walls of bacteria consist of peptidoglycans. Sometimes there is also an outer capsule. Note that the cell wall of prokaryotes differs chemically from the eukaryotic cell wall of plant cells and of protists.


Mitochondria Structure

Most mitochondria are surrounded by two membranes, which would result when one membrane-bound organism was engulfed into a vacuole by another membrane-bound organism. The mitochondrial inner membrane is extensive and involves substantial infoldings called cristae that resemble the textured, outer surface of alpha-proteobacteria. The matrix and inner membrane are rich with the enzymes necessary for aerobic respiration.

Figure (PageIndex<1>): Mitochondrial structure: This electron micrograph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane.

Mitochondria have their own (usually) circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria. Mitochondria also have special ribosomes and transfer RNAs that resemble these components in prokaryotes. These features all support the hypothesis that mitochondria were once free-living prokaryotes.


Mysterious workings of cholera bacteria uncovered

Researchers have found that an enzyme in the bacteria that causes cholera uses a previously unknown mechanism in providing the bacteria with energy. Because the enzyme is not found in most other organisms, including humans, the finding offers insights into how drugs might be created to kill the bacteria without harming humans.

Blanca Barquera, a Rensselaer associate professor of biology, led a team (including research professor Joel Morgan and postdoctoral fellow Oscar Juarez) whose findings were published in the June 28 edition of the Proceedings of the National Academy of Science.

The team studied Na+-NQR, an enzyme that is essentially two linked machines to create energy from food and electrically charge the cell membrane of Vibrio cholerae, powering many cellular functions.

Vibrio cholerae causes cholera, a disease transmitted primarily through contaminated drinking water. Cholera, in which severe diarrhea and vomiting lead to rapid dehydration, is a major cause of death in the developing world, and in the aftermath of catastrophes that compromise water systems.

The Rensselaer team found that the way in which the two machines are linked in Na+-NQR is different from other respiratory enzymes and likely involves much more movement of the protein than has been observed in other enzymes.

Their work stems from an interest in cellular respiration. Cellular respiration carries electrons from food to oxygen, in what amounts to a controlled burn. This process releases energy.

"Cellular respiration is remarkable," Barquera said. "It is one of the most efficient energy conversion processes known, and nevertheless, does not require high temperatures. This efficiency has drawn the attention of researchers."

In more complex organisms, like humans, the process of creating energy for a cell -- respiration -- takes place in specialized organelles within the cell called mitochondria.

But in bacteria, which lack mitochondria, respiration occurs in the cell membrane. Na+-NQR is a respiratory enzyme found on the cell membrane of Vibrio Cholerae.

The enzyme creates energy through respiration and uses that energy to pump ions out of the cell, electrically charging the cell membrane and providing power for all the functions of the cell. Unlike similar enzymes found in many animals and bacteria, Na+-NQR pumps sodium ions out of the cell, rather than protons.

Barquera's paper in PNAS describes the mechanism the enzyme uses to convert energy using sodium ions.

"Na+-NQR plays the same role as human respiratory proteins but it is much smaller," Barquera said. "We want to understand how it works, how it produces energy. If we understand how Na+-NQR works, we can learn the basic principles used by living organisms to convert energy and transport ions."

Researchers studied the enzyme by removing it from the inner cell membrane and studying it in a solution. Na+-NQR, which prefers an environment of water and oil, flourished in a solution similar to detergent, which mimics the bacterial membrane.

"We have the enzyme off of the membrane with all of its components," Barquera said. Once isolated, the researchers observed the enzyme as it moved sodium from the inside to the outside of the cell.

Their study revealed the protein itself is moving the ions along a path through the cell membrane.

"It works in a very different way from enzymes in other bacteria and mitochondria. The catch and release of ions is done by movement of the protein," Barquera said.

Barquera said that, by modifying the protein in various ways, the researchers had identified the site on the protein where the ions begin and end their travel along the protein.

Next they want to map the route the ion takes along the protein.

"We can see the in and out site. Now we want to know the path," Barquera said.


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