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NADH and FADH2, electron carrier molecules that were "loaded" during the citric acid cycle, are used in an electron transfer chain (involving NADH-Q reductase, succinate dehydrogenase, cytochrome reductase and cytochrome c oxidase) and to pump H+ across the membrane against a proton gradient.
A large protein complex called ATP synthase is embedded in that membrane and enables protons to pass through in both directions; it generates ATP from ADP and a phosphate when the proton moves with (down) the gradient, and it costs ATP to pump a proton against (up) the gradient. Because protons have already been pumped into the intermembrane space against the gradient, they now can flow back into the mitochondrial matrix via the ATP synthase, generating ATP in the process. The reaction is:
- ADP3- + H+ + Pi ↔ ATP4- + H2O
The synthase functions almost as a mechanical motor, with each NADH molecule contributing enough proton motive force to generate 2.5 ATP. Each FADH2 molecule is worth 1.5 ATP. All together, the 8 NADH and 2 FADH2 molecules contributed through oxidation of glucose (glycolysis, conversion of pyruvate to acetyl-CoA, and the Krebs cycle) account for 23 of the 30 total ATP energy carrier molecules. It is worth noting that these ATP values are maximum values and in reality proton leaks across the membrane cause somewhat lower values.
Photophosphorylation, which occurs when plants synthesize glucose during photosynthesis, also uses ATP synthase and a proton gradient to generate ATP. The process occurs across the thylakoid membrane when chlorophyll is energized by light and donates an excited electron to an electron transport chain.
There are a few well-known toxins that affect the process of oxidative phosphorylation and can lead to breakdown of the chain:
- Cyanide interrupts the electron transport chain in the inner membrane of the mitochondrion because it binds more strongly than oxygen to the Fe3+ (ferric iron ion) in cytochrome a3, preventing this cytochrome from combining electrons with oxygen.
- Oligomycin functions by inhibiting the ATP synthase protein and preventing it from generating ATP from the proton gradient.
- CCCP (m-chloro-carbonylcyanide-phenylhydrazine) destroys the proton gradient by allowing the protons to flow out of the membrane. Without the gradient, the ATP synthase cannot function and ATP synthesis breaks down.
- A detergent, or substance that destroys cellular membranes by breaking apart their lipid bilayers, will destroy the membrane used in the process and prevent a proton gradient.
- Rotenone prevents the transfer of electrons from Fe-S centers in Complex I (most notably) to ubiquinone. The electrons entering into Complex I are those derived from NADH, and provide the bulk of the reducing potential to the electron transport chain.
For each of these toxins, a backup will cause everything before it to break down as well. For example, if oligomycin is added, protons cannot pass through. As a result, the H+ pumps are unable to pass protons through because the gradient becomes too strong for them to overcome. NADH and FADH2 are then not oxidized and the citric acid cycle ceases to operate because there are no NAD+ and FAD coenzymes to be reduced.
Reactive oxygen speciesEdit
Several highly reactive, transient oxygen derivatives can be formed during this process:
A unique feature of the cytochrome c oxidase, complex IV, is its ability to maintain steric control over the reactive oxygen species created as it reduces oxygen to H2O.
- Interactive Molecular model of succinate dehydrogenase
- Interactive Molecular model of Coenzyme Q - cytochrome c reductase
- Interactive Molecular model of cytochrome c oxidase
1. Leninger, D., Cox, M. Principles of Biochemistry 3rd Ed., Worth Publications, New York, NT., 2001.
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