#33 Biochemistry Electron Transport/Oxidative Phosphorylation Lecture for Kevin Ahern's BB 451/551





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Published on Jan 25, 2012

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Highlights of Electron Transport

1. Oxidation is a process that involves the loss of electrons. Reduction is a process that involves the gain of electrons.
2. Electrons are carried to the electron transport system in the mitochondria by NADH and FADH2.
3. Mitochondria are the site of electron transport and oxidative phosphorylation.
4. Electrons from NADH enter the electron transport system through complex I.
5. Electrons from FADH2 enter the electron transport system through complex II.
6. Coenzyme Q accepts a pair of electrons from either complex I or complex II and passes electrons singly to cytochrome c through complex III.
7. The sequence of electrons passing from coenzyme Q is as follows: Coenzyme Q goes to Complex III goes to Cytochrome c goes to Complex IV goes to oxygen (to form water)
8. Oxygen is the terminal electron acceptor and is a limiting compound during periods of heavy exercise.
9. If oxygen is not available, electrons will NOT pass through the electron transport system and NADH and FADH2 will not be reoxidized. This is part of metabolic control.
10. Several compounds inhibit electron transport - rotenoneand amytal block all action of Complex I. Antimycin A blocks action of Complex III. Cyanide and carbon monoxide block action of complex IV.
11. Movement of electrons through Complex III is known as the Q cycle. This cycle begins with the binding of two molecules of CoQ (QH2 and Q) to Complex III. QH2 has two electrons and two protons.
12. After QH2 and Q bind, QH2 sends one electron to Q, creating Q- and one electron to cytochrome C. This converts QH2 to Q. Both cytochrome C and Q leave the complex, but Q- remains behind.
13. Next, another QH2 and another cytochrome C binds to Complex III. QH2 sends one electron to Q-, creating Q-2 and one electron to cytochrome C. Then Q-2 extracts two protons from the matrix and becomes QH2. Last, cytochrome C, QH2, and Q all leave the complex.
14. Electron transfer through complex IV occurs one electron at a time (since one electron arrives at a time from cytochrome c). Interruption of electron flow can result in production of reactive oxygen species.
15. In electron flow through complex IV, the first electron is transferred to copper and the second one is transferred to iron. Oxygen then binds to the iron first, followed by formation of a peroxide bridge between the iron and copper atoms. Addition of a third electron (to the oxygen on the copper) and binding of a proton from the matrix causes the O-O bond to be cleaved. A fourth electron then reduces the oxygen on the iron and a proton binds from the matrix as well.
16. During electron movement through Complex IV, four protons are taken from the matrix and combined with oxygen to form two water molecules. In addition, four other protons are taken from the matrix by the complex and pumped outside the mitochondrial matrix.

Oxidative Phosphorylation

1. ATP is created in oxidative phosphorylation by the movement of protons back into the mitochondrial matrix through complex V.
2. Two essential functions of electron transport - 1. Pump protons out of mitochondrial matrix and 2. Reoxidize NADH and FADH2 to NAD and FAD. In healthy, normal cells, oxidative phosphorylation is tightly coupled to electron transport.
3. Important aspects of the chemiosmotic process include:
a. Intact inner mitochondrial membrane
b. Electron transport creates a proton gradient
c. ATP is made by movement of protons back into the mitochondria
4. Coupling of electron transport and oxidative phosphorylation at a practical level means that the mitochondrial inner membrane remains impermeable to protons, except for those that enter via the ATP synthase.
5. The ATP synthase consists of a turbine-like structure containing 3 sites called Loose (L), Tight (T), and Open (O).

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