Electron Transport System and Oxidative Phosphorylation

22 vocabulary flashcards covering the components, carriers, and mechanisms of the mitochondrial electron-transport system.

Complex I (NADH-CoQ Oxidoreductase)

FMN accepts a hydride from NADH, but passes e- singly to iron-sulfate clusters; these clusters are close together, so e-s flow down and end at Co-Q

Complex II (Succinate-CoQ Oxidoreductase)

Does not pump H+s; contains FAD (from fumarate rxn of CAC) → FADH2 prostetic group attached to 3 iron-sulfur centers; cytochrome b acts as an e- sink to prevent free radicals

Complex III (CoQ–Cytochrome c Oxidoreductase)

Carries out the Q cycle; has cytochrome c, heme c

Complex IV (Cytochrome c Oxidase)

Final ETC complex; has Cyt c’s; CuA/CuA center with Heme a, Heme a3, CuB; molecular oxygen (O2)

Ubiquinone (Coenzyme Q)

Hydrophobic-tailed, soluble carrier that diffuses within the inner membrane, accepting up to two electrons and two protons. ubiquinone (Q) → semiquinone (QH) → ubiquinol (QH2)

Q Cycle components

1. Q and QH2 pool

2. Cytochrome c1 (bound)

3. Cytochrome c (docked, mobile)

4. Cytochrome b (with 2 b-type hemes - L and H)

5. Rieske 2Fe2S complex

6. Two sites for Q/QH/QH2 binding - QP (high QH2 affinity) + QN (high Q affinity)

7. electrons

8. protons

9. P side on intermembrane space

10. N side on matrix side

cytochromes

electron-transport heme proteins with heme groups that alternate between Fe(II) and Fe(III) oxidation states during e- transport; can hold H+

Q cycle step 1

1. QH2 binds at P site and donates one e- to Rieske-FeS-complex; e- moves to Cyt c1, then Cyt c

2. QH2’s protons move into IMS, and (now QH) donates other e- to Cyt bL, then Cyt bH

3. Cyt c leaves with its new e-

4. Q (former QH2) leaves P site and goes back into the pool

5. A different Q from the pool entered the N site and picked up the e- from Cyt bH and became QH

Q cycle step 2

1. New QH2 comes into P site - protons go into IMS and e- donated as before, so now Q leaves

2. QH in N site picks up another e- from Cyt bH; now it is QH2, so it leaves the N site

3. A new Cyt c picks up an e- from the second P site QH2 + leaves

Q cycle net gains

4 H+s pumped into the IMS; 2 H+ taken in from matrix; 2 e- Cyt c’s move onto Complex IV

Complex IV steps

1. 2 cyt c’s (each carry 1 e-) come into Complex IV one at a time

2. First e- from cyt c moves to CuA/CuA center, then to heme a, heme a3, then CuB; CuB is reduced from Cu2+ to Cu+

3. Second e- moves from Cyt c down to heme a3; iron in heme a3 is reduced from Fe3+ to Fe2+

4. Once CuB and heme a3 are both reduced by 1 e-, they can interact with O2, and a peroxide bridge is formed with O2 in the middle

5. 2 more Cyt c’s dock and donate e-s to CuB and heme a3

6. Two protons are taken from matrix to break peroxide bridge, forming CuB-OH and heme a3-OH

7. Two more protons are extracted from matrix to form water, and CuB and a3 are regenerated in their oxidized form

8. Separately, 4 H+ are pumped in from matrix into IMS (2 H+ per 2 Cyt c’s)

Complex IV net movement

4 Cyt c’s, 1 O2, 4 H+ (2 per 2 Cyt c’s), 2 H2O (only ½ O2 used per 2 Cyt c’s)