Complex I: NADH dehydrogenase

• aka NADH-CoQ reductase
• NADH delivers 2 electrons to the complex I and is oxidized to NAD+

CoQ (aka Q10 and ubiquinone)

• Lipid-soluble
• Mobile carrier
• NOT a protein

Complex II: succinate dehydrogenase

• aka Succinate-CoQ reductase
• Also part of citric acid cycle
• FADH2 delivers two electrons to complex II

Complex III: Cytochrome bc1 complex

• aka CoQ-cytochrome c reductase

Cytochrome C

• Water-soluble
• Mobile carrier

Complex IV: cytochrome c oxidase

• Produces one H2O from 2 H+ plus ½ O2 + 2e-
• Complexes I, III & IV pump H+ from matrix to intermembrane space
  (NOT complex II, cyt. C or CoQ)

COFACTORS

• Complex I – flavin mononucleotide (FMN).
• Complex II – FAD & FeS
• Complex III – Heme (Cyt. B & Cyt. C1) & FeS
• Complex IV – heme (Cyt. A & Cyt. A3) & Cu

REDOX REACTIONS

• NADH (2e-) reduces complex I
• FADH (2e-) reduces complex II
• CoQ (mobile carrier) transports 2e- from complex I & II to complex III
• Complex III (simplified diagram): Cyt. B & Cyt. C1 are e- transport proteins
• Cyt. C1 donates 2e- (1e- at a time) to Cyt. C (mobile-carrier)
• Cyt. C transports 4e- to complex IV (2 molecules of Cyt. C deliver 2e- each)
• Complex IV (simplified diagram w/o Cu centers): Cyt. A (4e-) to Cyt. A3 (4e-) to oxygen
• Oxygen (final e- acceptor) 4e- + 4H+ (from matrix) + O2 –> H2O

3 BYPASS REACTIONS

• Bypass complex I & produce less ATP

1. Succinate delivers e- to complex II via FADH2
2. Acyl CoA dehydrogenase (on matrix side) oxidizes fatty acyl CoA & produces FADH2
3. Cytosolic NADH delivers e- via glycerol-3-phosphate

• Enzyme glycerol-3-phosphate dehydrogenase (on intermembrane side) produces FADH2

CHEMIOSMOTIC GRADIENT

• Electrical gradient: from less positive in the matrix to more positive in the intermembrane space.
• pH gradient: from a lower pH in the intermembrane space to a higher pH in the matrix.