Expansion
Four complexes pumping protons, and ATP synthase using the gradient
Expansion
The complexes
- Complex I - NADH dehydrogenase. Pumps protons. Inhibited by rotenone, metformin at high dose
- Complex II - succinate dehydrogenase, also a Krebs cycle enzyme. Does not pump protons, which is why FADH2 yields less ATP than NADH
- Complex III - cytochrome bc1. Pumps protons. Inhibited by antimycin A
- Complex IV - cytochrome c oxidase. Pumps protons, and reduces oxygen to water. Inhibited by cyanide, carbon monoxide and azide
- ATP synthase (complex V) - uses the proton gradient to make ATP. Inhibited by oligomycin
Mobile carriers: coenzyme Q (ubiquinone) between I/II and III, and cytochrome c between III and IV.
Yields: NADH gives about 2.5 ATP, FADH2 about 1.5, because FADH2 enters at complex II and therefore bypasses one proton pumping step.
“Cyanide stops complex IV, so oxygen cannot be used at all.” That is why cyanide poisoning gives a high venous oxygen saturation with a profound lactic acidosis in a patient who is not cyanosed.
Uncouplers such as 2,4-dinitrophenol, aspirin in overdose and physiological thermogenin in brown fat dissipate the gradient as heat, so oxygen consumption rises while ATP production falls, causing hyperthermia.