Oxidative Phosphorylation - Eukaryotic Electron Transport Chains

Eukaryotic Electron Transport Chains

Further information: Electron transport chain and Chemiosmosis

Many catabolic biochemical processes, such as glycolysis, the citric acid cycle, and beta oxidation, produce the reduced coenzyme NADH. This coenzyme contains electrons that have a high transfer potential; in other words, they will release a large amount of energy upon oxidation. However, the cell does not release this energy all at once, as this would be an uncontrollable reaction. Instead, the electrons are removed from NADH and passed to oxygen through a series of enzymes that each release a small amount of the energy. This set of enzymes, consisting of complexes I through IV, is called the electron transport chain and is found in the inner membrane of the mitochondrion. Succinate is also oxidized by the electron transport chain, but feeds into the pathway at a different point.

In eukaryotes, the enzymes in this electron transport system use the energy released from the oxidation of NADH to pump protons across the inner membrane of the mitochondrion. This causes protons to build up in the intermembrane space, and generates an electrochemical gradient across the membrane. The energy stored in this potential is then used by ATP synthase to produce ATP. Oxidative phosphorylation in the eukaryotic mitochondrion is the best-understood example of this process. The mitochondrion is present in almost all eukaryotes, with the exception of anaerobic protozoa such as Trichomonas vaginalis that instead reduce protons to hydrogen in a remnant mitochondrion called a hydrogenosome.

Typical respiratory enzymes and substrates in eukaryotes.

Respiratory enzyme	Redox pair	Midpoint potential (Volts)
NADH dehydrogenase	NAD+ / NADH	−0.32
Succinate dehydrogenase	FMN or FAD / FMNH2 or FADH2	−0.20
Cytochrome bc1 complex	Coenzyme Q10ox / Coenzyme Q10red	+0.06
Cytochrome bc1 complex	Cytochrome box / Cytochrome bred	+0.12
Complex IV	Cytochrome cox / Cytochrome cred	+0.22
Complex IV	Cytochrome aox / Cytochrome ared	+0.29
Complex IV	O2 / HO−	+0.82
Conditions: pH = 7