Q7(b). Cytochromes: prosthetic group, haemoglobin, function

1. Prosthetic group: a non-protein unit, tightly (often covalently) bound to a protein, that is essential for its activity. In cytochromes it is a haem: Fe held by the four N atoms of a porphyrin ring (haem a, b or c; in cytochrome c the haem is bonded to the protein through two thioether links from cysteine).
2. Coordination. Cytochromes are six-coordinate: besides the four porphyrin N they have two stable axial bonds to amino-acid donors (histidine N and methionine S in cytochrome c). Haemoglobin and myoglobin are five-coordinate (one proximal histidine); the sixth site is empty or holds H₂O or O₂.
3. Oxidation state and spin. The cytochrome Fe uses the Fe³⁺/Fe²⁺ couple and is low spin in both states. In haemoglobin the Fe stays Fe(II); oxidation to Fe(III) (methaemoglobin) destroys O₂ binding.
4. Structure and binding. Haemoglobin is a tetramer and binds O₂ cooperatively; cytochromes are small single-chain proteins (or subunits of larger enzymes) and, with no vacant site, do not bind O₂ (except cytochrome c oxidase and P450, where the binding site is part of the catalytic centre).
5. Main function: electron transfer. Cytochromes carry electrons along the mitochondrial respiratory chain (cytochrome b → c₁ → c → cytochrome c oxidase, which reduces O₂ to water) and in photosynthesis. The haem buried in the protein, with little change in structure between Fe(II) and Fe(III), gives fast electron transfer.

Application. the energy released as electrons pass down the cytochrome chain pumps protons across the mitochondrial membrane and drives ATP synthesis; cyanide and CO are poisons because they bind to the iron of cytochrome c oxidase.