Cytochrome

Cytochromes are a group of heme-containing proteins that catalyze electron transfer in biological oxidation-reduction reactions. Each cytochrome contains a heme prosthetic group, an iron-containing porphyrin that cycles between Fe2+ and Fe3+ states as electrons are donated and accepted. This redox versatility allows cytochromes to shuttle electrons between enzymes in respiration, photosynthesis, and various metabolic pathways.

Cytochromes are widespread in living organisms. In mitochondria they participate in the respiratory chain (for example, cytochromes b and c1 in Complex III and cytochromes a and a3 in Complex IV). Cytochrome c is a small soluble carrier that transfers electrons from Complex III to Complex IV. In chloroplasts and bacterial membranes, similar cytochromes participate in photosynthetic electron transport. A different and vast family are the cytochromes P450, which are heme-containing monooxygenases that catalyze hydroxylation and other oxidation reactions; they are sometimes referred to as cytochromes due to their heme group but function as enzymes rather than simple electron carriers.

Structural differences distinguish cytochromes. C-type cytochromes (such as cytochrome c) have heme covalently linked to the polypeptide via two thioether bonds with cysteine residues and carry a single reactive Fe center coordinated by histidine as an axial ligand. Other cytochromes (a and b types) bind heme through non-covalent interactions and coordinate iron with histidine and methionine ligands.

Cytochromes are essential for energy production and redox balance; their malfunction can disrupt cellular respiration and, in metazoan cells, apoptotic signaling. They were first identified by David Keilin in the early 20th century.