Cytochromes are generally membrane-bound proteins that contain heme groups and carry out electron transport or catalyse reductive/oxidative reactions. They are found in the mitochondrial inner membrane and endoplasmic reticulum of eukaryotes, in the chloroplasts of plants, in photosynthetic microorganisms, and in bacteria.

The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states. For many cytochromes the metal ion present is that of iron, which interconverts between Fe2+ (reduced) and Fe3+ (oxidized) states (electron-transfer processes) or between Fe2+ (reduced) and Fe5+ (formal, oxidized) states (oxidative processes). Cytochromes are thus capable of performing oxidation and reduction. Because the cytochromes (as well as other complexes) are held within membranes in an organized way, the redox reactions are carried out in the proper sequence for maximum efficiency.

In the process of oxidative phosphorylation, which is the principal energy-generating process undertaken by organisms which need oxygen to survive, other membrane-bound and soluble complexes and cofactors are involved in the chain of redox reactions, with the additional net effect that protons (H+) are transported across the mitochondrial inner membrane. The resulting transmembrane proton gradient (protonmotive force) is used to generate ATP, which is the universal chemical energy currency of life. ATP is consumed to drive cellular processes that require energy (such as rotation of flagella, transport of molecules across the membrane, and synthesis of macromolecules).

Several kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, and reduction potential:

- Cytochrome a
- Cytochrome a3
- Cytochrome b
- Cytochrome c
- Cytochrome c1
- Cytochrome f

See the articles on mitochondria and chloroplasts for more information on electron transport and related metabolic pathways. See cytochrome P450 oxidase for more on steroidogenesis and detoxification enzymes.


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