cytochrome

A class of hemoprotein whose principal biologic function is electron and/or hydrogen transport by virtue of a reversible valency change of the heme iron. Cytochromes are classified in four groups (a, b, c, and d) according to spectrochemical characteristics; many variants exist, particularly among bacteria and in green plants and algae, one being a variant of the c type c. called c. f. The mitochondrial system of cytochromes provides electron transport through c. c oxidase to molecular oxygen as the terminal electron acceptor (respiration). [cyto- + G. chroma, color]

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cy·to·chrome 'sīt-ə-.krōm n any of several intracellular hemoprotein respiratory pigments that are enzymes functioning in electron transport as carriers of electrons

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n.

a compound consisting of a protein linked to haem. Cytochromes act as electron transfer agents in biological oxidation-reduction reactions, particularly those associated with the mitochondria in cellular respiration. See electron transport chain.

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cy·to·chrome (siґto-krōm) [cyto- + -chrome] any electron transfer hemoprotein having a mode of action in which the transfer of a single electron is effected by a reversible valence change of the central iron atom of the heme prosthetic group between the +2 and +3 oxidation states; classified as cytochromes a when the heme contains a formyl side chain, cytochromes b when protoheme (or a closely similar heme) is not covalently bound to the protein, cytochromes c when protoheme or other heme is covalently bound to the protein, and cytochromes d when the iron-tetrapyrrole has fewer conjugated double bonds than the hemes have. Well-known cytochromes have been numbered consecutively within groups and are designated by subscripts (beginning with no subscript), e.g., cytochromes c, c₁, c₂, etc. New cytochromes are named according to the wavelength in nanometers of the absorption maximum of the α-band of the iron (II) form in pyridine, e.g., c-555.