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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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.

Medical dictionary. 2011.

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