cytochrome, protein containing heme (see coenzyme) that participates in the phase of biochemical respiration called oxidative phosphorylation. Cytochromes act as carriers of hydride ions (sometimes considered to be the equivalent of electron pairs) in the series of complex enzymes known as the electron transport chain. As the hydride ions or their equivalent travel along the electron transport chain, each cytochrome is in turn reduced (accepts a hydride ion or pair of electrons) and then oxidized (donates the hydride ion or pair of electrons to the next acceptor in the chain); in the process the iron atom in the cytochrome heme shuttles between the ferrous and ferric states. The cytochromes were discovered in 1886.

Any of a group of cell proteins (hemoproteins) that serve a vital function in the transfer of energy within cells. Hemoproteins are linked to a nonprotein, iron-bearing component (a heme group), which can undergo the reversible oxidation-reduction reactions that yield energy for the cell. Cytochromes are subdivided into three classes depending on what wavelengths of light they absorb. At least 30 different cytochromes have been identified.

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