Canadian Medical Guide > Chemicals and Drugs > Enzymes and Coenzymes > Cytochromes Terms and Definitions

Cytochromes

Medical Definition:

Hemeproteins whose characteristic mode of action involves transfer of reducing equivalents which are associated with a reversible change in oxidation state of the prosthetic group. Formally, this redox change involves a single-electron, reversible equilibrium between the Fe(II) and Fe(III) states of the central iron atom (From Enzyme Nomenclature, 1992, p539). The various cytochrome subclasses are organized by the type of HEME and by the wavelength range of their reduced alpha-absorption bands.

Guide Notes:

GEN or unspecified; prefer specific cytochrome

Cytochromes Categories.

	Cytochrome a Group - Cytochromes (electron-transporting proteins) in which the heme prosthetic group is heme a, i.e., the iron chelate of cytoporphyrin IX. (From Enzyme Nomenclature, 1992, p539)
	Cytochrome b Group - Cytochromes (electron-transporting proteins) with protoheme (HEME B) as the prosthetic group.
	Cytochrome c Group - A group of cytochromes with covalent thioether linkages between either or both of the vinyl side chains of protoheme and the protein. (Enzyme Nomenclature, 1992, p539)
	Cytochrome d Group - Cytochromes (electron-transporting proteins) with a tetrapyrrolic chelate of iron as a prosthetic group in which the degree of conjugation of double bonds is less than in porphyrin. (From Enzyme Nomenclature, 1992, p539)
	Cytochrome P-450 Enzyme System - A superfamily of hundreds of closely related HEMEPROTEINS found throughout the phylogenetic spectrum, from animals, plants, fungi, to bacteria. They include numerous complex monooxygenases (MIXED FUNCTION OXYGENASES). In animals, these P-450 enzymes serve two major functions: (1) biosynthesis of steroids, fatty acids, and bile acids; (2) metabolism of endogenous and a wide variety of exogenous substrates, such as toxins and drugs (BIOTRANSFORMATION). They are classified, according to their sequence similarities rather than functions, into CYP gene families (>40% homology) and subfamilies (>59% homology). For example, enzymes from the CYP1, CYP2, and CYP3 gene families are responsible for most drug metabolism.
	Cytochromes f - Cytochromes f are found as components of the CYTOCHROME B6F COMPLEX. They play important role in the transfer of electrons from PHOTOSYSTEM I to PHOTOSYSTEM II.

Cytochromes Medical Definitions and Terms

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Cytochromes

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