cytochrome n : a class of hemoprotein whose principle biological function is as carriers of electrons
EtymologyFrom cyto- + ‘colour’.
- Any of various related hemoprotein compounds found in the
cells of most organisms,
which are an important part of cell respiration.
- 2001: The drug-metabolizing enzymes in the liver, called cytochrome-p450s, comprise a large family of some fifty or more related enzymes, which have evolved in a remarkable way — Leslie Iversen, Drugs: A Very Short Introduction (Oxford 2001, p. 33)
Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.
They are found either as monomeric proteins (e.g., cytochrome c) or as subunits of bigger enzymatic complexes that catalyze redox 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.
Structure and functionThe 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 Fe3+ (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 synthesis of macromolecules, active transport of molecules across the membrane, and assembly of flagella).
TypesSeveral kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, and reduction potential.
Three types of cytochrome are distinguished by their prosthetic groups:
The definition of cytochrome c is not defined in terms of the heme group. There is no "cytochrome e," but there is a cytochrome f, which is often considered a type of cytochrome c.
In mitochondria and chloroplasts, these cytochromes are often combined in electron transport and related metabolic pathways:
A completely distinct family of cytochromes is known as the cytochrome P450 oxidases, so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (with sodium dithionite) and complexed to carbon monoxide. These enzymes are primarily involved in steroidogenesis and detoxification.
cytochrome in Czech: Cytochrom
cytochrome in German: Cytochrome
cytochrome in Spanish: Citocromo
cytochrome in French: Cytochrome
cytochrome in Italian: Citocromi
cytochrome in Hebrew: ציטוכרום
cytochrome in Macedonian: Цитохром
cytochrome in Japanese: シトクロム
cytochrome in Polish: Cytochromy
cytochrome in Portuguese: Citocromo
cytochrome in Russian: Цитохромы
cytochrome in Chinese: 细胞色素