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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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.
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).
Three types of cytochrome are distinguished by their prosthetic groups:
|Cytochrome a||heme a|
|Cytochrome b||heme b|
|Cytochrome d||tetrapyrrolic chelate of iron|
|a and a3||Cytochrome c oxidase ("Complex IV")|
|b and c1||Coenzyme Q - cytochrome c reductase ("Complex III")|
|b6 and f||Plastoquinol—plastocyanin reductase|
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.