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Cytochrome b5
Cytochromes ''b''5 are ubiquitous electron transport hemoproteins found in animals, plants, fungi and purple phototrophic bacteria. The microsomal and mitochondrial variants are membrane-bound, while bacterial and those from erythrocytes and other animal tissues are water-soluble. The family of cytochrome ''b''5-like proteins includes (besides cytochrome ''b''5 itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome ''b''2 (L-lactate dehydrogenase; ), sulfite oxidase (), plant and fungal nitrate reductases (, , ), and plant and fungal cytochrome ''b''5/acyl lipid desaturase fusion proteins.
==Structure==
3-D structures of a number of cytochrome ''b''5 and yeast flavocytochrome ''b''2 are known. The fold belongs to the α+β class, with two hydrophobic cores on each side of a β-sheet. The larger hydrophobic core constitutes the heme-binding pocket, closed off on each side by a pair of helices connected by a turn. The smaller hydrophobic core may have only a structural role and is formed by spatially close N-terminal and C-terminal segments. The two histidine residues provide the fifth and sixth heme ligands, and the propionate edge of the heme group lies at the opening of the heme crevice. Two isomers of cytochrome ''b''5, referred to as the A (major) and B (minor) forms, differ by a 180° rotation of the heme about an axis defined by the α- and γ-meso carbons.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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