Cytochromes b₅ 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 b₅-like proteins includes (besides cytochrome b₅ itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome b₂ (L-lactate dehydrogenase; EC 1.1.2.3), sulfite oxidase (EC 1.8.3.1), plant and fungal nitrate reductases (EC 1.7.1.1, EC 1.7.1.2, EC 1.7.1.3), and plant and fungal cytochrome b₅/acyl lipid desaturase fusion proteins.

3-D structures of a number of cytochrome b₅ and yeast flavocytochrome b₂ 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 b₅, 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.

EC 1.6.2.2 cytochrome-b₅ reductase

NADH + H⁺ + 2 ferricytochrome b₅ → NAD⁺ + 2 ferrocytochrome b₅

EC 1.10.2.1 L-ascorbate—cytochrome-b₅ reductase

L-ascorbate + ferricytochrome b₅ → monodehydroascorbate + ferrocytochrome b₅

EC 1.14.18.2 CMP-N-acetylneuraminate monooxygenase

CMP-N-acetylneuraminate + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ → CMP-N-glycoloylneuraminate + 2 ferricytochrome b₅ + H₂O

EC 1.14.19.1 stearoyl-CoA 9-desaturase

stearoyl-CoA + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ → oleoyl-CoA + 2 ferricytochrome b₅ + H₂O

EC 1.14.19.3 linoleoyl-CoA 9-desaturase

linoleoyl-CoA + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ → γ-linolenoyl-CoA + 2 ferricytochrome b₅ + H₂O

• Cytochrome b
• Cytochrome b₅ deficiency
• P450-containing systems
• Cytochrome b₅, type A

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• PDB: 1B5A​ – Solution structure of rat cytochrome b₅ (form A)
• PDB: 1B5B​ – Solution structure of rat cytochrome b₅ (form B)
• PDB: 1CXY​ – X-ray structure of cytochrome b₅₅₈ from Ectothiorhodospira vacuolata
• Online Mendelian Inheritance in Man (OMIM): 250790 – Methemoglobinemia due to deficiency of cytochrome b₅