Cytochrome b5 explained

Cytochrome b5
Hgncid:	2570
Symbol:	CYB5A
Altsymbols:	CYB5
Entrezgene:	1528
Omim:	250790
Refseq:	NM_001914
Uniprot:	P00167
Pdb:	1JEX
Chromosome:	18
Arm:	q
Band:	23

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;), sulfite oxidase, plant and fungal nitrate reductases, and plant and fungal cytochrome b₅/acyl lipid desaturase fusion proteins.

Structure

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.

Cytochrome b₅ in some biochemical reactions

cytochrome-b₅ reductase

NADH + H⁺ + 2 ferricytochrome b₅ → NAD⁺ + 2 ferrocytochrome b₅ L-ascorbate—cytochrome-b₅ reductase

L-ascorbate + ferricytochrome b₅ → monodehydroascorbate + ferrocytochrome b₅ CMP-N-acetylneuraminate monooxygenase

CMP-N-acetylneuraminate + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ → CMP-N-glycoloylneuraminate + 2 ferricytochrome b₅ + H₂O stearoyl-CoA 9-desaturase

stearoyl-CoA + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ → oleoyl-CoA + 2 ferricytochrome b₅ + H₂O linoleoyl-CoA 9-desaturase

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

See also

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

References

• Lederer F . The cytochrome b5-fold: an adaptable module . Biochimie . 76 . 7 . 674–92 . 1994 . 7893819 . 10.1016/0300-9084(94)90144-9 .
• Napier JA, Michaelson LV, Sayanova O . The role of cytochrome b5 fusion desaturases in the synthesis of polyunsaturated fatty acids . Prostaglandins, Leukotrienes, and Essential Fatty Acids . 68 . 2 . 135–43 . February 2003 . 12538077 . 10.1016/S0952-3278(02)00263-6 .
• Rivera M, Barillas-Mury C, Christensen KA, Little JW, Wells MA, Walker FA . F. Ann Walker. Gene synthesis, bacterial expression, and 1H NMR spectroscopic studies of the rat outer mitochondrial membrane cytochrome b5 . Biochemistry . 31 . 48 . 12233–40 . December 1992 . 1333795 . 10.1021/bi00163a037 .
• Schenkman JB, Jansson I . The many roles of cytochrome b5 . Pharmacology & Therapeutics . 97 . 2 . 139–52 . February 2003 . 12559387 . 10.1016/S0163-7258(02)00327-3 .

External links

• – Solution structure of rat cytochrome b₅ (form A)
• – Solution structure of rat cytochrome b₅ (form B)
• – X-ray structure of cytochrome b₅₅₈ from Ectothiorhodospira vacuolata
• – Methemoglobinemia due to deficiency of cytochrome b₅