Cytochrome b5, Form A (CYB5A)

Cytochrome b5 is a membrane bound hemoprotein which function as an electron carrier for several membrane bound oxygenases. It has two isoforms produced by alternative splicing. Isoform 1 is bound to the cytoplasmic side of the endoplasmic reticulum. It has a C-terminal transmembrane alpha-helix. Isoform 2 was found in cytoplasm. Defects in CYB5A are the cause of type IV hereditary methemoglobinemia.

The derived amino acid sequence showed that the previous designation of several amino acids was in error. Liver and erythrocyte cytochrome b5 isoforms have the same amino acid sequence except at position 98, which is thr and pro for liver and erythrocyte proteins, respectively.