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 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.

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.