B‐Type Cytochrome Electron Carriers: Cytochromesb562andb5, and Flavocytochromeb2

Abstract

Abstractb‐Type cytochromes form a class of proteins that contain a heme (iron‐protoporphorin IX) bound non‐covalently to its polypeptide chain with two of its side chains coordinated to the iron. One of these side chains is always histidine and the other can be histidine or methionine. Cytochromes b5and b562are simple mono‐domain proteins that contain heme as the only prosthetic group and histidine and methionine, respectively, as the second heme ligand. Flavocytochrome b2also contains histidine as the second heme ligand, but contains a second prosthetic group, flavin mononucleotide (FMN), that resides in a separate domain. Cytochromes b5and b562each mediate intracellular electron transfer between redox partners, the former within the endoplasmic reticulum of mammalian cells and the latter within bacteria. Flavocytochrome b2catalyzes the oxidation of lactic acid within yeast mitochondria, with the cytochrome domain mediating electron transfer from substrate‐reduced FMN to exogenous cytochrome c. The structural, electrochemical, and catalytic properties of these three proteins are presented, along with the results of mutagenesis.