Homology between bakers' yeast cytochrome b2 and liver microsomal cytochrome b5

The amino-acid sequence of the hemebinding region of bakers' yeast cytochrome b(2) [L-(+)-lactate dehydrogenase, EC 1.1.2.3] has been determined. It shows a strong similarity with the sequence of microsomal cytochrome b(5), and appears to be compatible with the same kind of peptide-chain folding, in agreement with data obtained previously by various physiochemical methods. The comparison shows that the fifth and sixth heme ligands must be histidine residues, thus substantiating previous conclusions drawn in particular from photooxidation experiments and nuclear magnetic resonance studies. The data reported in this paper suggest a common origin for the two proteins. Implications for their biochemical evolution are presented.