CYTOCHROME B(5), ITS FUNCTIONS, STRUCTURE AND MEMBRANE TOPOLOGY

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b(5). In addition, one section is devoted to a description of a recently discovered function of cytochrome b(5), namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory in both the catalytic heme-binding and membrane-binding domain of cytochrome b(5). These studies have shown that: 1) the membrane binding domain of cytochrome b(5) spans the bilayer; 2) cytochrome b(5) lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b(5) with its redox partners.