The FRE1 ferric reductase of Saccharomyces cerevisiae is a cytochrome b similar to that of NADPH oxidase

Plasma membrane preparations from strains of the yeast Saccharomyces cerevisiae gave a reduced minus oxidized spectrum characteristic of a b-type cytochrome and very similar to the spectrum of flavocytochrome b(558) of human neutrophils. The magnitude of the signal correlated with the level of ferric reductase activity and the copy number of the FRE1 gene, indicating that the FRE1 protein is a cytochrome b. Sequence similarities with the flavin binding site of flavocytochrome b(558) and other members of the ferredoxin-NADP reductase family, together with increased levels of noncovalently bound FAD and iodonitrotetrazolium violet reductase activity in membranes from a yeast strain overexpressing ferric reductase, suggested that the FRE1 protein may also carry a flavin group. Potentiometric titrations indicated that FRE1, like neutrophil NADPH oxidase, has an unusually low redox potential, in the region of -250 mV, and binds CO.