Yeast flavohemoglobin is an ancient protein related to globins and a reductase family.

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Abstract
The hemoglobin of yeast is a two-domain protein with both heme and flavin prosthetic groups. The nucleotide sequences of the cDNA and genomic DNA encoding the protein from Saccharomyces cerevisiae show that introns are absent and that both domains are homologous with a flavoheme protein from Escherichia coli. The heme domains are also homologous with those of O2-binding heme proteins from several other distantly related bacteria, plants, and animals; all appear to be members of the same globin superfamily. Although the homologous hemoglobin of the bacterium Vitreoscilla sp. is a single-domain protein, several bacteria have related O2-binding heme proteins whose second domains have different structures and enzymatic activities: dihydropteridine reductase (E. coli), cytochrome c reductase (Alcaligenes eutrophus), and kinase in the O2 sensor of Rhizobium meliloti. This indicates that one evolutionary pathway of hemoglobin is that of a multipurpose domain attached to a variety of unrelated proteins to form molecules with different functions. The flavin domain of yeast hemoglobin is homologous with members of a flavoprotein family that includes ferredoxin reductase, nitric oxide synthase, and cytochrome P-450 reductase. The correspondence of yeast and E. coli flavohemoglobins indicates that the two-domain protein has been conserved intact for as long as 1.8 billion years, the estimated time of divergence of prokaryotes and eukaryotes provided that cross-species gene transfer has not occurred.