Structural Studies of Yeast Flavocytochrome b2: Cooperative Roles of the α and β Globules in the Formation of the Flavin-Binding Sites

The purpose of the study reported here was the localization of the heme binding sites on the two globular fragments, α and β, of the ‘cleaved’ form of the flavocytochrome b2 chain. These fragments were partially resolved by means of molecular sieving under denaturing conditions (3 M or 6 M guanidine in the presence of 2-mercaptoethanol). They were then renatured in the presence of excesses of FMN and protoheme. The protoheme was found to be quantitatively bound to the α subunit, confirming previous findings. The flavin binds neither to α alone nor to β alone, but only to the reassociated αβ protomer. The results are discussed in terms of the possible occurrence of gene fusion in the formation of the complex flavocytochrome chain of this very particular L-lactate cytochrome c reductase found specifically in yeasts.