Spontaneous Dissociation of a Cytochrome Core and a Biglobular Flavoprotein after Mild Trypsinolysis of the Bifunctional Saccharomyces cerevisiae Flavocytochrome b2

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Abstract
S. cerevisiae flavocytochrome b2 is known as a bifunctional enzyme which behaves as the association of an FMN flavodehydrogenase with its specific acceptor, a b5-like cytochrome. Mild trypsinolysis gives rise to 3 complementary fragments (n, X, .beta.''), both prosthetic groups being still bound. After such proteolysis the separation of a biglobular flavoprotein domain (carrying FMN) from a cytochrome domain (with the heme) is obtained by molecular sieving under non-denaturing conditions. The marked lack of affinity between the tetrameric flavoprotein (X, .beta.'')4 and the monomeric cytochrome core (n) leads to the hypothesis that the 2 domains are not tightly associated in the native molecule and might move relative to each other. Their respective mobility is possibly required for the catalytic mechanism. The comparison with previous trypsinolysis studies on the flavocytochrome b2 from Hansenula anomala suggests the presence of 2 common zones of hypersensitivity to proteases, along the protomeric polypeptide chain, and strongly supports the validity of the triglobular model for both flavocytochromes.