Baker's Yeast Flavocytochrome b2 [l‐(+)‐Lactate Dehydrogenase]

Abstract

Baker''s yeast flavocytochrome b2 has been suggested to be a polyglobular protein formed after a gene fusion process. One of the potential globules, the heme-binding core, can exist in the absence of the rest of the protein. Using [rabbit] antibodies elicited against the whole enzyme and against the core, it is shown that part of the core surface, and in particular the mouth of the heme-binding crevice, must be exposed in the complete enzyme molecule. Antibodies inhibit the activity of intact and proteolytically-cleaved enzymes, which normally show a number of differences in some kinetic parameters. Antibodies against the core induce a modification of some kinetic constants for the cleaved enzyme (in particular the Km for the substrate and Ki for D-(-)-lactate), bringing them back to values similar to those for the intact enzyme. These results can be interpreted as a tightening of the cleaved enzyme by anti-core antibodies. The conformational effect is transmitted from the heme-binding region to other parts of the molecule. This implies some intimate contacts between the core and the rest of the protein.