Baker's yeast flavocytochrome b2 has been suggested before to be a polyglobular protein formed after a gene fusion process. It is known that one of the potential globules, the heme-binding core, can exist in the absence of the rest of the protein. Using antibodies elicited against the whole enzyme and against the core, we show in this paper 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. Interestingly, 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.