In a decarboxylation step, it is the organic part of the molecule that is, in fact, the leaving group, 'pushed off' by the electrons on the carboxylate. When a pyruvate molecule binds, rearrangement o...In a decarboxylation step, it is the organic part of the molecule that is, in fact, the leaving group, 'pushed off' by the electrons on the carboxylate. When a pyruvate molecule binds, rearrangement of the active site architecture causes the previous step to go in reverse (step 5), freeing the CO 2 and generating a biotin base to deprotonate the alpha-carbon of pyruvate so that it can condense, in an aldol-like fashion, with CO 2 to form oxaloacetate (steps 6-7).