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The enzyme, 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase, catalyzes several reactions, the natural ones being (i) the exchange of hydrogen atoms of the methyl groups of pyruvate with protons of the solvent (C-H synthesis) and (ii) the reversible condensation of pyruvate with D-glyceraldehyde-3-phosphate (C-C synthesis). Previous work has provided chemical evidence for the occurrence of a protein-bound carboxylate group adjacent to the Schiff's base-forming lysine in the active site geometry. This carboxylate could provide the basic group postulated to participate in proton activation catalyzed by aldolases. With the use of three-dimensional models, it is shown that simple rotation about a carbon-carbon bond of the side chain will allow the base to assume the two positions necessary for proton activation in either the C-H synthesis or the C-C synthesis catalyzed by KDPG aldolase. This single base hypothesis provides a model wherein all reagents can approach a single face of the active site and is consistent with the stereochemistry thought to occur in the aldolase reaction.

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