2-keto-3-deoxy-6-phosphogluconate aldolases as catalysts for stereocontrolled carbon-carbon bond formation

The pyruvate aldolases use pyruvate as the nucleophilic component in stereoselective aldol condensations, producing a 4-hydroxy-2-ketobutyrate framework. We have examined the 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolases from Pseudomonas putida, Escherichia coli, and Zymomonas mobilis for utility as synthetic reagents. Unlike other pyruvate aldolases examined to date, the KDPG aldolases accept short-chain, non-carbohydrate electrophilic aldehydes as substrates, providing a general methodology for the construction of the 4-hydroxy-2-ketobutyrate skeleton. The three aldolases differ markedly with respect to enzyme stability, pH optima, stability in organic cosolvent mixtures, substrate specificity, and diastereoselectivity during aldol condensation. All three enzymes show broad substrate specificity with regard to the electrophilic component. The primary requirements for substrate activity appear to be minimal steric hindrance and the presence of electron-withdrawing substituents at C2. The aldolases from Pseudomonas and Escherichia are also specific for the D-stereochemical configuration at C2, while the enzyme from Zymomonas displays no stereochemical discrimination with regard to the electrophilic substrate. Nucleophiles other than pyruvate are accepted as nucleophilic substrates by all three enzymes, provided the electrophile is sufficiently reactive, In preparative scale reactions with three unnatural electrophiles, the three enzymes show varying degrees of stereochemical fidelity, In most cases, a single diastereomer of the aldol adduct was produced, although in one case, a diastereomeric excess of 50% was observed. In all cases, the diastereoselectivity is exclusively kinetic in origin, despite the reversibility of some reactions. The enzymes are remarkably tolerant of added cosolvent: all three showed >60% of native activity in 30% DMSO and DMF. By appropriate choice of enzyme, the KDPG aldolases offer exceptional utility for stereocontrolled carbon-carbon bond formation under a wide range of experimental conditions.