Acetohydroxyacid synthase: a new enzyme for chiral synthesis of R-phenylacetylcarbinol.

We have found that acetohydroxyacid synthase (AHAS) is an efficient catalyst for the enantiospecific (> or =98% enantiomeric excess) synthesis of (R)-phenylacetylcarbinol (R-PAC) from pyruvate and benzaldehyde, despite the fact that its normal physiological role is synthesis of (S)-acetohydroxyacids from pyruvate and a second ketoacid. (R)-phenylacetylcarbinol is the precursor of important drugs having alpha and beta adrenergic properties, such as L-ephedrine, pseudoephedrine, and norephedrin. It is currently produced by whole-cell fermentations, but the use of the isolated enzyme pyruvate decarboxylase (PDC) for this purpose is the subject of active research and development efforts. Some of the AHAS isozymes of Escherichia coli have important advantages compared to PDC, including negligible acetaldehyde formation and high conversion of substrates (both pyruvate and benzaldehyde) to PAC. Acetohydroxyacid synthase isozyme I is particularly efficient. The reaction is not limited to condensation of pyruvate with benzaldehyde and other aromatic aldehydes may be used.