Can the combination of electrochemical regeneration of NAD+, selectivity of L-alpha-amino-acid dehydrogenase, and reductive amination of alpha-keto-acid be applied to the inversion of configuration of a L-alpha-amino-acid?

Abstract

The inversion of configuration of L-alanine can be carried out by combining its selective oxidation in the presence of NAD+ and L-alanine dehydrogenase, electrochemical regeneration of the NAD+ at a carbon felt anode, and reductive amination of pyruvate, i.e., reduction of its imino derivative at a mercury cathode, the reaction mixture being buffered with concentrated ammonium/ammonia (1.28M / 1. 28M). The dehydrogenase exhibits astonishing activity and stability under such extreme conditions of pH and ionic strength. The main drawback of the process is its slowness. At best, the complete inversion of a 10 mM solution of L-alanine requires 140 h. A careful and detailed quantitative analysis of each of the key steps involved shows that the enzyme catalyzed oxidation is so thermodynamically uphill that it can be driven efficiently to completion only when both the coenzyme regeneration and the pyruvate reduction are very effective. The first condition is easily fulfilled. Under the best conditions, it is the rate of the chemical reaction producing the imine which controls the whole process kinetically. Copyright 1999 John Wiley & Sons, Inc.