A MECHANISTIC INVESTIGATION OF THE MICROBIAL CHIRAL INVERSION OF 2-PHENYLPROPIONIC ACID BY VERTICILLIUM-LECANII

Previous investigations, have described the development of nongrowing suspensions of Verticillium lecanii as a microbial model of the mammal ian chiral inversion of the 2-arylpropionic acids (2-APAs). Mechanisti c studies in mammals have shown that inversion involves loss of the al pha-methine proton but retention of the original atoms at the p-methyl position, and a mechanism has been proposed involving enzymatic epime risation of acyl-CoA thioester derivatives of the substrate. Inversion of the 2-APAs by V. lecanii exhibits extensive intersubstrate variati on in the presence, rate, extent, and direction of inversion, which ar e different from those observed in mammalian systems, possibly indicat ing differences in the mechanism of inversion between mammalian and mi crobial cells. This study involved the investigation of proton/deuteri um exchange by H-1-nuclear magnetic resonance following incubation of deuterated derivatives of 2-phenylpropionic acid (2-PPA), a model comp ound, in cell suspensions of V. lecanii and incubation of undeuterated 8-PPA in cell suspensions containing D2O. The results indicated that the inversion of 2-PPA by V. lecanii also involved exchange of the alp ha-methine proton but complete retention of the original atoms at the beta-methyl position. No kinetic deuterium isotope effect was observed , indicating that: loss of the alpha-methine proton is not the rate-li miting step of the inversion process. This suggests that the observed differences between microbial and mammalian systems probably involve t he stereoselective acyl-CoA thioester formation step and not the subse quent epimerisation of the resultant diastereomers. Chirality 9:254-26 0, 1997. (C) 1997 Wiley-Liss, Inc.