Redesigning the mechanism of the lipase- catalysed aminolysis of esters

In previous works, the lipase B from Candida antarctica (CAL-B) catalysed the resolution of several 2phenylcycloalkanamines by aminolysis of ethyl acetate. In these processes, the size of the cycle and the stereochemistry of the chiral centres of the amines had a strong influence on both the enantiomeric ratio and the reaction rate of these reactions. In this paper, molecular modelling approach has been used to revise the lipase-catalysed aminolysis mechanism. Thus, complexes of CAL-B with the phosphonamidate analogs related to substrates in the kinetic resolution of several 2-phenylcycloalkanamines by this enzyme were built and minimised. This computational study suggests the formation of zwitterionic species (named TI-Z), resulting from the direct His-unassisted attack of the amine to the carbonyl group of the acyl-enzyme, as the most plausible intermediate for the CAL-B-catalysed aminolysis. This proposal slightly differs from the commonly accepted serine-mediated mechanism, where the removal of the proton from the amine occurs simultaneously to the nucleophile attack to the acyl-enzyme complex (TI-2). Subsequently, His-assisted deprotonation of the resulting ammonium group takes place, and a molecule of water could be necessary in some cases to facilitate the transfer of the proton to the catalytic histidine. © 2016 Elsevier Science. All rights reserved