Recent advances in the generation of chiral intermediates using enzymes

Biologically active peptides and enzymatic approaches to their production

This review briefly surveys various classes of biologically active and flavor peptides that have been isolated and characterized in recent years, and analyzes emerging trends and advances in biotechnological methods for their production.

Enzymic catalysis in heterogeneous eutectic mixtures of substrates

Enzymic catalysis in heterogeneous eutectic mixtures offers advantages over conventional reactions conducted in organic solvents. Such benefits include the avoidance of bulk solvents and the attainment of greatly improved productivities. A wide range of proteases have been shown to retain their catalytic activity in eutectic mixtures of substrates, and have been used to synthesize various model and bioactive oligopeptides. Further work is required to establish what factors affect the kinetics and productivity of biocatalysis in eutectic media, and to what extent this approach can be applied to other enzyme-catalysed reactions of interest.

Enzymatic production of optically pure (2'R-cis)-2'-deoxy-3'-thiacytidine (3TC, lamivudine): a potent anti-HIV agent

Although equipotent in terms of antiviral activity, the two enantiomers of 2'-deoxy-3'-thiacytidine (BCH 189) differ markedly in their cytotoxicity. (2'R-cis)-2'-deoxy-3'-thiacytidine (3TC) is substantially less toxic than its optical antipode, and is undergoing development for the therapy of HIV infection. Cytidine deaminase from Escherichia coli is shown here to deaminate 2'-deoxy-3'-thiacytidine enantioselectively to leave 3TC essentially optically pure. This reaction has been used to develop a process for production of 3TC in multikilogram amounts. The production of cytidine deaminase was enhanced by strain improvement, fermentation development, and finally by cloning and overexpression of the gene. The enzyme was immobilized on Eupergit-C, which allowed it to be reused many times. The biotransformation conditions were optimized so that the best use could be made of the catalyst. A robust scaleable product isolation process was developed to yield the crystalline product. Overall, yields through the resolution process of 76% were obtained. All aspects of this process are capable of substantial further scaleup with only minor modifications.

Some recent developments in the use of enzyme catalysed reactions in organic synthesis

The following processes are discussed in this article: enzyme-catalysed hydrolyses of carboxylic acid esters and amides, phosphate esters, nitriles and epoxides; esterification and inter-esterification reactions catalysed by enzymes; reduction of ketones to secondary alcohols using whole-cell systems or isolated dehydrogenases; oxidation of alicyclic and aromatic substrates using mono-oxygenases and dioxygenases in bacteria and fungi including enzyme-catalysed Baeyer-Villiger oxidations; aldol reactions, formation of optically active cyanohydrins and enzyme-catalysed acyloin type reactions. The use of these biocatalytic methods for the stereo-controlled preparation of important target structures is reviewed and some of the future directions for the biotransformation area are discussed.