Synthetic Applications of Nitrile-Converting Enzymes

The use of nitrile hydratases (RCN + H2O → RCONH2) and nitrilases (RCN + 2 H2O → RCOO- + NH4 +) in biocatalysis attracts profound attention from the beginning of 1980s since it provides a useful alternative to the conventional nitrile hydrolysis using strong acid or base catalysts. In the past 5 years the potential of this branch of biotransformations has been broadened significantly by both the isolation of new mesophilic and thermophilic nitrile-metabolizing microorganisms and the purification of new nitrile-converting enzymes, including thermostable ones. Currently, the scope of products afforded by nitrile-converting biocatalysts encompass hundreds of compounds such as aliphatic, alicyclic, aromatic and heterocyclic carboxylic acids and their amides. New nitrile hydratases show high enantioselectivity towards racemic substituted 2-arylpropionitriles, 2-arylbutyronitriles and prochiral 3-arylglutaronitriles. Synthesis of optically active α-hydroxy and α-fluoro amides and acids employs whole-cell biocatalysts or purified nitrilases. (E)-Selectivity for a,ß-unsaturated nitriles and cis- or trans-selectivity for alicyclic nitriles is also demonstrated with some nitrile-converting enzymes. Regioselectivity and chemoselectivity of the biocatalysts enable preparation of diverse functional group-containing products such as cyanocarboxylic acids, cyanoamides, hemiesters and their amides and carboxylic acids and amides with ether groups. Stabilization of the nitrile-converting biocatalysts is achieved by entrapment in hydrogels or by lyophilization. Organic-aqueous mixtures are suitable as reaction media for numerous nitrile-converting enzymes.