Using enzyme cascades in biocatalysis: Highlight on transaminases and carboxylic acid reductases

Biocatalysis, the use of enzymes in chemical transformations, is an important green chemistry tool. Cascade reactions combine different enzyme activities in a sequential set of reactions. Cascades can occur within a living (usually bacterial) cell; in vitro in 'one pot' systems where the desired enzymes are mixed together to carry out the multi-enzyme reaction; or using microfluidic systems. Microfluidics offers particular advantages when the product of the reaction inhibits the enzyme(s). In vitro systems allow variation of different enzyme concentrations to optimise the metabolic 'flux', and the addition of enzyme cofactors as required. Cascades including cofactor recycling systems and modelling approaches are being developed to optimise cascades for wider industrial scale use. Two industrially important enzymes, transaminases and carboxylic acid reductases are used as examples regarding their applications in cascade reactions with other enzyme classes to obtain important synthons of pharmaceutical interest.