Engineering Biocatalysts for Synthesis Including Cascade Processes

Abstract

1 When preparing an Editorial summary for a special issue it is always interesting to look back at previous recent editorials to gain a sense of how far the field has advanced and how the focus of research has moved forward. In 2011, Advanced Synthesis & Catalysis published a themed issue, containing 31 papers, many of which addressed the challenge of engineering biocatalysts such that they could be used for chemical synthesis (see: Biocatalysis – A Gateway to Industrial Biotechnology).1 During the past decade, there has been a global effort to bring new biocatalyst platforms into practical usage, both in academic and industrial laboratories, resulting in the emergence of new classes of enzymes, such as transaminases, amine oxidases, ene reductases, nitrilases, Baeyer–Villiger monooxygenases and aldolases. Many, if not all, of these new biocatalysts have been subjected to extensive protein engineering and directed evolution in order to tailor their properties such as substrate scope, stereoselectivity, stability and catalytic activity. Coupled with the emergence of powerful sequence-based algorithms for mining genomes, the field of biocatalysis is now better equipped than ever before to both discover new enzymes and also engineer them to make them fit for purpose. As the toolbox of practical biocatalysts continues to rapidly expand, so the emphasis of research starts to shift towards application of these new enzymes to target molecule synthesis, including combinations of biocatalysts in cascade processes in vitro. The latter approach, which embraces both “systems biocatalysis” and also “synthetic biology”, represents a logical development for the field since in essence it seeks to emulate the way in which Nature constructs the diverse array of natural products such as alkaloids, terpenes, steroids, polyketides, etc. With respect to cascade reactions, biocatalysts possess one major advantage compared to non-enzyme-based catalysts, namely that they essentially all work under the same reaction conditions, i.e., aqueous solvent, ambient temperature and pressure, pH 5.0–8.0. When using biocatalysts there is no need to resort to very low or high temperatures or to switch solvents in order to maximise activity. Other than most chiral organocatalysts or ligands of metal catalysts, which after use must often laboriously be synthesised again, a biocatalyst can be re-produced from its DNA template with comparably low effort by using the intrinsic biological machinery. We should regard these aspects of biocatalysis as precious gifts, which will greatly aid the development of the field as researchers seek to construct more structurally complex target molecules by employing combinations of engineered biocatalysts. This special issue presents 34 papers from leading researchers in biocatalysis and is comprised of 5 reviews, 6 communications and 23 full papers. The 5 reviews from leading experts in their fields cover emerging topics as diverse as designed cascade reactions, imine reductases, dynamic kinetic resolutions, sialic acid derivatives and creation of artificial metalloenzymes. The original research papers address the entire range of contemporary themes in biocatalysis including new enzyme classes for synthesis as well as methods for their discovery, development and ultimately scale-up and application. As highlighted above, there is an increasing focus on combining two or more enzymes together in a single reaction flask in order to solve challenges ranging from cofactor recycling, in situ racemisation, synthesis of functionalised carbohydrates to in vivo pathway engineering. Readers of this issue will not only gain a timely insight into the current themes and state-of-the-art of this highly dynamic field but may also themselves reflect upon what new challenges the field of biocatalysis should tackle in the next 5 years.