Introduction and Aims of the Book

1.1 Introduction

Biocatalysis, by which we mean the use of enzymes and microorganisms as catalysts for chemical transformations, has had a long and distinguished history, particularly in the brewing, baking and animal feed industries. In addition, the development of fermentation-based processes for antibiotic production relies squarely on the fundamental properties of enzymes for the production of natural products. In the latter half of the 20th century, biocatalysis started to be viewed as a potential route to unnatural or synthetic compounds. For example, scientists in Schering in Berlin in Germany discovered that semi-synthetic steroids could be manipulated with exquisite selectivity using enzymes. In the 1970s, the use of hydrolytic enzymes (e.g. lipases and proteases) and oxidoreductases (e.g. alcohol dehydrogenases) started to gain popularity for the synthesis of chiral building blocks, particularly within the pharmaceutical industry where stereochemical purity in the final product was a high priority. The discovery that some enzymes (e.g. lipases and proteases) could be used under low water conditions in organic solvents extended the range of these biocatalysts to non-polar organic substrates. However, by the early 1990s, the range of biocatalysts being employed by organic chemists was still relatively narrow and confined to hydrolases and oxidoreductases. A typical international conference on biocatalysis at that time would focus ca. 80–90% of the presentations on these two classes of enzymes.

In the 1990s, several major developments occurred within the field of biocatalysis that together changed the face of the discipline and resulted in a rapid diversification of the enzymes available to synthetic chemists. Firstly the use of molecular biology protocols as tools to clone, express and manipulate novel genes, and hence produce new enzymes, became much more widespread and meant that there was increasingly less reliance on the use of wild-type crude cell extracts for biocatalysis. Secondly, the use of protein engineering and directed evolution emerged as powerful strategies for optimisation of the properties of enzymes, particularly in the context of improving stability, stereoselectivity, substrate tolerance and catalytic activity. Thirdly, the availability of sequenced genomes meant that there was an explosion in the number of gene/protein sequences that could be mined and used as the basis for discovering new enzymes. At the beginning of 2000, the cost of DNA sequencing and gene synthesis began to drop considerably, as a result of technological advances, which meant that ordering synthetic genes became cost effective and a rapid way of generating novel biocatalysts for screening. Together, these new technologies resulted in a significantly broader range of enzymes becoming available to organic chemists. Nowadays, we have a plethora of different methods and approaches available to us for the discovery and development of new biocatalysts (Figure 1.1). The challenge has shifted from simply discovering new enzymes to trying to curate the existing databases in order to try to understand which sequences might code for which enzyme activity. De novo protein design has made major strides forward, and allows us to generate new enzyme activities from alternative scaffolds. Enzyme evolution has developed to the point where it is now routinely applied to optimise enzyme performance and represents one of the most powerful algorithms available to those interested in the development of new biocatalysts. Other scientific advances in the scale-up of biocatalytic processes provided greater levels of confidence that once a biocatalyst had been identified for a specific chemical transformation, there was a good chance of developing a practical large-scale process. Biocatalysis is now a maturing technology for the manufacture of a range of chemical products across a wide range of industries from pharmaceuticals and biofuels to polymers and personal healthcare products. Compared to traditional synthesis methodologies, biocatalysis offers a number of advantages that can contribute to more sustainable manufacturing processes. In addition, the emerging field of synthetic biology offers the potential for increasingly efficient synthesis by combining multiple biocatalytic reactions all within a single host organism.

Today, biocatalysts are increasingly being considered as an option when planning synthetic strategies for the construction of target organic molecules, particularly in cases where it is important to try to achieve some type of selectivity (e.g. stereo-, regio-, and chemoselectivity). The use of biocatalysts as an alternative to chemical reagents and catalysts can also provide benefits in terms of a reduced number of synthetic steps, lower costs of goods, reduced use of harmful solvents and improved safety profiles. Together, these benefits can lead to a more sustainable overall process with a lower overall environmental impact. However, a major barrier remains preventing the more widespread application of biocatalysis, namely a general lack of awareness and understanding amongst the synthetic organic community regarding the types of biocatalysts that are available and the various ways in which they can be applied in target molecule synthesis.

During the past ten years, a number of excellent books and reviews have been published highlighting the various biocatalysts that are now available and detailing the types of transformations that can be accomplished. Typically, these books are organised according to different classes of enzyme (e.g. hydrolases, dehydrogenases, transferases and lyases) and the various reactions are presented based upon the type of synthetic transformation that can be achieved (e.g. hydrolysis, reverse hydrolysis, C–C bond formation, C–X bond formation, oxidation, etc.). This approach provides the reader with an excellent overview of where specific biocatalysts can now be applied in organic synthesis, together with an insight into the tolerance of enzymes for unnatural substrates, which allows consequently for the synthesis of non-natural target molecules. However, as more new biocatalysts become available for application in synthesis, inevitably the question arises as to what is the best way of preparing a particular target molecule, or a building block with specific arrangements of functionality and chirality, given the fact that there may be several available options. This situation is particularly true for the synthesis of chiral alcohols, amines, carboxylic acids, esters, amides, etc., for which there are now several available biocatalytic systems that could in principle be used, and certainly many more than were...