Chapter 12: Enzyme Biotransformations and Reactors

Enzyme-catalysed biotransformations, either using whole cells or free enzymes, are increasingly being exploited in industrial chemistry. They can offer remarkable reaction, stereo- and regio-selectivity, and work in benign aqueous systems. Some enzymes are remarkably robust, while others are relatively fragile, but may be stabilized by immobilization or used in whole cell systems. The use of purified (or partially purified) enzymes avoids the possibility of side reactions (of substrate or product), but incurs the additional cost of purification. This is why, historically, some of the most commonly used enzymes (e.g. lipases, proteases and glycoside hydrolases) are naturally extracellular. However, advances in molecular biology and protein engineering mean that production of any enzyme can be engineered into commonly used hosts (e.g. yeast or Escherichia coli). Methods are available to modify substrate recognition and reaction selectivity, allowing tuning of an enzyme to a novel substrate. Together with improvements in immobilization technology and enzyme reactor design, this is opening up new possibilities for single and multi-step biocatalytic processes.