Chapter 11: Biocatalysis in Organic Chemistry

Biocatalysis in Organic Chemistry begins by contrasting enzymatic biotransformations with traditional chemical synthesis, highlighting the ability of enzymes to catalyze reactions at ambient temperatures and pressures while achieving superior stereospecificity and enantioselectivity, often eliminating the need for complex protection and deprotection steps. The text explains fundamental stereochemical concepts, including chirality, enantiomers, and the calculation of enantiomeric excess, emphasizing the importance of producing homochiral drugs to avoid toxicity or unwanted side effects seen in racemic mixtures. Significant historical and industrial examples are analyzed, such as the revolution in steroid manufacturing where microbial hydroxylation by fungi like Rhizopus arrhizus drastically reduced the synthesis steps for cortisone from bile acids and plant sterols. The discussion expands to the synthesis of chiral intermediates for pharmaceuticals and agrochemicals, including beta-adrenergic receptor agonists and herbicides like chloropropionic acid, utilizing whole-cell biocatalysts. A major section focuses on the discovery and optimization of novel enzymes through the exploration of microbial diversity and environmental DNA (metagenomics), including extremophiles that provide thermostable enzymes like Taq DNA polymerase. The chapter details modern protein engineering techniques used to optimize biocatalysts, distinguishing between rational design methods like site-directed mutagenesis—which require structural knowledge—and directed evolution strategies such as DNA shuffling, error-prone PCR, and gene site saturation mutagenesis. These evolutionary methods are illustrated through case studies involving the improvement of glyphosate tolerance, lipase enantioselectivity, and the development of nitrilases and aldolases (DERA) for statin drug production. Finally, the summary covers large-scale industrial applications, describing the enzymatic production of high-fructose corn syrups using glucose isomerase, the transesterification of fats to create cocoa butter substitutes using regiospecific lipases, the lipase-catalyzed synthesis of polyesters, and the highly efficient production of acrylamide using nitrile hydratase from Rhodococcus rhodochrous.