Enzymes are remarkable catalysts. Their potential to catalyse specific reactions with a high degree of regio- and stereo-selectivity was exploited in the early days of steroid manufacture where, despite relatively poor productivity, the exquisite selectivity of the conversions meant that it was possible to economically convert readily available precursors into valuable intermediates for final product synthesis. This avoided the attrition of multi-step de novo chemical synthesis, a factor which was inherent in most commercially successful biotransformation processes prior to the introduction of molecular cloning. Finding the best enzyme for the reaction was largely based on extensive microbial isolation and screening programmes, and the chosen isolate or a mutant thereof would also be the production host.

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