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The modern era of pharmaceuticals, based on synthetic compounds rather than natural extracts, emerged around the middle of the 20^th century. Subsequently, the vast majority of pharmaceutical active ingredients have been synthesised using reactive stoichiometric reagents or simple catalysts such as metals or strong acids. Although the role of enzymes as catalysts in biological systems has been known since the second half of the 19^th century, until quite recently their application in pharmaceutical synthesis had been restricted to a few niche applications such as the hydroxylation of steroids.

There are multiple reasons why enzymes were previously considered unsuitable for pharmaceutical synthesis, the primary ones being 

• that the availability of enzymes was limited ...