Enzyme Promiscuity and the Evolution of New Protein Functions

This chapter introduces the various categories of promiscuity and their possible role in enzyme evolution. It also reviews some of the well-known examples of enzymes for which the promiscuous function is industrially relevant, leading to conversions of nonnatural compounds. Enzyme promiscuity is becoming an increasingly important phenomenon in the field of biocatalysis, both directly and indirectly: directly, because many desirable conversions are “unnatural,” so expansion of the existing functions of protein catalysts is necessary to convert these “promiscuous” substrates; indirectly, through the observation that promiscuous enzymes may be particularly evolvable, opening a wide range of possibilities for engineering these potentially valuable side activities. Candida antarctica lipase B (CALB) is a typical representative of the α/β-hydrolase fold family of enzymes comprising not only a number of other esterases, but also haloalkane dehalogenases and epoxide hydrolases. Many of these natural products have unique bioactivity and are thus of great interest to the pharmaceutical industry. Halohydrin lyases or haloalcohol dehalogenases are found in various bacteria that are able to use halogenated hydrocarbons as a sole source of carbon and energy. The improvement of several of the promiscuous reactions of CALB was obtained by removing the catalytic nucleophile, naturally resulting in decreased performance for the native activity.