7.22 Enzyme Catalytic Promiscuity: Expanding the Catalytic Action of Enzymes to New Reactions

The use of enzymes as catalysts for organic synthesis requires that they catalyze nonbiochemical reactions. This promiscuity can be an ability to accept substrates beyond their natural biochemical substrates (substrate promiscuity) or an ability to catalyze new reactions (catalytic promiscuity). This chapter focuses on this second type of promiscuity. Catalytically promiscuous reactions are additional abilities of enzymes beyond their normal reaction. The promiscuous reactions share mechanistic steps with the normal reaction. Chemists can use catalytically promiscuous reactions for synthesis. Protein engineering can improve the existing catalytically promiscuous reaction and, by adding new mechanistic steps, create new catalytic promiscuity. This chapter reviews examples of catalytic promiscuity, organized by reaction type, with an emphasis on stereoselective carbon-carbon bond forming reactions such as aldol and Michael additions.