Promiscuous acyltransferases for ester and amide synthesis in aqueous solution

Hydrolases are well-known for hydrolyzing esters, amides, carbamates, peptides, or acid anhydrides in the presence of water. However, some of them are also capable of catalyzing the reverse reaction (condensation) under certain conditions in aqueous systems. Hence, these enzymes are called promiscuous hydrolases/acyltransferases. This review deals with their discovery, background information on their mechanism of action, and significant improvements by enzyme engineering to both enhance product formation and decrease the undesired hydrolysis of the targeted acyl products. Their applications in biocatalysis are exemplified by the synthesis of a wide range of esters or amides in aqueous systems, including preparative-scale processes and the combination of hydrolases/acyltransferases with other enzymes in cascade reactions to utilize alternative feedstocks from renewable resources, for example. Complementary, the use of ATP-dependent amide synthesizing enzymes is covered. Together, promiscuous hydrolases/acyltransferases represent practically useful alternatives to well-established chemical reactions, operating in aqueous solutions that also appeal to different industries.