NAD(P)H-Dependent Enzymes for Reductive Amination : Active Site Description and Carbonyl-Containing Compound Spectrum.

The biocatalytic asymmetric synthesis of amines from carbonyl compounds and amine precursors presents an important advance in sustainable synthetic chemistry. Oxidoreductases (ORs) that catalyze the NAD(P)H-dependent reductive amination of carbonyl compounds directly to amines using amine donors present advantages complementary to those of amine transaminases (ATAs) with respect to selectivity, stability and substrate scope. Indeed some ORs accept alkyl and aryl amines as reaction partners enabling access to chiral secondary amine products that are not directly accessible using ATAs. Moreover, superior atom economy can usually be achieved as no sacrificial amines are required as with ATAs. In recent years a number of ORs that apparently catalyze both imine formation and imine reduction in the reductive amination of carbonyls has been identified using structure informed protein engineering, sequence analysis from natural biodiversity and increasingly a mixture of both. In this review we summarize the development of such enzymes from the engineering of amino acid dehydrogenases (AADHs) and opine dehydrogenases (OpDHs) to become amine dehydrogenases (AmDHs), which are active toward ketones devoid of any requisite carboxylate and/or amine functions, through to the discovery of native AmDHs and reductive aminases (RedAms), and the engineering of all of these scaffolds for improved or altered activity. Structural and mechanistic studies have revealed similarities, but also differences in the determinants of substrate binding and mechanism in the enzymes. The survey reveals that a complementary approach to enzyme discovery that utilizes both natural genetic resources and engineering can be combined to deliver biocatalysts that have significant potential for the industrial synthesis of chiral amines.