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

Abstract

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)Hdependent 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.

Biocatalysis making waves in organic chemistry

“…Thus, imine reductases (IREDs), [334][335][336] amine dehydrogenases (AmDHs) [337][338][339][340][341] and reductive aminases (RedAms) 342,343 catalyse the formation of the imine and its subsequent reduction (Scheme 41). 344,345 These reductive enzymes, together with amino acid dehydrogenases (AaDHs) that transform keto acids to amino acids, and PLP-dependent transaminases (TAs) that catalyse transamination, give access to primary, secondary and tertiary amines and have been applied on gram to kilogram scale. Transaminases in particular have been used in many processes already, for example in the synthesis of sitagliptin.…”

Enzymatic strategies for asymmetric synthesis

“…105,106 The field of enzymatic imine reduction is a prime example of how rapidly the field of biocatalysis can evolve. 107 Within the last decade, the portfolio of imine reductases (often denoted IREDs) grew from a few examples known from specific biosynthetic pathways with little application potential in biocatalysis -most prominent being the case of dihydrofolate reductase involved in the synthesis of tetrahydrofolic acid -to large collections of stereocomplementary enzymes, for which specific sequence motifs could be identified. 108 The stereoselectivity has been assigned to two distinct stereocomplementary superfamilies.…”

Directed Evolution of a Ketone Synthase for Efficient and Highly Selective Functionalization of Internal Alkenes by Accessing Reactive Carbocation Intermediates

“…The ketone product is accessible by stabilizing a carbocation intermediate and the reaction proceeds via an electron/hydride transfer pathway. c, Given that this new enzyme function can be accessed, many challenging functionalization reactions of internal alkenes can be realized in combination with established biocatalysts such as ketoreductases 20,21 (i), w-transaminases 20 (ii) and imine reductases 22,23 (iii).…”