Asymmetricr eductivea mination of ketones using w-transaminases (w-TAs)offers apromising alternative to the chemocatalytic synthesiso f chiral amines.O ne fundamental challenge to the biocatalytic strategyi st he very low enzyme activities for most ketones compared with native substrates (i.e., <1% relativet op yruvate). Here we have demonstrated that as ingle point mutation in the active site of the (S)-selective w-TAf rom Ochrobactrum anthropi could induce ar emarkable acceleration of the amination reactionw ithout anyl oss in stereoselectivity and enzyme stability.M olecular modeling of quinonoid intermediates,a lanine scanning mutagenesis and kinetic analysis revealedthat the W58 residue acted as as tericb arrier to binding and catalytic turnover of ketone substrates. Removal of the steric strain by W58L substitution, which was selected by partials aturation mutagenesis,l ed to dramatica ctivity improvements for structurally diverse ketones (e.g.,3 40-fold increase in k cat /K M for acetophenone). TheW 58Lm utant afforded an efficient synthesis of enantiopure amines (i.e., >99% ee)u sing isopropylamine as an amino donor.

Isopropylamine is an ideal amino donor for reductive amination of carbonyl compounds by ω-transaminase (ω-TA) owing to its cheapness and high volatility of a ketone product. Here we developed asymmetric synthesis of unnatural amino acids via ω-TA-catalyzed amino group transfer between α-keto acids and isopropylamine.

Flee to the hills: An unfavorable equilibrium in the amino group transfer between amino acids and keto acids catalyzed by α‐transaminases was successfully overcome by coupling with a ω‐transaminase reaction as an equilibrium shifter, leading to efficient asymmetric synthesis of diverse unnatural amino acids, including L‐tert‐leucine and D‐phenylglycine.

Production of structurally diverse chiral amines via biocatalytic transamination is challenged by severe steric interference in a small active site pocket of ω‐transaminase (ω‐TA). Herein, we demonstrated that structure‐guided remodeling of a large pocket by a single point mutation, instead of excavating the small pocket, afforded desirable alleviation of the steric constraint without deteriorating parental activities toward native substrates. Molecular modeling suggested that the L57 residue of the ω‐TA from Ochrobactrum anthropi acted as a latch that forced bulky substrates to undergo steric interference with the small pocket. Removal of the latch by a L57A substitution allowed relocation of the small pocket and dramatically improved activities toward various arylalkylamines and alkylamines (e.g., 1100‐fold increase in kcat/KM for α‐propylbenzylamine). This approach may provide a facile strategy to broaden the substrate specificity of ω‐TAs.magnified image

Substrate binding pockets of ω‐transaminase (ω‐TA) consist of a large (L) pocket capable of dual recognition of hydrophobic and carboxyl substituents, and a small (S) pocket displaying a strict steric constraint that permits entry of a substituent no larger than an ethyl group. Despite the unique catalytic utility of ω‐TA enabling asymmetric reductive amination of carbonyl compounds, the severe size exclusion occurring in the S pocket has limited synthetic applications of ω‐TA to access structurally diverse chiral amines and amino acids. Here we report the first example of an ω‐TA whose S pocket shows a non‐canonical steric constraint and readily accommodates up to an n‐butyl substituent. The relaxed substrate specificity of the (S)‐selective ω‐TA, cloned from Paracoccus denitrificans (PDTA), afforded efficient asymmetric syntheses of unnatural amino acids carrying long alkyl side chains such as L‐norvaline and L‐norleucine. Molecular modeling using the recently released X‐ray structure of PDTA could pinpoint an exact location of the S pocket which had remained dubious. Entry of a hydrophobic substituent in the L pocket was found to have the S pocket accept up to an ethyl substituent, reminiscent of the canonical steric constraint. In contrast, binding of a carboxyl group to the L pocket induced a slight movement of V153 away from the small‐pocket‐forming residues. The resulting structural change elicited excavation of the S pocket, leading to formation of a narrow tunnel‐like structure allowing accommodation of linear alkyl groups of carboxylate‐bearing substrates. To verify the active site model, we introduced site‐directed mutagenesis to six active site residues and examined whether the point mutations alleviated the steric constraint in the S pocket. Consistent with the molecular modeling results, the V153A variant assumed an elongated S pocket and accepted even an n‐hexyl substituent. Our findings provide precise structural information on substrate binding to the active site of ω‐TA, which is expected to benefit rational redesign of substrate specificity of ω‐TA.magnified image