Snapshots of the Catalytic Cycle of the Industrial Enzyme α-Amino-ε-Caprolactam Racemase (ACLR) Observed Using X-ray Crystallography

α-Amino-e-caprolactam racemase (ACLR) is a PLP-dependent enzyme exploited in industry for the racemization of amino acid amides in dynamic kinetic resolutions that produce homochiral amino acids. We report high-resolution structures of wild-type and variant ACLRs from Rhizobium freirei, generated using the chiral substrate ACL and representative of covalent intermediates in the catalytic cycle. These complexes substantiate suggestions of a two-base mechanism, in which the PLP-binding K267 and D210 serve as proton donors in the racemization, and they constitute a robust basis on which to engineer ACLRs for improved activity as industrial biocatalysts.