No abstract

An efficient one-pot biocatalytic cascade was developed for synthesis of d-amino acids from readily available l-amino acids via stereoinversion.

Pullulanases are well‐known debranching enzymes hydrolyzing α‐1,6‐glycosidic linkages. To date, engineering of pullulanase is mainly focused on catalytic pocket or domain tailoring based on structure/sequence information. Saturation mutagenesis‐involved directed evolution is, however, limited by the low number of mutational sites compatible with combinatorial libraries of feasible size. Using Bacillus naganoensis pullulanase as a target protein, here we introduce the ‘evolutionary coupling saturation mutagenesis’ (ECSM) approach: residue pair covariances are calculated to identify residues for saturation mutagenesis, focusing directed evolution on residue pairs playing important roles in natural evolution. Evolutionary coupling (EC) analysis identified seven residue pairs as evolutionary mutational hotspots. Subsequent saturation mutagenesis yielded variants with enhanced catalytic activity. The functional pairs apparently represent distant sites affecting enzyme activity.

l-Isoleucine dioxygenase (IDO) directly catalyzes the C–H bond hydroxylation of several hydrophobic aliphatic amino acids. However, the ambiguous selectivity of IDO prevents its application in chiral hydroxy amino acid production. The hydroxylation of l-norleucine by IDO, which produces 4-hydroxynorleucine and 5-hydroxynorleucine with obvious regioselectivity, was used to investigate the mechanism of IDO regioselectivity. Along with computational structural analysis and high-throughput screening, the IDO structure revealed single-site variants (T244A, T244G, and T244S) with enhanced regioselectivity; for example, the 4-hydroxynorleucine purity in regioisomeric products increased from 78.9% (by wild-type IDO) to 95.1%, 96.6%, and 95.3%, respectively. Molecular dynamics simulations showed that mutating T244 into smaller amino acids fine-tuned the substrate binding pose. For asymmetric catalysis requiring precise positioning, this change expanded the most frequent distances between the substrate C4 or C5 and Fe2+, giving a maximum 4-hydroxynorleucine purity of 96.6%. We improved the understanding of the regioselectivity of Fe­(II)/2-ketoglutarate-dependent dioxygenases and provide a route for diversifying C–H hydroxylation-based active compounds.