Title: Molecular Insights into the Regioselectivity of the Fe(II)/2-Ketoglutarate-Dependent Dioxygenase-Catalyzed C–H Hydroxylation of Amino Acids

Abstract: 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.