Engineering Bacillus subtilis Isoleucine Dioxygenase for Efficient Synthesis of (2S,3R,4S)-4-Hydroxyisoleucine

Isoleucine dioxygenase (IDO)-catalyzed hydroxylation of isoleucine is a promising method for the synthesis of the diabetic drug (2S,3R,4S)-4-hydroxyisoleucine [(2S,3R,4S)-4-HIL]. However, the low activity of IDO significantly limits its practical application. In this work, a high-throughput screening method was developed and directed evolution was performed on the IDO from Bacillus subtilis, resulting in a double mutant with improvements in specific activity, protein expression level, and fermentation titer of 3.2-, 2.8-, and 9.4-fold, respectively. l-Isoleucine (228 mM) was completely converted to (2S,3R,4S)-4-HIL by the best variant with a space-time yield of up to 80.8 g L^-1 d^-1, which is the highest record reported so far. With a further increase of the substrate loading to 1 M, a high conversion of 91% could also be achieved. At last, enzymatic synthesis of (2S,3R,4S)-4-HIL was successfully carried out on a 3 L scale, indicating tremendous potential of the IDO variant I162T/T182N for green and efficient production of (2S,3R,4S)-4-HIL.