Rational Design of Cyclohexanone Monooxygenase and a Whole-Cell System for Biocatalytic Redox-Neutral Cascade Synthesis of Lactones

Whole-cell redox-neutral cascade reactions are promising for the sustainable synthesis of bulky lactones. In this study, the cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 (CHMO_Ac) was rationally designed based on enhancing product release and consensus sequence design to enhance the activity and stability of the enzyme. A four-point mutant MU4 (G14A/A43G/M400L/F432I) showed improved activity and stability for the oxidation of cyclohexanone to ε-caprolactone at a high substrate loading. A newly NADPH-dependent alcohol dehydrogenase from Pseudomonas monteilii (PmADH) with high cyclohexanol oxidation activity was introduced for constructing the redox-neutral cascade for the synthesis of ε-caprolactone from cyclohexanol. To improve the intracellular environment for cascade synthesis of ε-caprolactone, CRISPR/Cas9-mediated strong promoter insertion to the endogenous catalase gene katE in the NADP self-sufficient Escherichia coli strain BK-1 (Ptrc-nadK/Ptrc-pncB) was done for mitigating the oxidative stress to obtain the strain E. coli BKE-3 harboring MU4 and PmADH. Finally, 776 mM cyclohexanol could be converted into ε-caprolactone by the BKE-3 whole-cell catalytic system with a >99% conversion rate without cyclohexanone accumulation, which was the highest record for the redox-neutral cascade synthesis of ε-caprolactone. The BKE-3 whole-cell catalytic system also showed high efficiency for the synthesis of various lactones. This work provides an efficient redox-neutral catalytic platform for the sustainable synthesis of lactones featuring high substrate loading and atomic economy.