Improvement of 1,3-propanediol oxidoreductase (DhaT) stability against 3-hydroxypropionaldehyde by substitution of cysteine residues

1,3-propanediol oxidoreductase (DhaT), which catalyzes the conversion of 3-hydroxypropionaldehyde (3-HPA) to 1,3-propanediol (1,3-PD) with the oxidation of NADH to NAD+, is a key enzyme in the production of 1,3-PD from glycerol. DhaT is known to be severely inactivated by its physiological substrate, 3-HPA, due to the reaction of 3-HPA with the thiol group of the cysteine residues. In this study, using site-directed mutagenesis, four cysteine residues in Klebsiella pneumoniae J2B DhaT were substituted to alanine, the amino acid commonly found in cysteine’s positions in other DhaT, individually and in combination. Among the total of 15 mutants developed, a double mutant (C28A_C107A) and a triple mutant (C28A_C93A_C107A) exhibited approximately 50 and 16% higher activity than the wild-type counterpart, respectively, after 1 h incubation with 10 mM 3-HPA. According to detailed kinetic studies, the double mutant had slightly better kinetic properties (Vmax, Kcat, and Km for both 3-HPA and NADH) than wild-type DhaT. This study shows that DhaT stability against 3-HPA can be increased by cysteine-residue removal, albeit to a limited extent.