Structural insights into the mechanism underlying the dual cofactor specificity of glyoxylate reductase from Acetobacter aceti in the β-hydroxyacid dehydrogenase family

The β-hydroxyacid dehydrogenase family exhibits diverse cofactor preferences: some enzymes favor NAD, others favor NADP, and a subset can utilize both NAD and NADPH. Glyoxylate reductase from Acetobacter aceti JCM 20276 (AacGR) exhibits a dual cofactor specificity for NADPH and NADH in its catalytic reduction of glyoxylate to glycolate. In contrast to conventional cofactor-discriminating motifs, NRX and DXX, found in NADP- and NAD-specific enzymes, respectively, AacGR has a TPS motif in the equivalent position. Here we report X-ray crystallographic analysis of AacGR in its ligand-free form, and in complexes with NADPH and NADH, revealing critical interactions: Ser41 of the TPS motif interacted with the 2'-phosphate group of NADPH, while no analogous interaction occurred with the ribose hydroxy groups of NADH. Moreover, the TPS motif resided within a characteristic β-turn-like structure adjacent to a long flexible loop. Site-directed mutagenesis and kinetic analyses suggest that Ser41 facilitates NADPH binding, while the lack of a direct interaction of the TPS motif with NADH may allow for NADH utilization. The conformational dynamics of the TPS-containing β-turn-like structure along with the flexible loop likely govern the dual cofactor specificity and catalytic turnover of AacGR.