Crystal structure of rice L-galactose dehydrogenase.

L-Galactose dehydrogenase plays a key role in the biosynthesis of L-ascorbic acid (vitamin C) in plants. Here, we report crystal structures of rice (Oryza sativa) L-galactose dehydrogenase in both apo and NAD+-bound forms at 1.2 and 1.8 Å resolution, respectively. Rice L-galactose dehydrogenase adopts a conserved (β/α)8-barrel fold with a well preserved NAD+-binding cleft, as in other plant L-galactose dehydrogenases. Structural comparisons revealed conformational flexibility in the first and second loop regions, which form the lid and side wall of the NAD+-binding site. These loops undergo significant rearrangement upon NAD+ binding, likely facilitating cofactor and substrate uptake. These findings suggest a flexible loop-mediated regulation of cofactor and substrate access.