Crystal structure of Azospirillum brasilense L-arabinose 1-dehydrogenase (NADP-bound form)

In Azospirillum brasilense, a gram-negative nitrogen-fixing bacterium, l-arabinose is converted to α-ketoglutarate through a nonphosphorylative metabolic pathway. In the first step in the pathway, l-arabinose is oxidized to l-arabino-γ-lactone by NAD(P)-dependent l-arabinose 1-dehydrogenase (AraDH) belonging to the glucose-fructose oxidoreductase/inositol dehydrogenase/rhizopine catabolism protein (Gfo/Idh/MocA) family. Here, we determined the crystal structures of apo- and NADP-bound AraDH at 1.5 and 2.2 A resolutions, respectively. A docking model of l-arabinose and NADP-bound AraDH and structure-based mutational analyses suggest that Lys91 or Asp169 serves as a catalytic base and that Glu147, His153, and Asn173 are responsible for substrate recognition. In particular, Asn173 may play a role in the discrimination between l-arabinose and d-xylose, the C4 epimer of l-arabinose.