L-Aspartate oxidase is present in the anaerobic hyperthermophilic archaeon Pyrococcus horikoshii OT-3:: characteristics and role in the de novo biosynthesis of nicotinamide adenine dinucleotide proposed by genome sequencing

A gene encoding the L-aspartate oxidase homologue was identified via genome sequencing in the anaerobic hyperthermophilic archaeon Pyrococcus horikoshii OT-3. We succeeded in expressing the encoding gene in Escherichia coli and purified the product to homogeneity. Characterization of the protein revealed that it is the most thermostable L-aspartate oxidase detected so far. In addition to the oxidase activity, the enzyme catalyzed L-aspartate dehydrogenation in the presence of an artificial electron acceptor such as phenazine methosulfate, 2,6-dichlorophenol-indophenol, and ferricyanide. L-Aspartate oxidase is known to function as the first enzyme in the de novo NAD biosynthetic pathway in prokaryotes. By a similarity search in public databases, the genes that encode the homologue of all other enzymes involved in the pathway were identified in the P horikoshii OT-3 genome. This suggests that P horikoshii OT-3 may use the de novo NAD biosynthetic pathway under anaerobic conditions.