Catalyzes the oxidation of L-aspartate to iminoaspartate, the first step in the de novo biosynthesis of NAD(+) (PubMed:2841129, PubMed:7033218, PubMed:2187483, PubMed:8706749, PubMed:8706750, PubMed:11294641). Can use either oxygen or fumarate as electron acceptors, which allows the enzyme to be functional under aerobic and anaerobic conditions (PubMed:8706750, PubMed:11294641, PubMed:12200425). In vivo, fumarate is used under anaerobic conditions, and oxygen is the predominant electron acceptor under aerobic conditions due to the lower fumarate levels (PubMed:20149100). In vitro, fumarate is a more efficient electron acceptor and is kinetically superior to oxygen (PubMed:12200425, PubMed:20149100). L-aspartate + O2 = H2O2 + iminosuccinate fumarate + L-aspartate = iminosuccinate + succinate Binds 1 FAD per subunit. Inhibited by the product iminoaspartate (PubMed:8706749). Competitively inhibited by mesotartrate (PubMed:2187483). NAD acts as a competitive inhibitor to FAD (PubMed:7033218, PubMed:2187483). Inhibited by iodoacetic acid, diethylpyrocarbonate and tetranitromethane (PubMed:2187483). kcat is 29.4 min(-1) for L-aspartate oxidase activity with oxygen as electron acceptor. kcat is 16.0 min(-1) for reduced benzyl viologen:fumarate oxidoreductase activity (PubMed:11294641). kcat is 40.0 min(-1) with L-aspartate as substrate. kcat is 0.21 min(-1) with N-acetyl-L-aspartate as substrate. kcat is 0.46 min(-1) with N-formyl-L-aspartate as substrate. kcat is 10.4 min(-1) with 3-OH-erythro-L-aspartate as substrate (PubMed:20600565). Optimum pH is 8.0. Optimum temperature is 40 degrees Celsius. Cofactor biosynthesis; NAD(+) biosynthesis; iminoaspartate from L-aspartate (oxidase route): step 1/1. Monomer (PubMed:2187483, PubMed:8706749). Homodimer (PubMed:10425677). Both the monomeric and dimeric forms of the enzyme are catalytically active (PubMed:10425677). Folds into three domains: the FAD-binding domain, the capping domain and the C-terminal helical domain. Mutants lacking this gene generate 25% less H(2)O(2) than the parent strain. The chemistry is similar to that of typical amino acid oxidases in which the transfer of the hydride from C2 of L-aspartate to FAD is rate-limiting and occurs in a concerted manner with respect to deprotonation of the alpha-amine. NadB may have structurally evolved from succinate dehydrogenase/fumarate reductase-type enzymes to gain the new functionality of oxidizing amino acids while retaining the ability to reduce fumarate. Belongs to the FAD-dependent oxidoreductase 2 family. NadB subfamily.