DADH catalyzes the flavin-dependent oxidative deamination of d-amino acids to the corresponding α-keto acids and ammonia. Here we report the first X-ray crystal structures of DADH at 1.06 Å resolution and its complexes with iminoarginine (DADH(red)/iminoarginine) and iminohistidine (DADH(red)/iminohistidine) at 1.30 Å resolution. The DADH crystal structure comprises an unliganded conformation and a product-bound conformation, which is almost identical to the DADH(red)/iminoarginine crystal structure. The active site of DADH was partially occupied with iminoarginine product (30% occupancy) that interacts with Tyr53 in the minor conformation of a surface loop. This flexible loop forms an "active site lid", similar to those seen in other enzymes, and may play an essential role in substrate recognition. The guanidinium side chain of iminoarginine forms a hydrogen bond interaction with the hydroxyl of Thr50 and an ionic interaction with Glu87. In the structure of DADH in complex with iminohistidine, two alternate conformations were observed for iminohistidine where the imidazole groups formed hydrogen bond interactions with the side chains of His48 and Thr50 and either Glu87 or Gln336. The different interactions and very distinct binding modes observed for iminoarginine and iminohistidine are consistent with the 1000-fold difference in k(cat)/K(m) values for d-arginine and d-histidine. Comparison of the kinetic data for the activity of DADH on different d-amino acids and the crystal structures in complex with iminoarginine and iminohistidine establishes that this enzyme is characterized by relatively broad substrate specificity, being able to oxidize positively charged and large hydrophobic d-amino acids bound within a flask-like cavity.

amino acid ͉ arginine dehydrogenase ͉ racemase A lthough L-amino acids are the predominant amino acids in protein synthesis, D-amino acids serve as specialized components of many types of machineries in living organisms. In mammals, D-serine and D-aspartate are associated with cell aging and neural signaling (1, 2). In bacteria, some D-amino acids are essential ingredients of cell wall synthesis (3). Endogenous D-amino acids are produced by racemization from the prevalent L-amino acids through the action of racemases. Amino acid racemases are classified into 2 groups: pyridoxal 5Ј phosphate-dependent and phosphate-independent enzymes (4). Completely different reaction mechanisms have been proposed for these 2 groups of enzymes for the spatial rearrangement of ␣-hydrogen in the corresponding amino acids. Nevertheless, racemization of amino acids reported so far is catalyzed by a single enzyme.When provided in excess, some D-amino acids can be used as nutrients to support growth by bacteria. In most cases, D-amino acid oxidase or dehydrogenase catalyzes the oxidative deamination as the first step in catabolism. Pseudomonas aeruginosa, an opportunistic human pathogen with an enormous catabolic capacity, is capable of growing on D-arginine as the sole source of carbon and nitrogen (5). The presence of an inducible D-arginine dehydrogenase activity in this organism was initially reported by Haas and coworkers (6), and 2-ketoarginine derived from this reaction could be converged into the arginine transaminase (ATA) pathway (7,8), 1 of the 4 pathways for L-arginine catabolism in pseudomonads (Fig. 1). In fact, it has been proposed that L-arginine might be converted into D-arginine via racemization (6), reminiscent of L-alanine utilization through a catabolic alanine racemase and D-alanine dehydrogenase in Escherichia coli and many bacteria (9, 10). Existence of an arginine racemase in P. aeruginosa was supported by growth complementation of arginine auxotrophs with D-arginine (6). However, the activity of P. aeruginosa arginine racemase has never been demonstrated in vitro, presumably because of the instant decomposition of both L-and D-arginine in extracts.Under aerobic conditions, L-arginine is preferentially catabolized by the arginine succinyltransferase (AST) pathway, followed by the ATA pathway (7,11). Enzymes of the AST pathway are encoded by the aruCFGDBE operon (12), which is induced by exogenous L-arginine in the presence of a functional arginine regulator, ArgR (13). The ArgR protein belongs to the AraC family of transcriptional regulators. Depending on the location of its binding sites, ArgR serves as a repressor or activator of ArgR regulon in arginine and glutamate metabolism. Thus, when the AST pathway is absent or remains uninduced (e.g., in the argR mutant), the ATA pathway then takes charge as the auxiliary route of L-arginine utilization.

ELONGATED HYPOCOTYL 5 (HY5), a basic domain/leucine zipper (bZIP) transcription factor, acts as a master regulator of transcription to promote photomorphogenesis. At present, it's unclear whether HY5 uses additional mechanisms to inhibit hypocotyl elongation. Here, we demonstrate that HY5 enhances the activity of GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2), a key repressor of brassinosteroid signaling, to repress hypocotyl elongation. We show that HY5 physically interacts with and genetically acts through BIN2 to inhibit hypocotyl elongation. The interaction of HY5 with BIN2 enhances its kinase activity possibly by the promotion of BIN2 Tyr 200 autophosphorylation, and subsequently represses the accumulation of the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1). Leu 137 of HY5 is found to be important for the HY5-BIN2 interaction and HY5-mediated regulation of BIN2 activity, without affecting the transcriptional activity of HY5. HY5 levels increase with light intensity, which gradually enhances BIN2 activity. Thus, our work reveals an additional way in which HY5 promotes photomorphogenesis, and provides an insight into the regulation of GSK3 activity.