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Amino acid oxidases (AOs) are flavin adenine dinucleotide (FAD)-dependent dimeric enzymes that stereo specifically catalyse the deamination of an Ą-amino acid leading to an Ą-keto acid. Putative Leptospira interrogans recombinant L-amino acid oxidase (Li-rLAO; lacking 20 residues corresponding to the N-terminal signal sequence) was cloned, expressed, purified, and its three-dimensional structure was determined by X-ray crystallography at a resolution of 1.8 A. The active site could be easily identified by the presence of electron density corresponding to a non-covalently bound FAD in both protomers of the dimeric enzyme. Structural analysis of Li-rLAO revealed that its polypeptide fold is similar to those of the previously determined homologous structures as available in the Protein Data Bank. However, a substrate-binding residue found at the active site of other previously determined homologous structures was not conserved in Li-rLAO, suggesting that its specificity may differ from those of earlier reported structures. Not surprisingly, Li-rLAO showed no activity for most amino acids and amines; it exhibited a low activity only with L-arginine as the substrate. The catalytic properties of Li-rLAO could be rationalized in terms of its three-dimensional structure. [ABSTRACT FROM AUTHOR]