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This paper reports the biochemical characterization of the flavoprotein L-aspartate oxidase from Escherichia coli. Modification of a previously published procedure allowed overexpression of the holoenzyme in an unproteolysed form. L-Aspartate oxidase is a monomer of 60 kDa containing 1 mol of non-covalently bound FAD/mol protein. A polarographic and two spectrophotometric coupled assays have been set up to monitor the enzymatic activity continuously. L-Aspartate oxidase was subjected to product inhibition since iminoaspartate, which results from the oxidation of L-aspartate, binds to the enzyme with a dissociation constant (Kd) equal to 1.4 μM. The enzyme binds FAD by a simple second-order process with Kd 0.67 μM. Site-directed mutagenesis of the residues E43, G44, S45, F47 and Y48 located in the putative binding site of the isoallossazinic portion of FAD reduces the affinity for the coenzyme. [ABSTRACT FROM AUTHOR]