Monoamine oxidase B (MAO B), an integral protein of the outer mitochondrial membrane, catalyzes the oxidative deamination of various neuroactive and vasoactive amines. A covalently bound FAD cofactor at Cys-397 of human MAO B is required for the oxidation of the amine substrates. In addition to the covalent binding site, MAO B also contains a noncovalent FAD binding region (residues 6-34) known as the dinucleotide binding motif. Previously, we have shown that Glu-34 is required for catalytic activity, presumably by forming a hydrogen bond between the carboxylate group of glutamate and the 2'-hydroxyl group of ribose in the AMP moiety of FAD. In this work, we have identified a third FAD binding site in MAO B (residues 39-46) by sequence comparisons to other flavoenzymes. The conserved sequence contains a tyrosine residue (Tyr-44) which, based on the X-ray crystal structure of ferredoxin-NADP+ reductase, is postulated to participate in FAD binding through van der Waals contact with the isoalloxazine ring and a hydrogen bond to the 3'-hydroxy of the ribityl moiety. To test the postulated role of this tyrosine residue, site-directed mutants that encode substitutions at Tyr-44 were prepared and expressed in mammalian COS-7 cells. Variant MAO B enzymes were then characterized with respect to enzymatic activity and [14C]FAD incorporation. Substitution of tyrosine with phenylalanine had no effect on MAO B activity or the level of [14C]FAD incorporation compared to the wild-type enzyme, indicating that the hydroxyl group of the tyrosine residue was not essential at residue 44.(ABSTRACT TRUNCATED AT 250 WORDS)