Investigation of structure and function of a catalytically efficient variant of the human flavin-containing monooxygenase form 3 (FMO3)

protein with similarity to flavin-containing monooxygenases; 1w4x, phenylacetone monooxygenase. This article has not been copyedited and formatted. The final version may differ from this version. Abstract To characterize the contribution of amino acid 360 to the functional activity of the human flavin-containing monooxygenase form 3 (FMO3) and form 1 (FMO1) in the oxygenation of drugs and chemicals, we expressed four FMO3 variants (i.e., Ala 360-FMO3, His 360-FMO3, Gln 360-FMO3 and Pro 360-FMO3) and one FMO1 variant (i.e., Pro 360-FMO1) and compared them to wild-type enzymes (Leu 360-FMO3 and His 360-FMO1), respectively. The amino acid substitutions were introduced into wild-type FMO3 or FMO1 cDNA by site directed mutagenesis. The thermal stability of variants of Leu 360 FMO3 was also studied and the thermal stability was significantly different from that of wild-type FMO3. The influence of different substrates to modulate the catalytic activity of FMO3 variants was also examined. Selective functional substrate activity was determined with mercaptoimidazole, chlorpromazine and