tailieunhanh - Báo cáo khoa học: Structure–activity relationships of wheat flavone O-methyltransferase – a homodimer of convenience

Wheat flavoneO-methyltransferase catalyzes three sequential methylations of the flavone tricetin. Like other flavonoidO-methyltransferases, the pro-tein is a homodimer. We demonstrate, using analytical ultracentrifugation, that perchlorate dissociates the dimer into monomers. The resulting mono-mers retain all their catalytic capacity, including the ability to catalyze the three successive methylations. | ỊFEBS Journal Structure-activity relationships of wheat flavone O-methyltransferase - a homodimer of convenience Jack A. Kornblatt1 Jian-Min Zhou2 and Ragai K. Ibrahim2 1 Enzyme Research Group Department of Biology Concordia University Montreal Canada 2 Plant Biochemistry Laboratory Department of Biology Concordia University Montreal Canada Keywords analyticalultracentrifugation dimer-monomer equilibrium flavone O-methyltransferase isothermal titration calorimetry multiple methylations Correspondence J. A. Kornblatt Department of Biology Concordia University 7141 Sherbrooke St West Montreal QC Canada H4B 1R6 Fax 1 514 848 2881 Tel 1 514 848 2424 ext 3404 E-mail krnbltt@ Website http bioweb Received 4 July 2007 revised 11 January 2008 accepted 4 March 2008 doi Wheat flavone O-methyltransferase catalyzes three sequential methylations of the flavone tricetin. Like other flavonoid O-methyltransferases the protein is a homodimer. We demonstrate using analytical ultracentrifugation that perchlorate dissociates the dimer into monomers. The resulting monomers retain all their catalytic capacity including the ability to catalyze the three successive methylations. We show using isothermal titration calorimetry that the binding constant for S does not change significantly as the protein dissociates. The second substrate tricetin binds to the dimers but could not be tested with the monomers. CD UV and fluorescence spectroscopy show that there are substantial changes in the structure of the protein as it dissociates. The fact that there are differences between the monomers and dimers even as the monomers maintain activity may be the result of the very low catalytic capacity of this enzyme. Maximal turnover numbers for the dimers and monomers are only about 6-7 per minute. Even though the binding pockets for S-adenosyl-i-methionine tricetin selgin and tricin are intact selection of a .