The catalytic reaction of copper amine oxidase proceeds through a ping-pong mechanism comprising two half-reactions. In the initial half-reaction, the substrate amine reduces the Tyr-derived cofactor, topa quinone (TPQ), to an aminoresorcinol form (TPQamr) that is in equilibrium with a semiquinone radical (TPQsq) via an intramolecular electron transfer to the active-site copper. We have analyzed this reductive half-reaction in crystals of the copper amine oxidase from Arthrobacter globiformis. Anerobic soaking of the crystals with an amine substrate shifted the equilibrium toward TPQsq in an "on-copper" conformation, in which the 4-OH group ligated axially to the copper center, which was probably reduced to Cu(I). When the crystals were soaked with substrate in the presence of halide ions, which act as uncompetitive and noncompetitive inhibitors with respect to the amine substrate and dioxygen, respectively, the equilibrium in the crystals shifted toward the "off-copper" conformation of TPQamr. The halide ion was bound to the axial position of the copper center, thereby preventing TPQamr from adopting the on-copper conformation. Furthermore, transient kinetic analyses in the presence of viscogen (glycerol) revealed that only the rate constant in the step of TPQamr/TPQsq interconversion is markedly affected by the viscogen, which probably perturbs the conformational change. These findings unequivocally demonstrate that TPQ undergoes large conformational changes during the reductive half-reaction.

From the Department of Biochemistry Osaka Medical College Takatsuki Osaka 569 8686 Japan

the Advanced Photon Technology Division RIKEN SPring 8 Center Sayo gun Hyogo 679 5148 Japan

the Department of Chemistry Osaka Medical College Takatsuki Osaka 569 8686 Japan and

the Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka 567 0047 Japan

the Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka 567 0047 Japan the Center of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University 783 71 Olomouc Czech Republic

the Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka 567 0047 Japan the Department of Chemistry Osaka Medical College Takatsuki Osaka 569 8686 Japan and

the School of Science and Technology Kwansei Gakuin University Sanda Hyogo 669 1337 Japan the Advanced Photon Technology Division RIKEN SPring 8 Center Sayo gun Hyogo 679 5148 Japan

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