Mechanism of the insect enzyme, tyramine beta-monooxygenase, reveals differences from the mammalian enzyme, dopamine beta-monooxygenase.
Tyramine beta-monooxygenase (TbetaM) catalyzes the synthesis of the neurotransmitter, octopamine, in insects. Kinetic and isotope effect studies have been carried out to determine the kinetic mechanism of TbetaM for comparison with the homologous mammalian enzymes, dopamine beta-monooxygenase and peptidylglycine alpha-hydroxylating monooxygenase. A new and distinctive feature of TbetaM is very strong substrate inhibition that is dependent on the level of the co-substrate, O(2), and reductant as well as substrate deuteration. This has led to a model in which tyramine can bind to either the Cu(I) or Cu(II) forms of TbetaM, with substrate inhibition ameliorated at very high ascorbate levels. The rate of ascorbate reduction of the E-Cu(II) form of TbetaM is also reduced at high tyramine, leading us to propose the existence of a binding site for ascorbate to this class of enzymes. These findings may be relevant to the control of octopamine production in insect cells.
Hess
Corinna R
CR
Department of Chemistry , University of California, Berkeley, California 94720.
McGuirl
Michele M
MM
Division of Biological Sciences and the Biomolecular Structure and Dynamics Program, University of Montana, Missoula, Montana 59812.
Klinman
Judith P
JP
Department of Chemistry , University of California, Berkeley, California 94720; Molecular and Cell Biology, University of California, Berkeley, California 94720. Electronic address: klinman@berkeley.edu.
D000818:Animals; D001205:Ascorbic Acid / Q000737:chemistry; D001671:Biochemistry / Q000295:instrumentation / Q000379:methods; D004299:Dopamine beta-Hydroxylase / Q000737:chemistry* / Q000502:physiology; D004330:Drosophila; D029721:Drosophila Proteins / Q000737:chemistry* / Q000502:physiology; D007313:Insecta; D007554:Isotopes / Q000737:chemistry; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000737:chemistry* / Q000502:physiology; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry*; D013045:Species Specificity; D013379:Substrate Specificity; D014439:Tyramine / Q000737:chemistry*
D029721:Drosophila Proteins; D007554:Isotopes; D006899:Mixed Function Oxygenases; D004299:Dopamine beta-Hydroxylase; C527639:tyramine beta-monooxygenase, Drosophila; D001205:Ascorbic Acid; D010100:Oxygen; D014439:Tyramine