Mechanism of modulation of dopamine .beta.-monooxygenase by pH and fumarate as deduced from initial rate and primary deuterium isotope effect studies

Dopamine beta-monooxygenase catalyzes a reaction in which 2 mol of protons are consumed for each turnover of substrate. Studies of the pH dependence of initial rate parameters (Vmax and Vmax/Km) and their primary hydrogen isotope effects show that at least two ionizable residues are involved in catalysis. One residue (B1, pK = 5.6-5.8) must be protonated prior to the carbon-hydrogen bond cleavage step, implying a role for general-acid catalysis in substrate activation. A second protonated residue (B2, pK = 5.2-5.4) facilitates, but is not required for, product release. Recent measurement of the intrinsic isotope effect for dopamine beta-monoxygenase [Miller, S. M., & Klinman, J. P. (1983) Biochemistry (preceding paper in this issue)] allows an analysis of the pH dependence of rate constant ratios and in selected instances individual rate constants. We demonstrate large changes in the rate-determining step as well as an unprecedented inversion in the kinetic order of substrate release from ternary complex over an interval of 2 pH units. Previously, fumarate has been used in dopamine beta-monooxygenase assays because of its property of enzyme activation. Studies of the pH behavior in the presence of saturating concentrations of fumarate have shown two causes of the activation: (i) fumarate perturbs the pK of B1 to pK = 6.6-6.8 such that the residue remains protonated and the enzyme optimally active over a wider pH range; (ii) fumarate decreases the rate of dopamine release from the ternary enzyme-substrate complex, increasing the equilibrium association constant for dopamine binding. Both effects are consistent with a simple electrostatic stabilization of bound cationic charges by the dianionic form of fumarate.