Measurement of Intrinsic Rate Constants in the Tyrosine Hydroxylase Reaction

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Abstract
Tyrosine hydroxylase (TyrH) is a pterin-dependent mononuclear non-heme aromatic amino acid hydroxylase that catalyzes the conversion of tyrosine to dihydroxyphenylalanine (DOPA). Chemical quench analyses of the enzymatic reaction show a burst of DOPA formation, followed by a linear rate equal to the kcat value at both 5 and 30 °C. The effects of increasing solvent viscosity confirm that kcat is ∼84% limited by diffusion, most probably due to slow product release, and that tyrosine has a commitment to catalysis of 0.45. The effect of viscosity on the kcat/Km for 6-methyltetrahydropterin is greater than the theoretical limit, consistent with the coupling of pterin binding to the movement of a surface loop. The absorbance changes in the spectrum of the tetrahydropterin during the first turnover, the kinetics of DOPA formation during the first turnover, and the previously described kinetics for formation and decay of the Fe(IV)O intermediate [Eser, B. E., Barr, E. W., Frantom, P. A., Saleh, L., Bollinger, J. M., Jr., Krebs, C., and Fitzpatrick, P. F. (2007) J. Am. Chem. Soc. 129, 11334−11335] were analyzed globally, yielding a single set of rate constants for the TyrH reaction. Reversible binding of oxygen is followed by formation of Fe(IV)O and 4a-hydroxypterin with a rate constant of 13 s−1 at 5 °C. Transfer of oxygen from Fe(IV)O to tyrosine to form DOPA follows with a rate constant of 22 s−1. Release of DOPA and/or the 4a-hydroxypterin with a rate constant of 0.86 s−1 completes the turnover.

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