Steady state kinetics of wild-type (closed circles) and T283M (open circles) TyrH as a function of tyrosine concentration. Conditions: 300 μM BH4, 100 μg/mL catalase, 10 μM ferrous ammonium sulfate, 1 mM dithiothreitol, 50 mM HEPES, pH 7.0, 30°C. The lines are from fits of the data to Equation (1).