Figure 3.

Ammonium ion contamination effects from deamination of L-Ser in the tryptophan synthase-catalyzed pyruvate side reaction. (A) Effects of L-Ser - α₂β₂ incubation on the indoline reaction (incubation times are indicated). Final concentrations: α₂β₂, 10 µM; L-Ser 50 mM, indoline 5 mM. (B) Effects of NH₄⁺ preincubation with α₂β₂. Final concentrations: α₂β₂, 10 µM; L-Ser 50 mM, indoline 5 mM, and NH₄⁺ as indicated. Time courses in (A) and (B) were measured using SWSF instrumentation. (C) Comparison of the effect of L-Ser incubation time (black circles) on amplitude with the effects of added [NH₄⁺] (white circles) on the fast phase of the indoline reaction (α₂β₂ preincubated with NH₄⁺). Amplitudes taken from (A) and (B). To correct for fast phase amplitude loss in the instrument dead-time, the total amplitude of the fast phase was estimated as the sum of the observed fast phase and the y-intercept value for the reaction of α₂β₂ and L-Ser, with indoline. Traces were fit to the sum of two exponentials. The NH₄⁺ concentrations for time resolved data points (black circles) were estimated using k_cat = 0.098 s⁻¹ for pyruvate formation at each incubation time in (A).