The anthranilate aggregate of Escherichia coli: kinetics of inhibition by tryptophan of phosphoribosyltransferase

Abstract

The kinetic mechanism of the phosphoribosyltransferase reaction is shown to be rapid equilibrium random bi bi with an enzyme–anthranilate–pyrophosphate abortive complex. We present a rate equation that not only predicts the observed kinetic patterns but also accomodates the fact that feedback inhibition is partial, even though tryptophan (Ki = 0.5 μM) and phosphoribosylpyrophosphate (Km = 50 μM) are competitive. Neither ligand completely abolishes the effect of the other. Instead, the binding of one ligand leads to a mutual elevation in the dissociation constant of the opposing ligand by a factor of two to three. Tryptophan inhibition is noncompetitive with respect to anthranilate (Km = 0.58 μM) and does not diminish the rate of interconversion of ternary complexes. Tryptophan cooperativity, with respect to the inhibition of phosphoribosyltransferase, conforms to the concerted Monod–Wyman–Changeux formulation (kinetic Hill coefficient = 2), whereas tryptophan as an inhibitor of anthranilate synthase more closely conforms to a Koshland model of sequential cooperativity with a kinetic Hill coefficient of 1.4. The aggregrate contains only one class of tryptophan sites. Thus the first tryptophan molecule bound to the aggregate maximally inhibits both phosphoribosyltransferase active centers and one of the two anthranilate synthase catalytic sites. The remaining anthranilate synthase subunit thereupon is converted into a form with less (but not zero) affinity for chorismate and a greater affinity for a second molecule of tryptophan.