The reactions of L-histidine (L-His) and L-tryptophan (L-Trp) with the α2β2 complex of Escherichia coli tryptophan synthase are introduced as probes both of β-subunit catalysis and of ligand-mediated α-β allosteric interactions. Binding of DL-α-glycerol 3-phosphate (GP), an analogue of 3-indole-D-glycerol 3′-phosphate (IGP), to the α-catalytic site increases the affinity of α2β2 for L-His 4.5-fold and the affinity for L-Trp 17-fold and brings about a redistribution of β-bound intermediates that favors the quinonoids derived from each amino acid. Inorganic phosphate (Pi) (presumably via binding to the α-catalytic site) influences the distribution of L-His intermediates as does GP. Previous binding studies [Heyn, M. P., & Weischet, W. O. (1975) Biochemistry 14, 2962-2968] indicate that when the phosphoryl group subsite of the α-catalytic site is occupied by GP or Pi, a high-affinity indole subsite is induced at the α-catalytic site. Interaction of benzimidazole (BZ), an analogue of indole, with this site also shifts the distribution of β-bound L-His intermediates in favor of the L-His quinonoid. In the absence of Pi or GP, BZ interacts primarily at the β-catalytic site and competes with L-His for the β-subunit indole subsite. Since L-His and GP (or Pi) are substrate analogues and L-Trp is the physiological product, these allosteric effects likely take place with the natural substrates. Accordingly, the β-site becomes a higher affinity site for L-Ser, and L-Ser is in a more chemically reactive form when IGP (or D-glyceraldehyde 3-phosphate) is bound at the α-catalytic site. Hence, we postulate that, in vivo, ligand binding at the α-catalytic site confers changes to the β-catalytic site which increase the probability of L-Trp formation via an increased likelihood of L-Ser condensation with indole. © 1990, American Chemical Society. All rights reserved.
