Allosteric effects acting over a distance of 20-25 .ANG. in the Escherichia coli tryptophan synthase bienzyme complex increase ligand affinity and cause redistribution of covalent intermediates

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Abstract
The reactions of L-histidine (L-His) and L-tryptophan (L-trp) with the .alpha.2.beta.2 complex of Escherichia coli tryptophan synthase are introduced as probes both of .beta.-subunit catalysis and of ligand-mediated .alpha.-.beta. allosteric interactions. Binding of DL-.alpha.-glycerol 3-phosphate (GP), an analogue of 3-indole-D-glycerol 3''-phosphate (IGP), to the .alpha.-catalytic site increases the affinity of .alpha.2.beta.2 for L-His 4.5-fold and the affinity for L-Trp 17-fold and brings about a redistribution of .beta.-bound intermediates that favors the quinonoids derived from each amino acid. Inorganic phosphate (Pi) (presumably via binding to the .alpha.-catalytic site) influences the distribution of L-His intermediates as does GP. Previous binding studies [Heyn, M. P., and Weischet, W. O. (1975) Biochemistry 14, 2962-2968] indicate that when the phosphoryl group subsite of the .alpha.-catalytic site is occupied by GP or Pi, a high-affinity indole subsite is induced at the .alpha.-catalytic site. Interaction of benzimidazole (BZ), an analogue of indole, with this site also shifts the distribution of .beta.-bound L-His intermediates in favor of the L-His quinonoid. In the absence of Pi or GP, BZ interacts primarily at the .beta.-catalytic site and competes with L-His for the .beta.-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 .beta.-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 .alpha.-catalytic site. Hence, we postulate that, in vivo, ligand binding at the .alpha.-catalytic site confers changes to the .beta.-catalytic site which increase the probability of L-TRp formation via an increased likelihood of L-Ser condensation with indole.