Allosteric regulation of substrate channeling: Salmonella typhimurium tryptophan synthase

The regulation of the synthesis of L-tryptophan (L-Trp) in enteric bacteria begins at the level of gene expression where the cellular concentration of L-Trp tightly controls expression of the five enzymes of the Trp operon responsible for the synthesis of L-Trp. Two of these enzymes, trpA and trpB, form an alpha beta beta alpha bienzyme complex, designated as tryptophan synthase (TS). TS carries out the last two enzymatic processes comprising the synthesis of L-Trp. The TS alpha-subunits catalyze the cleavage of 3-indole D-glyceraldehyde 3 & PRIME;-phosphate to indole and D-glyceraldehyde 3-phosphate; the pyridoxal phosphate-requiring beta-subunits catalyze a nine-step reaction sequence to replace the L-Ser hydroxyl by indole giving L-Trp and a water molecule. Within alpha beta dimeric units of the alpha beta beta alpha bienzyme complex, the common intermediate indole is channeled from the alpha site to the beta site via an interconnecting 25 & ANGS;-long tunnel. The TS system provides an unusual example of allosteric control wherein the structures of the nine different covalent intermediates along the beta-reaction catalytic path and substrate binding to the alpha-site provide the allosteric triggers for switching the alpha beta beta alpha system between the open (T) and closed (R) allosteric states. This triggering provides a linkage that couples the allosteric conformational coordinate to the covalent chemical reaction coordinates at the alpha- and beta-sites. This coupling drives the alpha- and beta-sites between T and R conformations to achieve regulation of substrate binding and/or product release, modulation of the alpha- and beta-site catalytic activities, prevention of indole escape from the confines of the active sites and the interconnecting tunnel, and synchronization of the alpha- and beta-site catalytic activities. Here we review recent advances in the understanding of the relationships between structure, function, and allosteric regulation of the complex found in Salmonella typhimurium.