Crystal structures of a mutant (beta K87T) tryptophan synthase alpha(2)beta(2) complex with ligands bound to the active sites of the alpha- and beta-subunits reveal ligand-induced conformational changes

Three-dimensional structures are reported for a mutant (beta K87T) tryptophan synthase alpha(2) beta(2) complex with either the substrate L-serine (beta K87T-Ser) or product L-tryptophan (beta K87T-Trp) at the active site of the beta-subunit, in which both amino acids form external aldimines with the coenzyme, pyridoxal phosphate, We also present structures with L-serine bound to the beta site and either alpha-glycerol 3-phosphate (beta K87T-Ser-GP) or indole-3-propanol phosphate (beta K87T-Ser-IPP) bound to the active site of the alpha-subunit. The results further identify the substrate and product binding sites in each subunit and provide insight into conformational changes that occur upon formation of these complexes. The two structures having ligands at the active sites of both alpha- and beta-subunits reveal an important new feature, the ordering of alpha-subunit loop 6 (residues 179-187), Closure of loop 6 isolates the active site of the alpha-subunit from solvent and results in interaction between alpha Thr183 and the catalytic residue alpha Asp60, Other conformational differences between the wild type and these two mutant structures include a rigid-body rotation of the alpha-subunit of similar to 5 degrees relative to the beta-subunit and large movements of part of the beta-subunit (residues 93-189) toward the rest of the beta-subunit. Much smaller differences are observed in the beta K87T-Ser structure. Remarkably, binding of tryptophan to the beta active site results in conformational changes very similar to those observed in the beta K87T-Ser-GP and beta K87T-Ser-IPP structures, with exception of the disordered alpha-subunit loop 6, These large-scale changes, the closure of loop 6, and the movements of a small number of side chains in the alpha-beta interaction site provide a structural base for interpreting the allosteric properties of tryptophan synthase.