CRYSTAL-STRUCTURES OF A MUTANT (BETA-K87T) TRYPTOPHAN SYNTHASE ALPHA(2)BETA(2) COMPLEX WITH LIGANDS BOUND TO THE ACTIVE-SITES OF THE ALPHA-SUBUNITS AND BETA-SUBUNITS REVEAL LIGAND-INDUCED CONFORMATIONAL-CHANGES

Three-dimensional structures are reported for a mutant (beta K87T) try ptophan 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 extern al aldimines with the coenzyme, pyridoxal phosphate, We also present s tructures with L-serine bound to the beta site and either alpha-glycer ol 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 resu lts further identify the substrate and product binding sites in each s ubunit and provide insight into conformational changes that occur upon formation of these complexes. The two structures having ligands at th e active sites of both alpha- and beta-subunits reveal an important ne w feature, the ordering of alpha-subunit loop 6 (residues 179-187), Cl osure of loop 6 isolates the active site of the alpha-subunit from sol vent and results in interaction between alpha Thr183 and the catalytic residue alpha Asp60, Other conformational differences between the wil d type and these two mutant structures include a rigid-body rotation o f the alpha-subunit of similar to 5 degrees relative to the beta-subun it and large movements of part of the beta-subunit (residues 93-189) t oward the rest of the beta-subunit. Much smaller differences are obser ved 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 struc tures, with exception of the disordered alpha-subunit loop 6, These la rge-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 st ructural base for interpreting the allosteric properties of tryptophan synthase.