Backbone assignments and conformational dynamics in the S. typhimurium tryptophan synthase alpha-subunit from solution-state NMR

摘要

Backbone assignments for the isolated alpha-subunit of Salmonella typhimurium tryptophan synthase (TS) are reported based on triple resonance solution-state NMR experiments on a uniformly H-2,C-13,N-15-labeled sample. From the backbone chemical shifts, secondary structure and random coil index order parameters (RCI-S-2) are predicted. Titration with the 3-indole-d-glycerol 3 '-phosphate analog, N-(4 '-trifluoromethoxybenzenesulfonyl)-2-aminoethyl phosphate (F9), leads to chemical shift perturbations indicative of conformational changes from which an estimate of the dissociation constant is obtained. Comparisons of the backbone chemical-shifts, RCI-S-2 values, and site-specific relaxation times with and without F9 reveal allosteric changes including modulation in secondary structures and loop rigidity induced upon ligand binding. A comparison is made to the X-ray crystal structure of the alpha-subunit in the full TS alpha beta beta alpha bi-enzyme complex and to two new X-ray crystal structures of the isolated TS alpha-subunit reported in this work.