Structural basis for the impaired channeling and allosteric inter−subunit communication in the A169L/C170W mutant of tryptophan synthase

We determined the 2.25 Å resolution crystal structure of the βA169L/βC170W mutant form of the tryptophan synthase α2β2 complex fromSalmonella typhimurium complexed with the α-active site substrate analogue 5-fluoro-indole-propanol-phosphate to identify the structural basis for the changed kinetic properties of the mutant (Anderson, K. S., Kim, A. Y., Quillen, J. M., Sayers, E., Yang, X. J., and Miles, E. W. (1995) J. Biol. Chem. 270, 29936–29944). Comparison with the wild-type enzyme showed that the βTrp170 side chain occludes the tunnel connecting the α- and β-active sites, explaining the accumulation of the intermediate indole during a single enzyme turnover. To prevent a steric clash between βLeu169 and βGly135, located in the β-sheet of the COMM (communication) domain (βGly102-βGly189), the latter reorganizes. The changed COMM domain conformation results in a loss of the hydrogen bonding networks between the α- and β-active sites, explaining the poor activation of the α-reaction upon formation of the aminoacrylate complex at the β-active site. The 100-fold reduced affinity for serine seems to result from a movement of βAsp305 away from the β-active site so that it cannot interact with the hydroxyl group of a pyridoxal phosphate-bound serine. The proposed structural dissection of the effects of each single mutation in the βA169L/βC170W mutant would explain the very different kinetics of this mutant and βC170F