Multiple replacements at position 211 in the alpha subunit of tryptophan synthase as a probe of the folding unit association reaction.

Equilibrium and kinetic studies on the folding of a series of amino acid replacements at position 211 in the alpha subunit of tryptophan synthase from Escherichia coli were performed in order to determine the role of this position in the rate-limiting step in folding. Previous studies [Beasty, A. M., Hurle, M. R., Manz, J. T., Stackhouse, T., Onuffer, J. J., & Matthews, C. R. (1986) Biochemistry 25, 2965-2974] have shown that the rate-limiting step corresponds to the association/dissociation of the amino (residues 1-188) and carboxy (residues 189-268) folding units. In terms of the secondary structure, the amino folding unit consists of the first six strands and five alpha helices of this alpha/beta barrel protein. The carboxy folding unit comprises the remaining two strands and three alpha helices; position 211 is in strand 7. Replacement of the wild-type glycine at position 211 with serine, valine, and tryptophan at most alters the rate of dissociation of the folding units; association is not changed significantly. In contrast, glutamic acid and arginine dramatically decelerate and accelerate, respectively, both association and dissociation. The difference in effects is attributed to long-range electrostatic interactions for these charged side chains; steric effects and/or hydrogen bonding play lesser roles. When considered with previous data on replacements at other positions in the alpha subunit [Hurle, M. R., Tweedy, N. B., & Matthews, C. R. (1986) Biochemistry 25, 6356-6360], it is clear that beta strands 6 (in the amino folding unit) and 7 (in the carboxy folding unit and containing position 211) dock late in the folding process.