Intermediate Trapping via a Conformational Switch in the Na+-Activated Tryptopha

Tryptophan synthase has been well studied for many years and serves as the paradigm for allosteric regulation of substrate channeling in multienzyme complexes. The 22 tryptophan synthase bienzyme complex from Salmonella typhimurium catalyzes the final two steps in the biosynthesis of tryptophan. The enzyme consists of two -dimers joined -end to -end in a nearly linear fashion. The and active sites in the bienzyme complex are shown by the crystal structure to be connected by a 25-30 Å tunnel. The -subunit catalyzes the cleavage of 3-in dole-D-glycerol 3'-phosphate (IGP)1 to indole and D-glyceraldehyde 3-phosphate (G3P), while the -subunit catalyzes the condensation of the indole generated in the -reaction with L-serine (L-Ser) to form L-tryptophan (L-Trp). The combined - and -reactions are designated as the -reaction. Rapid kinetic studies have shown that indole is channeled through the connecting tunnel between the - and -sites.

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