The Tryptophan Synthase Multienzyme Complex: Exploring Structure-Function Relationships with X-Ray Crystallography and Mutagenesis

The bifunctional tryptophan synthase ₂₂ complex that catalyzes the final two reactions in tryptophan biosynthesis is a classic example of a multienzyme complex that "channels" a metabolic intermediate (indole) between two active sites. The three-dimensional structure of the ₂₂ complex from Salmonella typhimurium reveals that the four polypeptide sub-units are arranged in an extended order forming a complex 150 Å long. The active sites of the neighboring and subunits are separated by about 30 Å and appear to be connected by a tunnel, which may facilitate the intramolecular transfer of indole. The active site of the subunit, which is centrally located near one end of an eight-fold / barrel structure, contains the sites of most of the missense mutations which were identified as key residues by Yanofsky and colleagues in early genetic studies. Site-directed muta-genesis is being used to replace residues found in the active sites of the and subunits in order to probe the mechanism of catalysis. Recombinant DNA technology should also be useful in analyzing protein-protein interaction, protein folding and the channeling phenomenon.

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