Substitution Of A Surface-Exposed Residue Involved In An Allosteric Network Enhances Tryptophan Synthase Function In Cells

Networks of noncovalent amino acid interactions propagate allosteric signals throughout proteins. Tryptophan synthase (TS) is an allosterically controlled bienzyme in which the indole product of the alpha subunit (alpha TS) is transferred through a 25 angstrom hydrophobic tunnel to the active site of the beta subunit (beta TS). Previous nuclear magnetic resonance and molecular dynamics simulations identified allosteric networks in alpha TS important for its function. We show here that substitution of a distant, surface-exposed network residue in alpha TS enhances tryptophan production, not by activating alpha TS function, but through dynamically controlling the opening of the indole channel and stimulating beta TS activity. While stimulation is modest, the substitution also enhances cell growth in a tryptophan-auxotrophic strain of Escherichia coli compared to complementation with wild-type alpha TS, emphasizing the biological importance of the network. Surface-exposed networks provide new opportunities in allosteric drug design and protein engineering, and hint at potential information conduits through which the functions of a metabolon or even larger proteome might be coordinated and regulated.