Aliphatic alcohols stabilize an alternative conformation of the tryptophan synthase alpha 2 beta 2 complex from...

The conformational changes that accompany association of the tryptophan synthase alpha and beta 2 subunits are probed by investigating the effects of solvents on the catalytic and spectroscopic properties of tryptophan synthase. Low concentrations of ethanol, propanol, and butanol inhibit conversion of L-serine and indole to L-tryptophan by the alpha 2 beta 2 complex. Inhibition depends on the concentration, chain length, and hydrophobicity of the alcohol. In contrast, these alcohols increase the rates of conversion of beta-chloro-L-alanine and indole to L-tryptophan and of L-serine to pyruvate. Thus, alcohols alter the substrate specificity and reaction specificity of the alpha 2 beta 2 complex. These altered specificities are similar to those of the free beta 2 subunit in aqueous solution. The spectroscopic properties of enzyme-substrate intermediates formed by the alpha 2 beta 2 complex in 3 M ethanol and 0.5 M 1-butanol also resemble the intermediates formed by the free beta 2 subunit in aqueous solution. Experiments using gel-filtration, membrane ultrafiltration, and limited proteolysis show that the alpha 2 beta 2 complex does not dissociate at low concentrations of alcohols. Our results provide evidence that alcohols can stabilize an alternative conformation of the alpha 2 beta 2 complex which is similar to the open conformation of the free beta 2 subunit in aqueous solution.