title: Aliphatic alcohols stabilize an alternative conformation of the tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium.
abstract: 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.
authors.affiliation: Laboratory of Biochemical Pharmacology, NIDDK, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: S A
authors.initials: SA
authors.lastname: Ahmed
authors.forename: E W
authors.initials: EW
authors.lastname: Miles
mesh_terms: D020001:1-Butanol; D000433:1-Propanol / Q000494:pharmacology; D000438:Alcohols / Q000494:pharmacology*; D000440:Butanols / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004795:Enzyme Stability / Q000187:drug effects; D000431:Ethanol / Q000494:pharmacology; D007211:Indoles / Q000378:metabolism; D011487:Protein Conformation; D012486:Salmonella typhimurium / Q000201:enzymology*; D012694:Serine / Q000378:metabolism; D013053:Spectrophotometry; D014364:Tryptophan / Q000096:biosynthesis; D014367:Tryptophan Synthase / Q000187:drug effects* / Q000378:metabolism
chemical_list: D000438:Alcohols; D000440:Butanols; D007211:Indoles; D000431:Ethanol; D012694:Serine; C030374:indole; D014364:Tryptophan; D020001:1-Butanol; D000433:1-Propanol; D014367:Tryptophan Synthase