title: Monovalent cations affect dynamic and functional properties of the tryptophan synthase alpha 2 beta 2 complex.
abstract: Monovalent cations affect both conformational and catalytic properties of the tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium. Their influence on the dynamic properties of the enzyme was probed by monitoring the phosphorescence decay of the unique Trp-177 beta, a residue located near the beta-active site, at the interface between alpha- and beta-subunits. In the presence of either Li+, Na+, Cs+, or NH4+, the phosphorescence decay is biphasic and the average lifetime increases indicating a decrease in the flexibility of the N-terminal domain of the beta-subunit. Since amplitudes but not lifetimes are affected, cations appear to shift the equilibrium between preexisting enzyme conformations. The effect on the reaction between indole and L-serine was studied by steady state kinetic methods at room temperature. We found that cations: (i) bind to the L-serine--enzyme derivatives with an apparent dissociation constant, measured as the concentration of cation corresponding to one-half of the maximal activity, that is in the millimolar range and decreases with ion size; (ii) increase kcat with the order of efficacy Cs+ > K+ > Li+ > Na+; (iii) decrease KM for indole, Na+ being the most effective and causing a 30-fold decrease; and (iv) cause an increase of the kcat/KM ratio by 20-40-fold. The influence on the equilibrium distribution between the external aldimine and the alpha-aminoacrylate, intermediates in the reaction of L-serine with the beta-subunits of the enzyme, was found to be cation-specific.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Istituto di Scienze Biochimiche, Università di Parma, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Peracchi
authors.forename: A
authors.initials: A
authors.lastname: Mozzarelli
authors.forename: G L
authors.initials: GL
authors.lastname: Rossi
mesh_terms: D000641:Ammonia / Q000494:pharmacology; D002414:Cations, Monovalent / Q000494:pharmacology*; D002586:Cesium / Q000494:pharmacology; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D008163:Luminescent Measurements; D046911:Macromolecular Substances; D011188:Potassium / Q000494:pharmacology; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism; D012413:Rubidium / Q000494:pharmacology; D012486:Salmonella typhimurium / Q000201:enzymology*; D012964:Sodium / Q000494:pharmacology; D013053:Spectrophotometry; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002414:Cations, Monovalent; D046911:Macromolecular Substances; D011994:Recombinant Proteins; D002586:Cesium; D000641:Ammonia; D008094:Lithium; D012964:Sodium; D014367:Tryptophan Synthase; D012413:Rubidium; D011188:Potassium