title: Monovalent metal ions play an essential role in catalysis and intersubunit communication in the tryptophan synthase bienzyme complex.
abstract: This investigation shows that the alpha 2 beta 2 tryptophan synthase bienzyme complex from Salmonella typhimurium is subject to monovalent metal ion activation. The effects of the monovalent metal ions Na+ and K+ were investigated using rapid scanning stopped-flow (RSSF), single-wavelength stopped-flow (SWSF), and steady-state techniques. RSSF measurements of individual steps in the reaction of L-serine and indole to give L-trytophan (the beta-reaction) as well as the reaction of 3-indole-D-glycerol 3'-phosphate (IGP) with L-serine (the alpha beta-reaction) demonstrate that monovalent metal ions such as Na+ and K+ change the distribution of intermediates in both the transient and steady states. Therefore the metal ion effect alters relative ground-state energies and the relative positions of ground- and transition-state energies. The RSSF spectra and SWSF time courses show that the turnover of indole is significantly reduced in the absence of either Na+ or K+. The alpha-aminoacrylate Schiff base species, E(A-A), is in a less active state in the absence of monovalent metal ions. Na+ decreases the steady-state rate of IGP cleavage (the alpha-reaction) to about 30% of the value obtained in the absence of metal ions. Steady-state investigations show that in the absence of monovalent metal ions the alpha- and alpha beta-reactions have the same activity. Na+ binding gives a 30-fold stimulation of the alpha-reaction when the beta-site is in the E(A-A) form.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, University of California at Riverside 92521-0129, USA.
authors.forename: E U
authors.initials: EU
authors.lastname: Woehl
authors.forename: M F
authors.initials: MF
authors.lastname: Dunn
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D002414:Cations, Monovalent / Q000494:pharmacology*; D005994:Glycerophosphates / Q000378:metabolism; D007700:Kinetics; D046911:Macromolecular Substances; D008433:Mathematics; D008962:Models, Theoretical; D011188:Potassium / Q000494:pharmacology*; D012486:Salmonella typhimurium / Q000201:enzymology*; D012694:Serine; D012964:Sodium / Q000494:pharmacology*; D013053:Spectrophotometry; D013997:Time Factors; D014364:Tryptophan / Q000378:metabolism; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002414:Cations, Monovalent; D005994:Glycerophosphates; D046911:Macromolecular Substances; C008117:indoleglycerol phosphate; D012694:Serine; D014364:Tryptophan; D012964:Sodium; D014367:Tryptophan Synthase; D011188:Potassium