title: Monovalent cations partially repair a conformational defect in a mutant tryptophan synthase alpha 2 beta 2 complex (beta-E109A).
abstract: We are using the tryptophan synthase alpha 2 beta 2 complex as a model system to investigate how ligands, protein-protein interaction, and mutations regulate enzyme activity, reaction specificity, and substrate specificity. The rate of conversion of L-serine and indole to L-tryptophan by the beta 2 subunit alone is quite low, but is activated by certain monovalent cations or by association with alpha subunit to form an alpha 2 beta 2 complex. Since monovalent cations and alpha subunit appear to stabilize an active conformation of the beta 2 subunit, we have investigated the effects of monovalent cations on the activities and spectroscopic properties of a mutant form of alpha 2 beta 2 complex having beta 2 subunit glutamic acid 109 replaced by alanine (E109A). The E109A alpha 2 beta 2 complex is inactive in reactions with L-serine but active in reactions with beta-chloro-L-alanine. Parallel experiments show effects of monovalent cations on the properties of wild type beta 2 subunit and alpha 2 beta 2 complex. We find that CsCl stimulates the activity of the E109A alpha 2 beta 2 complex and of wild type beta 2 subunit with L-serine and indole and alters the equilibrium distribution of L-serine reaction intermediates. The results indicate that CsCl partially repairs the deleterious effects of the E109A mutation on the activity of the alpha 2 beta 2 complex by stabilizing a conformation with catalytic properties more similar to those of the wild type alpha 2 beta 2 complex. This conclusion is consistent with observations that monovalent cations alter the catalytic and spectroscopic properties of several pyridoxal phosphate-dependent enzymes by stabilizing alternative conformations.
authors.affiliation: Laboratory of Biochemical Pharmacology, NIDDK, National Institutes of Health, Bethesda, Maryland 20892-0830, USA.
authors.forename: S B
authors.initials: SB
authors.lastname: Ruvinov
authors.forename: S A
authors.initials: SA
authors.lastname: Ahmed
authors.forename: P
authors.initials: P
authors.lastname: McPhie
authors.forename: E W
authors.initials: EW
authors.lastname: Miles
mesh_terms: D001665:Binding Sites; D002586:Cesium / Q000494:pharmacology*; D003461:Crystallography; D008094:Lithium / Q000494:pharmacology; D009154:Mutation; D011487:Protein Conformation; D011732:Pyridoxal Phosphate / Q000737:chemistry; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D013053:Spectrophotometry; D013329:Structure-Activity Relationship; D014367:Tryptophan Synthase / Q000737:chemistry* / Q000502:physiology
chemical_list: D000644:Quaternary Ammonium Compounds; D002586:Cesium; D011732:Pyridoxal Phosphate; D008094:Lithium; D014367:Tryptophan Synthase