title: Binding of Ca2+ to the (Ca(2+)-Mg2+)-ATPase of sarcoplasmic reticulum: kinetic studies.
abstract: Stop-flow fluorescence and rapid-filtration methods have been used to establish the kinetics of Ca2+ binding to, and dissociation from, the (Ca(2+)-Mg2+)-ATPase of skeletal-muscle sarcoplasmic reticulum and to define the effects of H+ and Mg2+ on Ca2+ binding and dissociation rates. The kinetics have been interpreted in terms of the scheme: E2 E2<==>E1<==>E1Ca<==>E1'Ca<==>E1'Ca2. The kinetics of the E2<==>E1 E1 transition have been determined by measuring the rate of change of the fluorescence of the ATPase labelled with 4-nitrobenzo-2-oxa-1,3-diazole after a pH jump or the addition of Ca2+ to the labelled ATPase in the presence of thapsigargin or thapsivillosin A. It has been shown that Mg2+ has a marked effect on Ca2+ dissociation at pH 7.2 and that changes in the tryptophan fluorescence of the ATPase follow the same time course as the dissociation of 45Ca2+. It is proposed that the effect of Mg2+ follows from binding to a 'gating' site, as detected by changes in the fluorescence of the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin. The rate of dissociation of Ca2+ from the ATPase increases with increasing pH. The rate of dissociation of Ca2+ decreases with increasing Ca2+ concentration in the medium, with an apparent affinity for Ca2+ greater than that seen for the change in fluorescence amplitude. It is shown that this follows if the first, inner, Ca(2+)-binding site on the ATPase has a lower affinity for Ca2+ than the second, outer, site. Effects of H+ and Mg2+ on Ca2+ dissociation can be treated by the quasiequilibrium approach. Mg2+ and H+ also affect the rate of Ca2+ binding to the ATPase, and effects of H+ and Mg2+ on the E2<==>E1 equilibrium explain the results of experiments in which the concentrations of H+ and Mg2+ are jumped.
authors.affiliation: Department of Biochemistry, University of Southampton, Hants., U.K.
authors.forename: I M
authors.initials: IM
authors.lastname: Henderson
authors.forename: A P
authors.initials: AP
authors.lastname: Starling
authors.forename: M
authors.initials: M
authors.lastname: Wictome
authors.forename: J M
authors.initials: JM
authors.lastname: East
authors.forename: A G
authors.initials: AG
authors.lastname: Lee
mesh_terms: D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009132:Muscles / Q000201:enzymology; D010069:Oxadiazoles / Q000737:chemistry; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013379:Substrate Specificity; D014364:Tryptophan / Q000737:chemistry
chemical_list: D005456:Fluorescent Dyes; D010069:Oxadiazoles; C011514:4-nitrobenzofurazan; D014364:Tryptophan; D017301:Ca(2+) Mg(2+)-ATPase; D008274:Magnesium; D002118:Calcium