title: Binding of Ca2+ to the (Ca(2+)-Mg2+)-ATPase of sarcoplasmic reticulum: equilibrium studies.
abstract: Equilibrium fluorescence methods have been used to establish a model for Ca2+ binding to the (Ca(2+)-Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum and to define the effects of H+ and Mg2+ on Ca2+ binding. The basic scheme proposed is: E2 <--> E1 <--> E1Ca <--> El'Ca <--> E1'Ca2. The E1 conformation of the ATPase initially has one high-affinity binding site for Ca2+ exposed to the cytoplasmic side of the sarcoplasmic reticulum, but in the E2 conformation this site is unable to bind Ca2+; Ca2+ does not bind to luminal sites on E2. The second, outer, Ca(2+)-binding site on the ATPase is formed after binding of Ca2+ to the first, inner, site on E1 and the E1Ca <--> E1'Ca conformation change. The pH- and Mg(2+)-dependence of the E2 <--> E1 equilibrium has been established after changes in the fluorescence of the ATPase labelled with 4-nitrobenzo-2-oxa-1,3-diazole. It is proposed that Mg2+ from the cytoplasmic side of the sarcoplasmic reticulum can bind to the first Ca(2+)-binding site on both E1 and E2. It is proposed that the change in tryptophan fluorescence intensity after binding of Ca2+ follows from the E1Ca <--> E1'Ca change. The pH- and Mg(2+)-dependence of this change defines H(+)- and Mg(2+)-binding constants at the two Ca(2+)-binding sites. It is proposed that the change in tryptophan fluorescence observed on binding Mg2+ follows from binding at the second Ca(2+)-binding site. Effects of pH and Mg2+ on the fluorescence of the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin are proposed to follow from binding to a site on the ATPase, the 'gating' site, which affects the affinity of the first Ca(2+)-binding site for Ca2+ and affects the rate of dissociation of Ca2+ from the ATPase.
authors.affiliation: Department of Biochemistry, University of Southampton, Hants., U.K.
authors.forename: I M
authors.initials: IM
authors.lastname: Henderson
authors.forename: Y M
authors.initials: YM
authors.lastname: Khan
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 / Q000737:chemistry / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002722:Chlorobenzenes / Q000737:chemistry; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D009132:Muscles / Q000201:enzymology; D010069:Oxadiazoles / Q000737:chemistry; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D014364:Tryptophan / Q000737:chemistry
chemical_list: D002722:Chlorobenzenes; D005456:Fluorescent Dyes; D010069:Oxadiazoles; C011514:4-nitrobenzofurazan; D014364:Tryptophan; D017301:Ca(2+) Mg(2+)-ATPase; C004944:chlorfenethol; D002118:Calcium