Effects of K+ on the binding of Ca2+ to the Ca(2+)-ATPase of sarcoplasmic reticulum.
Equilibrium and kinetic fluorescence methods have been used to characterize the interactions between K+ and the Ca(2+)-ATPase of skeletal-muscle sarcoplasmic reticulum. K+ shifts the E2-E1 equilibrium of the ATPase towards E1 and increases the rate of Ca2+ binding to the ATPase, as detected by changes in tryptophan fluorescence intensity, suggesting that K+ increases the rate of the E2-E1 transition. The data are consistent with binding of K+ at the inner Ca(2+)-binding site on the ATPase in competition with H+ and Mg2+, with a higher affinity in the E1 than in the E2 conformation. K+ has no effect on the affinity for Mg2+, as detected by changes in tryptophan fluorescence intensity; since it has been proposed that the changes in tryptophan fluorescence follow from binding to Mg2+ at the outer Ca(2+)-binding site, this suggests that K+ is unable to bind at the outer Ca(2+)-binding site. K+ increases the rate of dissociation of Ca2+ from the Ca(2+)-bound ATPase and reduces the effect of Mg2+ on the fluorescence intensity of the ATPase labelled with 4-(bromomethyl)-6,7-dimethoxycoumarin. It is suggested that these effects of K+ are the result of binding at a 'gating' site on the ATPase, in competition with binding of H+. Binding of K+ at the inner Ca(2+)-binding site and at the gating site account for the observed effects of K+ on the affinity of the ATPase for Ca2+.
Lee
A G
AG
Department of Biochemistry, University of Southampton, Hants, U.K.
Baker
K
K
Khan
Y M
YM
East
J M
JM
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000187:drug effects* / Q000378:metabolism*; D003374:Coumarins; D005453:Fluorescence; D005456:Fluorescent Dyes; D007700:Kinetics; D008954:Models, Biological; D010069:Oxadiazoles; D011188:Potassium / Q000494:pharmacology*; D011487:Protein Conformation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects* / Q000201:enzymology*; D014364:Tryptophan
D003374:Coumarins; D005456:Fluorescent Dyes; D010069:Oxadiazoles; C011514:4-nitrobenzofurazan; C053769:4-bromomethyl-6,7-dimethoxycoumarin; D014364:Tryptophan; D000252:Calcium-Transporting ATPases; D011188:Potassium; D002118:Calcium