The effect of monovalent and divalent cations on the ATP‐dependent Ca2+‐binding and phosphorylation during the reaction cycle of the sarcoplasmic reticulum Ca2+‐transport ATPase

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Abstract
The coupling of Ca2+ movements and phosphate fluxes as well as the time-dependent occurrence of sequential reaction intermediates in the forward mode of the Ca, Mg-dependent ATPase reaction have been investigated using leaky vesicles (A23187) in the presence of varying Ca2+, Mg2+, and K+ concentrations. The employed ATP concentration of 2 .mu.M does not allow more than one reaction cycle to occur. The respective fractions of ADP-sensitive and ADP-insensitive phosphoenzyme have been determined. The chosen experimental conditions (0-1.degree. C, pH 6.0, absence of solubilizers) allow a prolonged time of observation and exclude interfering alterations of coupling and binding parameters, respectively. It is shown that under the experimental conditions K+ interacts with at least four different reaction steps (phosphoenzyme formation, E1P .fwdarw. E2P transition, E2P hydrolysis, and E2 .fwdarw. E1 transformation). Mg2+ represents the sole ionic co-factor for the formation of the substrate MgATP if it is present in high concentrations (5 mM). Additional Ca2+ is bound to the substrate as well as to unspecific sites otherwise occupied by Mg2+ if Mg2+ is reduced to 0.1 mM. In this case the E1P .fwdarw. E2P transition rate (including Ca2+ translocation and Ca2+ release from low-affinity sites) is little diminished. If, in the absence of K+, both Mg2+ and Ca2+ are deficient E2P hydrolysis is vastly retarded. We find Ca2 release to occur time-coincidently with E1P formation and not concomitantly with the camparably slow appearance of E2P; the molar amount of Ca2+ released, however, rather agreed with that of E2P formed. This suggests that under the prevailing conditions of a high proton concentration, phosphoenzyme states containing occluded Ca2+ or Ca2+ bound to low-affinity sites are transitional and not detectable. Preliminary findings on this subject have been published by us and colleagues from this laboratory [Hasselbach, W., Agostini, B., Medda, P., Migala, A. and Waas, W. (1985) in The sarcoplasmic reticulum calcium pump: Early and recent developments critically overviewed (Fleischer, S. and Tonomura, Y., eds) pp. 19-49, Academic Press, Orlando].

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