title: Conformational transitions in the Ca2+ + Mg2+-activated ATPase and the binding of Ca2+ ions.
abstract: We have studied the fluorescence of the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum labelled with fluorescein isothiocyanate. The change in intensity of fluorescein fluorescence caused by addition of Ca2+ to the labelled ATPase can be interpreted in terms of a two-conformation model for the ATPase, one conformation (E1) having a high affinity for Ca2+, the other (E2) a low affinity. Effects of Ca2+ as a function of pH allow an estimate of the effect of pH on the E1/E2 ratio, consistent with kinetic studies. A model is presented for binding of Ca2+ to the ATPase as a function of pH that is consistent both with the data on the E1/E2 equilibrium and with literature data on Ca2+ binding.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins; D007700:Kinetics; D008954:Models, Biological; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects; D011522:Protons; D013050:Spectrometry, Fluorescence; D013861:Thiocyanates; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
chemical_list: D005452:Fluoresceins; D011522:Protons; D013861:Thiocyanates; D014639:Vanadium; D014638:Vanadates; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D016650:Fluorescein-5-isothiocyanate; D002118:Calcium