Calmodulin X (Ca2+)4 is the active calmodulin-calcium species activating the calcium-, calmodulin-dependent protein kinase of cardiac sarcoplasmic reticulum in the regulation of the calcium pump.
Calcium-, calmodulin-dependent phosphorylation of cardiac sarcoplasmic reticulum increases the rate of calcium transport. The complex dependence of calmodulin-dependent phosphoester formation on free calcium and total calmodulin concentrations can be satisfactorily explained by assuming that CaM X (Ca2+)4 is the sole calmodulin-calcium species which activates the calcium-, calmodulin-dependent, membrane-bound protein kinase. The apparent dissociation constant of the E X CaM X (Ca2+)4 complex determined from the calcium dependence of calmodulin-dependent phosphoester formation over a 100-fold range of total calmodulin concentrations (0.01-1 microM) was 0.9 nM; the respective apparent dissociation constant at 0.8 mM free calcium, 1 mM free magnesium with low calmodulin concentrations (0.1-50 nM) was 2.60 nM. These results are in good agreement with the apparent dissociation constant of 2.54 nM of high affinity calmodulin binding determined by 125I-labelled calmodulin binding to sarcoplasmic reticulum fractions at 1 mM free calcium, 1 mM free magnesium and total calmodulin concentration ranging from 0.1 to 150 nM, i.e. conditions where approximately 98% of the total calmodulin is present as CaM X (Ca2+)4. The apparent dissociation constant of the calcium-free calmodulin-enzyme complex (E X CaM) is at least 100-fold greater than the apparent dissociation constant of the E X CaM X (Ca2+)4 complex, as judged from non-saturation 125I-labelled calmodulin binding at total calmodulin concentrations of up to 150 nM, in the absence of calcium.
D000818:Animals; D001921:Brain; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000302:isolation & purification / Q000494:pharmacology*; D002417:Cattle; D004285:Dogs; D007457:Iodine Radioisotopes; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008954:Models, Biological; D009206:Myocardium / Q000201:enzymology*; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D002147:Calmodulin; D007457:Iodine Radioisotopes; D010761:Phosphorus Radioisotopes; D011494:Protein Kinases; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium