Reversible loss of cooperative calcium ion binding by sarcoplasmic reticulum calcium adenosinetriphosphatase.
Incubation of rabbit skeletal muscle sarcoplasmic reticulum vesicles in solutions of very low [Ca2+] caused Ca2+ to bind noncooperatively, as determined by the dependence of the intrinsic tryptophan fluorescence intensity on added increments of Ca2+. Cooperative Ca2+ binding was obtained if the ATPase was incubated in [Ca2+] high enough (25 microM) to saturate the two high-affinity Ca2+ binding sites and then titrated with [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. The cooperative binding had an apparent association constant of 6.3 X 10(6) M-1 and a Hill coefficient of 2.6; these constants for the noncooperative binding case were 5.0 X 10(5) M-1 and 1.2, respectively. The transitions from the noncooperative to the cooperative Ca2+ binding forms of the enzyme were slow compared to the time required for Ca2+ binding to reach equilibrium. Thus, it appears that sarcoplasmic reticulum CaATPase is a hysteretic enzyme. Intrinsic association constants for Ca2+ binding and equilibrium constants for the transitions between the two forms in low and high [Ca2+] were estimated from analyses of a general scheme for cooperative and noncooperative binding.
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D008956:Models, Chemical; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence
D010070:Oxalates; D000001:Calcimycin; D004533:Egtazic Acid; D019815:Oxalic Acid; D000252:Calcium-Transporting ATPases; D002118:Calcium