title: Mechanistic origin of the kinetic cooperativity for the ATPase activity of sarcoplasmic reticulum.
abstract: The (Ca2+-Mg2+)-ATPase from sarcoplasmic reticulum presents negative cooperativity for the hydrolysis of Mg2+-ATP at different concentration ranges of this substrate. A kinetic model is proposed according to which Mg2+-ATP may bind to three different enzymatic species present during the catalytic cycle. E (K1 = 1 microM). E' approximately P.Ca2 (K9 = 500 microM) and EP (K7 = 20 microM), accelerating the release of Pi. The fact that each of these species has a different affinity for Mg2+-ATP allows a significant enhancement of the rate of Pi release to the medium at the different ranges of Mg2+-ATP concentration where the enzyme shows a kinetic cooperativity. The kinetic analysis of this mechanism yields an equation which is a ratio of two cubic polynomials (3:3 rate equations) with respect to Mg2+-ATP and which may explain the negative cooperativity of the enzyme at different concentration ranges of Mg2+-ATP.
mesh_terms: D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D007700:Kinetics; D008962:Models, Theoretical; D009132:Muscles / Q000201:enzymology; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases