Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
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 (K 1=1 µM), E′∼P.Ca2 (K 9=500 µM) and *EP (K 7=20 µM), 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.
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