Temperature dependence of the barley root plasma membrane-bound Ca2+- and Mg2+-dependent ATPase [barley root ATPase, lipid-protein interactions, Ca2+-lipid interactions]
1981
Caldwell, C.R. | Haug, A. (Michigan State Univ. (USA). MSU-DOE Plant Research Lab.)
Arrhenius plots of the maximal velocities for the Ca('2+)- and Mg('2+)-dependent ATPase activities found in a plasma membrane-rich microsome fraction isolated from the roots of barley (Hordeum vulgare L. cv. Conquest) were nonlinear. Arrhenius plot analyses using a relation which produced curvilinear Arrhenius plots accurately fit the data and allowed the calculation of the activation enthalpies and molar heat capacities of activation. The temperature dependence of the computed K(, m) values for the Ca('2+)- and Mg('2+)-dependent ATPase activities was complex, with the highest enzyme-substrate affinities being obtained near the barley seedling growth temperature (16 deg C). Using electron paramagnetic resonance spectroscopy with amphiphilic cationic and anionic spin probes, it was possible to demonstrate that temperature changes and increasing Ca('2+) concentrations could alter the mobility of the membrane lipid polar head groups. Inhibition of the ATPase activities by high levels of Ca('2+) may result from a Ca('2+)-induced reduction in the lipid polar head group mobility. The possible role of lipid polar head group-protein interactions in the complex temperature dependence of the barley root ATPase kinetic constants is discussed.
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