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Sarcoplasmic Reticulum, Calcium-Dependent Dinitrophenyl Phosphate Hydrolysis, Activation Volume, Binding Volume
Abstract:
The effect of hydrostatic pressure on the calcium-dependent hydrolysis of dinitrophenyl phosphate by the sarcoplasmic calcium transport enzyme has been studied. The magnesium dinitrophenyl phosphate complex is the true substrate of the enzyme (K = 7000 M-1) by which it is hydrolyzed at 20 degrees C with a turnover rate of 4 s-1. Activation by calcium ions occurs between 0.1 and 1 microM as observed for ATP hydrolysis. The activation volume of the enzyme saturated with both ligands exhibits pronounced pressure-dependence, rising from 25 ml/mol at atmospheric pressure to 80 ml/mol at 100 MPa. The apparent binding volumes for magnesium dinitrophenyl phosphate and calcium are likewise pressure-dependent. The volume changes connected with the binding of magnesium dinitrophenyl phosphate is quite small approaching zero at 100 MPa. The apparent binding volume for calcium greatly increases with pressure from 35 ml/mol at atmospheric pressure to 150 ml/mol at 70 MPa. A nearly constant binding volume of approximately 40 ml/mol results if the effect of pressure on the respective rate constants that contribute to the apparent binding constant, is taken into account. The pressure-dependence of enzyme activity at subsaturating calcium concentrations yields an activation volume of 250 ml/mol related to the rate of calcium binding indicating the occurrence of a transient large volume expansion of the enzyme complex. The volume changes observed for the calcium-dependent interaction of the enzyme with magnesium dinitrophenyl phosphate well agree with that found for magnesium p-nitrophenyl phosphate (W. Hasselbach and L. Stephan,Z. Naturforsch. 42 c, 641-652 (1987)) indicating that the found volume changes are intrinsic properties of the transport enzyme, independent of the respective energy donor.
