Activation and binding volumes of the sarcoplasmic reticulum transport enzyme activated by calcium or strontium

Effects of high hydrostatic pressures on living cells: a consequence of the properties of macromolecules and macromolecule-associated water

Sixty percent of the Earth's biomass is found in the sea, at depths greater than 1000 m, i.e., at hydrostatic pressures higher than 100 atm. Still more surprising is the fact that living cells can reversibly withstand pressure shifts of 1000 atm. One explanation lies in the properties of cellular water. Water forms a very thin film around macromolecules, with a heterogeneous structure that is an image of the heterogeneity of the macromolecular surface. The density of water in contact with macromolecules reflects the physical properties of their different domains. Therefore, any macromolecular shape variations involving the reorganization of water and concomitant density changes are sensitive to pressure (Le Chatelier's principle). Most of the pressure-induced changes to macromolecules are reversible up to 2000 atm. Both the effects of pressure shifts on living cells and the characteristics of pressure-adapted species are opening new perspectives on fundamental problems such as regulation and adaptation.

Volume changes in high-affinity calcium binding of the sarcoplasmic reticulum calcium-transport enzyme

The effect which hydrostatic pressure exerts on the hydrolysis of dinitrophenyl phosphate and nitrophenyl phosphate by the sarcoplasmic reticulum calcium-transport enzyme was determined. Activation volumes for substrate hydrolysis at saturating and non-saturating concentrations of calcium were determined and used to evaluate volume increments for initial calcium binding. A reaction scheme in which two unidirectional substrate-driven reactions transfer high-affinity into low-affinity calcium-binding sites was applied to determine binding-volume increments. It has been inferred from the pressure dependence of the volume-generating function, defined as the difference between the reciprocal reaction rates of the saturated and the unsaturated enzyme, that calcium binding proceeds in two steps. The two associated binding constants are endowed with large binding-volume increments of opposite signs (+84 to +207 ml/mol and -3 to -136 ml/mol). Under different experimental conditions, with respect to the temperature, degree of calcium saturation and absence or presence of Me2SO, they add up to the same integral volume increment of 73 +/- 3.5 ml/mol for the entry of two calcium ions into the reaction cycle. In aqueous media, the two binding constants contribute about equally to binding and to the observed binding-volume increment. The presence of Me2SO strongly favours the first binding step. The size of the integral volume increment is in line with that determined for the interaction of calcium with calmodulin [Kupke, D.W. & Dorrier, T.E. (1986) Biochem. Biophys. Res. Commun. 38, 199-204].