Activation volumes of the calcium dependent para-nitrophenyl phosphate hydrolysis of the sarcoplasmic reticulum calcium transport enzyme.

Abstract

The effect of pressure on the calcium dependent hydrolysis of para-nitrophenyl phosphate by the calcium transport enzyme of the sarcoplasmic reticulum was studied under different conditions: temperature, solutes, substrate and ion concentrations. The calcium transport enzyme exhibits a large positive activation volume which does neither depend on the enzyme's inhibition by high salt concentrations nor its activation by ethylene glycol. The activation volume further proves to be pressure-independent but exhibits a pronounced negative temperature coefficient. The volume changes connected with the entrance of para-nitrophenyl phosphate, calcium or magnesium ions into the substrate ion complex are quite small, indicating that the transfer of water connected with the binding of these ligands is compensated by volume changes of the protein, accompanying the transition of the enzyme from its activated into its ground state.