Hydrostatic pressure and ionic strength effects on the kinetics of lysozyme

The kinetics of the reaction catalyzed by egg-white lysozyme from hen eggs (EC 3.2.1.17) was investigated by measurement of the decrease in turbidity of suspensions of dried Micrococcus luteus cells. The substrate and NaCl concentration were varied, as well as the hydrostatic pressure (1-476 atm). A plot of the initial velocity against the reciprocal of the substrate concentration gave straight lines. From these plots values for the maximum velocity, V, and the apparent Michaelis constant, K(m), as well as the effect of NaCl concentration were determined. The maximum velocity showed a pronounced maximum when plotted over a range of NaCl concentration from 0-0.208 M. The volume change of activation, DeltaV([unk]), calculated from V, was negative, and decreased in absolute value from -8 to about 0 cm(3)/mol as the NaCl molarity was increased over the above range. This effect is explained as due to a different number of salt molecules being built into activated complexes with each of these activated complexes reacting at its own characteristic rate. Strong pressure-dependence of enzyme catalysis indicates appreciable conformational changes of the enzyme as it is incorporated into the activated complex. The volume change causing the pressure effect arises principally from changes in the number of ionized species with the accompanying electrostriction.