Pressure dependence of the photocycle kinetics of bacteriorhodopsin

The pressure dependence of the photocycle kinetics of bacteriorhodopsin from <i>Halobacterium salinarium</i> was investigated at pressures up to 4 kbar at 25°C and 40°C. The kinetics can be adequately modeled by nine apparent rate constants, which are assigned to irreversible transitions of a single relaxation chain of nine kinetically distinguishable states <i>P</i>1 to <i>P</i>9. All states except <i>P</i>1 and <i>P</i>9 consist of two or more spectral components. The kinetic states <i>P</i>2 to <i>P</i>6 comprise only the two fast equilibrating spectral states L and M. From the pressure dependence, the volume differences ΔV0LM between these two spectral states could be determined that range from ΔV0LM = -11.4 ± 0.7 ml/mol (<i>P</i>2) to ΔV0LM = 14.6 ± 2.8 ml/mol (<i>P</i>6). A model is developed that explains the dependence of ΔV0LM on the kinetic state by the electrostriction effect of charges, which are formed and neutralized during the L/M transition