Pressure dependence of the photocycle kinetics of bacteriorhodopsin

The pressure dependence of the photocycle kinetics of bacteriorhodopsin from Halobacterium salinarium was investigated at pressures up to 4 kbar at 25degreesC and 40degreesC. 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 P-1 to P-9 All states except P-1 and P-9 consist of two or more spectral components. The kinetic states P-2 to P-6 comprise only the two fast equilibrating spectral states L and M. From the pressure dependence, the volume differences DeltaVdegrees(LM),m between these two spectral states could be determined that range from DeltaVdegrees(LM) = -11.4 +/- 0.7 ml/mol (P-2) to DeltaVdegrees(LM) = 14.6 +/- 2.8 mL/mol (P-6). A model is developed that explains the dependence of DeltaVdegrees(LM) on the kinetic state by the electrostriction effect of charges, which are formed and neutralized during the L/M transition.