Pressure effects on the dark-adaptation of bacteriorhodopsin.

The influence of hydrostatic pressure (340 MPa) on the dark-adaptation kinetics and the relaxation of dark-adapted bacterio-rhodopsin following a pressure jump (0.1 MPa --> 340 MPa) have been studied. We have also measured the temperature dependence of the equilibrium isomeric ratio of all-trans and 13-cis retinal in dark-adapted bacteriorhodopsin at 340 MPa. The results show that hydrostatic pressure affects both the dark-adaptation rate and the dark equilibrium isomeric ratio. With increasing pressure, the fraction of all-trans isomers decreases. The kinetics have been analyzed with a two-state model. The description of the pressure dependence using transition state theory is inappropriate for two reasons; (a) pressure changes the viscosity of the protein and its environment, and (b) pressure changes the population of conformational substates within either isomeric form of bacteriorhodopsin. The temperature independent ratio of all-trans and 13-cis isomers indicates that the all-trans and 13-cis conformations have the same conformational volume.