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 bacteriorhodopsin followi ng a pressure jump (0.1 MPa --> 340 MPa) have been studied. We have al so measured the temperature dependence of the equilibrium isomeric rat io of all trans and 13-cis retinal in dark adapted bacteriorhodopsin a t 340 MPa. The results show that hydrostatic pressure affects both the dark-adaptation rate and the dark equilibrium isomeric ratio. With in creasing pressure, the fraction of all-trans isomers decreases. The ki netics have been analyzed with a two state model. The description of t he 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 conformat ional substates within either isomeric form of bacteriorhodopsin. The temperature independent ratio of all-trans and 13-cis isomers indicate s that the all trans and 13-cis conformations have the same conformati onal volume.