Volume and enthalpy changes in the early steps of bacteriorhodopsin photocycle studied by time-resolved photoacoustics

Abstract

We have studied the photoinduced volume changes, energetics, and kinetics in the early steps of the bacteriorhodopsin (BR) photocycle with pulsed, time-resolved photoacoustics. Our data show that there are two volume changes. The fast volume change ( < or = 200 ns) is an expansion (2.5 +/- 0.3 A3/molecule) and is observed exclusively in the purple membrane (PM), vanishing in the 3-[(3-cholamidopropyl)-dimethylammonio] -1-propane-sulfonate-sulfonate-solubilized BR sample; the slow change (approximately 1 micros) is a volume contraction (-3.7 +/- 0.3 A3/molecule). The fast expansion is assigned to the restructuring of the aggregated BR in the PM, and the 1-micros contraction to the change in hydrogen bonding of water at Asp 212 (Kandori et al. 1995. J. Am. Chem. Soc. 117:2118-2119). The formation of the K intermediate releases most of the absorbed energy as heat, with delta Hk = -36 +/- 8 kJ/mol. The activation energy of the K --> L step is 49 +/- 6 kJ/mol, but the enthalpy change is small, -4 +/- 10 kJ/mol. On the time scale we studied, the primary photochemical kinetics, enthalpy, and volume changes are not affected by substituting the solvent D2O for H2O. Comparing data on monomeric and aggregated BR, we conclude that the functional unit for the photocycle is the BR monomer, because both the kinetics (rate constant and activation energy) and the enthalpy changes are independent of its aggregation state.