Influence of an electrical potential on the charge transfer kinetics of bacteriorhodopsin

On the kinetics of voltage formation in purple membranes of Halobacterium salinarium

The kinetics of the bacteriorhodopsin photocycle, measured by voltage changes in a closed membrane system using the direct electrometrical method (DEM) of Drachev, L.A., Jasaitus, A.A., Kaulen, A.D., Kondrashin, A.A., Liberman, E.A., Nemecek, I.B., Ostroumov, S.A., Semenov, Yu, A. & Skulachev, V.P. (1974) Nature 249, 321-324 are sixfold slower than the kinetics obtained in optical studies with suspensions of purple membrane patches. In this study, we have investigated the reasons for this discrepancy. In the presence of the uncouplers carbonyl cyanide m-chlorophenylhydrazone or valinomycin, the rates in the DEM system are similar to the rates in suspensions of purple membrane. Two alternative explanations for the effects of uncouplers were evaluated: (a) the 'back-pressure' of the Deltamicro;H+ slows the kinetic steps leading to its formation, and (b) the apparent difference between the two systems is due to slow major electrogenic events that produce little or no change in optical absorbance. In the latter case, the uncouplers would decrease the RC time constant for membrane capacitance leading to a quicker discharge of voltage and concomitant decrease in photocycle turnover time. The experimental results show that the primary cause for the slower kinetics of voltage changes in the DEM system is thermodynamic back-pressure as described by Westerhoff, H.V. & Dancshazy, Z. (1984) Trends Biochem. Sci. 9, 112-117.

Electrogenic processes and protein conformational changes accompanying the bacteriorhodopsin photocycle

The possible mechanisms of electrogenic processes accompanying proton transport in bacteriorhodopsin are discussed on the basis of recent structural data of the protein. Apparent inconsistencies between experimental data and their interpretation are considered. Special emphasis is placed on the protein conformational changes accompanying the reprotonation of chromophore and proton uptake stage in the bacteriorhodopsin photocycle.

Evidence for the first phase of the reprotonation switch of bacteriorhodopsin from time-resolved photovoltage and flash photolysis experiments on the photoreversal of the M-intermediate

The kinetics of the photoreversal reaction of the M-intermediate of bacteriorhodopsin (bR) was investigated by time-resolved optical absorption spectroscopy and photovoltage measurements using double-flash excitation (a green flash (532 nm) followed by a blue flash (400 nm) after a variable delay). The sign of the photovoltage and the 1H/2H kinetic isotope effect indicate that the Schiff base is reprotonated by a group between the Schiff base and the extracellular surface, probably Asp85. Analysis of the kinetic data shows that the charge movement in 150 mM KCl at 12 degrees C is characterized by two components with time constants of approximately 100 ns and approximately 600 ns, respectively, which are independent of the delay time between the flashes and the pH. The amplitudes of the fast and slow components depend on the delay and the pH. The slower component starts to contribute to the charge movement only after delays longer than 100 micros, is absent at low pH, and increases in amplitude with a pKa of approximately 6. Because the proton release group deprotonates after 70-100 micros and has a transient pKa of 5.8, these results suggest the following assignment: the fast and the combination of fast and slow components represent photoreversal from two M states, with the release group protonated and deprotonated, respectively. The slow phase of the photoreversal starts from a state with the release group deprotonated, and with the pK of Asp85 elevated, and is probably due to the restoration of the pK of Asp85 to its initial low value. This provides further evidence for coupling between the pK's of Asp85 and the release group and suggests that proton release is the first step in the reprotonation switch. At alkaline pH the amplitude of the electrical signal from the back photoreaction decreases with an apparent pK of 8, without a corresponding decrease in the amount of M. At neutral pH the movement of the positively charged guanidinium group of Arg82 from a position near the release group on the surface to Asp85 makes a substantial contribution to the electrical photoreversal amplitude. Above the pK of the release group in the unphotolysed state (approximately 8), Arg82 stays near the surface, leading to a corresponding signal reduction.