The photochemical reaction cycle of proteorhodopsin at low pH

The proton acceptor group in the recently described retinal protein, proteorhodopsin has an unusually high pK(a) of 7.1. It was shown that at pH above this pK(a), illumination initiates a photocycle similar to that of bacteriorhodopsin, and the protein transports proton across the cell membrane. Recently it was reported that proteorhodopsin, unlike bacteriorhodopsin, transports protons at pH below the pK(a) of the proton acceptor, and this transport is in the reverse direction. We have investigated the photocycle of proteorhodopsin at such low pH. At pH 5, three spectrally distinct intermediates K, L, and N, and another spectrally silent one, PR', could be identified, but a deprotonated Schiff base containing M-like intermediate, characteristic for proton pumping activity, does not accumulate. All the reactions between the intermediates are close to equilibrium, except the last transition from PR' to PR, when the protein returns to its initial unexcited state in a quasiunidirectional reaction. The electric signal measurements indicate that although charge motions are detected inside the protein, their net dislocation is zero, indicating that contrary to the earlier reported, at low pH no charged particle is transported across the membrane.