Kinetic isotope effects reveal an ice-like and a liquid-phase-type intramolecular proton transfer in bacteriorhodopsin

The mechanism of the intramolecular proton transfer in the membrane protein bacteriorhodopsin (bR) is studied. The kinetic isotope effects after H/D exchange were determined for the individual photocycle reactions and used as an indicator. Significant differences in the kinetic isotope effects are observed between the intramolecular proton transfer on the release and the uptake pathways. The results suggest a fast intramolecular proton transfer mechanism in the proton release pathway, which is similar to the one proposed for ice, where the rate limiting step is the proton movement within the H bond. However, the reactions in the intramolecular proton uptake pathway occur in a mechanism similar to the one suggested for liquid water, where the rate limiting step is given by a rotational rearrangement of H bonded network groups. We propose that the experimental evidence for a proton wire mechanism given here for bacteriorhodopsin is of general relevance also for other proton transporting proteins.