Simultaneous monitoring of light-induced changes in protein side-group protonation, chromophore isomerization, and backbone motion of bacteriorhodopsin by time-resolved Fourier-transform infrared spectroscopy.

Abstract

Absorbance changes in the infrared and visible spectral range were measured in parallel during the photocycle of light-adapted bacteriorhodopsin, which is accompanied by a vectorial proton transfer. A global fit analysis yielded the same rate constants for the chromophore reactions, for protonation changes of protein side groups, and for the backbone motion. From this result we conclude that all reactions in various parts of the protein are synchronized to each other and that no independent cycles exist for different parts. The carbonyl vibration of Asp-85, indicating its protonation, appears with the same rate constant as the Schiff base deprotonation. The carbonyl vibration of Asp-96 disappears, indicating most likely its deprotonation, with the same rate constant as for the Schiff base reprotonation. This result supports the proposed mechanism in which the protonated Schiff base, a deprotonated aspartic acid (Asp-85) on the proton-release pathway, and a protonated aspartic acid (Asp-96) on the proton-uptake pathway act as internal catalytic proton-binding sites.