Correlation of photochemical cycle, H⁺ release and uptake, and electric events in bacteriorhodopsin

Abstract

The kinetics of 3 photoinduced responses of bacteriorhodopsin have been compared: spectral changes, pH shifts in a suspension of open purple membrane sheets and electric potential generation by the sheets incorporated into a lipid‐impregnated collodion film. In the presence of a pH‐buffer, the H + release by bacteriorhodops in was shown to correlate with the formation of the M412 intermediate and the microsecond phase of the potential generation. The H + /M412 ratio is equal to 0.7±0.1 if the ionic strength of the solution is high. In the absence of the buffer, the H + release proved to be much slower than spectral and electric responses. The kinetics of H + uptake by bacteriorhodopsin is close to M412 decay and to the electrogeneous millisecond phase in both the presence and absence of the pH buffer. The bacteriorhodopsin‐induced proton release phase accounts for about 20%, and the uptake phase for about 80% of the overall potential. This is compatible with the model assuming that the proton start‐out point ‐ possibly, the protonated Schiff base connecting lysine 216 with retinal ‐ is closer to the outer rather than the inner (cytoplasmic) surface of the bacterial membrane.