Efficient Approach to Determine the pKa of the Proton Release Complex in the Photocycle of Retinal Proteins

This work utilizes a photoelectrochemical approach to study the pH dependence of proton release and uptake in the photocycles of two retinal proteins, bacteriorhodopsin (BR) and archaerhodopsin 4 (AR4). By detecting photoinduced potentials that originate from the proton concentration changes (Delta H+) generated by proteins near the indium tin oxide (ITO) electrode, we show that the kinetics of release and uptake can be followed in a broad pH range, and the pK(a) of the proton release complex (PRC) can be easily determined under different conditions. Nonoriented protein films were deposited on the electrode, and photovoltage in an electrochemical cell was detected after illumination with a green flash. The kinetics of proton release and uptake could be measured as light-induced decreases and increases of the photopotential. A kinetic analysis was performed, and a formula describing proton fluxes of wild-type BR and AR4 and D96N mutant of BR was derived. Three components-fast proton release, slow proton release, and proton uptake-were found in the wild-type retinal proteins; two components, fast and slow proton releases, were found in the D96N mutant. The pH dependence of the fraction of fast release over the whole release was used to determine the pKa for proton release in the photocycles of these retinal proteins. Measurements were also performed in conventional buffer solutions and crown ether. The presence of buffer in 10-50 mM concentration did not abolish the light-induced signals, indicating that the electrode response is much less sensitive to buffers than pH-sensitive dyes in a suspension due to a higher protein/buffer ratio near the electrode. This feature enables us to study effects of chemicals with high buffer capacity, and significant effects of buffers and crown ether on proton pumping behaviors of retinal proteins were revealed. In comparison with the classic pH-sensitive dye approach, the photoelectrochemical approach is convenient and efficient for measurements of transient proton concentration changes (Delta H+) generated by a proton pump and thus might be utilized as a powerful tool for the investigation of light-driven proton pumping mechanisms in a wide pH range.