Strongly hydrogen-bonded water molecule is observed only in the alkaline form of proteorhodopsin

Proteorhodopsin (PR), an archaeal-type rhodopsin found in marine bacteria, functions as a light-driven proton pump. The proton-pumping activity of PR is highly pH-dependent, its exact mechanism being still controversial. The present FTIR spectra are very similar at pH 10 and 5 in the 1800-900 cm−1 region, but significantly different in the 2700-2000 cm−1 region. This implies that the structure and structural changes are almost identical between the alkaline and acid forms of PR except for water-containing hydrogen-bonding network. In addition, different hydrogen-bonding strength of internal water molecule may be correlated with the pH-dependent proton-pumping activity of PR.