Bacteriorhodopsin's Intramolecular Proton-Release Pathway Consists of a Hydrogen-Bonded Network

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Abstract
In its proton-pumping photocycle, bacteriorhodopsin releases a proton to the extracellular surface at pH 7 in the transition from intermediate L to intermediate M. The proton-release group, named XH, was assigned in low-temperature FT-IR studies to a single residue, E204 [Brown, L. S., Sasaki, J., Kandori, H., Maeda, A., Needleman, R., and Lanyi, J. K. (1995) J. Biol. Chem.270, 27122−27126]. The time-resolved room-temperature step-scan FT-IR photocycle studies on wild-type and E204Q-, and E204D-mutated bacteriorhodopsin, which we present here, show in contrast that the FT-IR data give no evidence for deprotonation of E204 in the L-to-M transition. Therefore, it is unlikely that E204 represents XH. On the other hand, IR continuum absorbance changes indicate intramolecular proton transfer via an H-bonded network to the surface of the protein. It appears that this H-bonded network is spanned between the Schiff base and the protein surface. The network consists at least partly of internally bound water molecules and is stabilized by E204 and R82. Other not yet identified groups may also contribute. At pH 5, the intramolecular proton transfer to the surface of the protein seems not to be disturbed. The proton seems to be buffered at the surface and later in the photocycle released into the bulk during BR recovery. Intramolecular proton transfer via a complex H-bonded network is proposed to be a general feature of proton transfer in proteins.