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FORMATION AND DECAY KINETICS OF BACTERIORHODOPSIN'S N INTERMEDIATE: SOFTENING THE PROTEIN CONFORMATION WITH ALCOHOLS AFFECTS INTRA‐PIROTEIN PROTON TRANSFER AND RETINAL ISIOMERIZATION
Author(s) -
Fukuda Kazuya,
Kouyama Tsutomu
Publication year - 1992
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1992.tb09729.x
Subject(s) - bacteriorhodopsin , chemistry , ethanol , alcohol , retinal , isomerization , methanol , proton , kinetics , photochemistry , hydrogen bond , reaction rate constant , crystallography , medicinal chemistry , stereochemistry , membrane , organic chemistry , molecule , biochemistry , catalysis , physics , quantum mechanics
— The trans photocycle of bacteriorhodopsin was investigated in the presence of organic solvents with a hydrogen‐bonding group; i.e . methanol, ethanol, 1‐propanol and so on. These alcohols scarcely or only slightly affected the L→M and Ob̊ 570 transitions, but they perturbed the M→N and N→O transitions greatly. The rate of the M→N transition increased linearly with increasing alcohol concentration and, at maximal alcohol concentrations under which the native protein conformation was retained, the M→N transition was accelerated by a factor of ∼5. This alcohol effect was reversible. It is suggested that a long‐distance proton transfer involved in the M→N transition (Asp.96→retinal) becomes easier when the protein conformation is softened bv partially breaking hydrogen‐bonding networks in the protein. Another significant effect of alochol is inhibition of the N→O transition at weakly acidic pH, which was slowed down maximally by a factor of ˜10. This alcohol effect was less significant at alkaline pH, where reprotonation of Asp‐96 from the cytoplasmic membrane surface is a rate‐limiting reaction. It is suggested that, at acidic pH, thi: cis‐to‐trans isomerization involved in the N→O transition is a rate‐limiting reaction and that this step is inhibited in the presence of a high concentration of alcohols.