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THE PURPLE MEMBRANE PROTON PUMP: A MECHANISTIC PROPOSAL FOR THE SOURCE OF THE SECOND PROTON *
Author(s) -
SELTZER STANLEY,
EHRENSON STANTON
Publication year - 1984
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.1751-1097.1984.tb03429.x
Subject(s) - bacteriorhodopsin , deprotonation , chromophore , chemistry , bathochromic shift , proton , excited state , protonation , photochemistry , photoexcitation , ring (chemistry) , deuterium , brønsted–lowry acid–base theory , membrane , atomic physics , organic chemistry , catalysis , fluorescence , ion , biochemistry , physics , quantum mechanics
— A mechanistic proposal is presented to account for the observation that two protons are pumped for each turnover in the photocycle of bacteriorhodopsin. One of the protons pumped presumably arises from the deprotonation of the Schiff base as suggested by others. Photoexcitation of retinal is known to result in substantial positive charge development in the six membered ring of the chromophore. It is suggested that the other proton pumped comes from a proteinic Brönsted acid group located close to the cyclohexenyl ring of the bound chromophore. Calculations indicate that the acidity of such a suitably placed Brönsted acid can be increased 3 to 5.5 p K units by virtue of a purely electrostatic effect if 0.2 units of additional positive charge develops atC–5 of the chromophore in its excited state. Two other observations: (a) the bathochromic shift and (b) the deuterium isotope effect in the formation of the first intermediate (bR 568 → hv K 610 ) are also shown to be consistent with this mechanistic proposal.