Effect of salt on the tyrosine and protonated Schiff base deprotonation kinetics in bacteriorhodopsin

  • Dupuis P
  • El-Sayed ' M
  • El-Sayed M
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Abstract

It is observed that the number of protons pumped per M,,, formed in the bacteriorhodopsin photocycle almost doubles at high salt concentrations. In this paper, we studied the rates of deprotonation of tyrosine and of the two components of the protonated Schiff base at high salt concentrations, and at different pHs and temperatures. It is found that at pH = 7, increasing the salt concentration decreases the rates of deprotonation of the protonated Schiff base and tyrosine. 'This strongly suggests that the observed gain in the quantum yield of proton release per M412 formed when the salt concentration is increased is not a result of an increase in these rates. However, at high pH, e.g. 9.6, increasing the salt concentration increases the rate of deprotonation of the Schiff base by about 50%. Under all conditions, the slow component of the Schiff base is found to lose its proton prior to tyrosine. PAUL DUPUIS et M. A. EL-SAYED. Can. J. Chem. 63, 1699 (1985). A de fortes concentrations en sel, le nombre de protons pompes pour chaque Ma,? forme au cours du photocycle de la bacteriorhodopsine est pratiquement double. Dans cette publication,nous rapportons les resultats d'une etude, i des fortes concentrations en sel, a divers pH et i diverses temperatures, sur les vitesses de dkprotonation de la tyrosine et des deux composantes de la base de Schiff protonee. On a trouve qu'i un pH de 7 I'augmentation de la concentration en sel diminue les vitesses de deprotonation de la base de Schiff protonee et de la tyrosine. Ceci suggkre fortement que I'augmentation observee du rendement quantique des protons lib&& pour chaque M412 forme lorsqu'on augmente la concentration du sel n'est pas dG ? i une augmentation de ces vitesses. Cependant, si on op&re i un pH d'environ 9,6 et que I'on augmente la concentration du sel, on provoque aussi une augmentation de la vitesse de deprotonation de la base de Schiff de I'ordre de 50%. On a trouve que, quelles que soient les conditions, la composante lente de la base de Schiff perd son proton avant la tyrosine. [Traduit par le journal]

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Authors

  • Paul Dupuis

  • M A El-Sayed '

  • M A El-Sayed

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