Thermodynamics and energy coupling in the bacteriorhodopsin photocycle

G. Váró, Janos K. Lanyi

Research output: Contribution to journalArticle

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Abstract

Time-resolved absorption changes of photoexcited bacteriorhodopsin were measured with a gated multichannel analyzer between 100 ns and 100 ms at six temperatures between 5 and 30°C. The energetics of the chromophore reaction cycle were analyzed on the basis of a model containing a single cycle and reversible reactions. The calculated thermodynamic parameters provide insights to general principles of the active transport. They indicate that in this light-driven proton pump the free energy is retained after absorption of the photon as the enthalpy of the pKa shift in the chromophore which allows deprotonation of the Schiff base. Part of the excess free energy is dissipated at the "switch" step where the reaction and transport cycles are coupled, and the rest at the chromophore recovery step. All other reactions take place near equilibrium. The "switch" step is the M1 → M2 transition in the reaction cycle [Váró, G., & Lanyi, J. K. (1991) Biochemistry (preceding paper in this issue)]. It provides for return of the chromophore pKa to its initial value so the Schiff base will become a proton acceptor, for reordering access of the Schiff base from one side of the membrane to the other, and for unidirectionality of the proton transfer. Conformational energy of the protein, acquired during the "switch" step, drives the completion of the photocycle.

Original languageEnglish
Pages (from-to)5016-5022
Number of pages7
JournalBiochemistry
Volume30
Issue number20
Publication statusPublished - 1991

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Bacteriorhodopsins
Schiff Bases
Chromophores
Thermodynamics
Protons
Switches
Free energy
Proton Pumps
Active Biological Transport
Photons
Biochemistry
Deprotonation
Proton transfer
Enthalpy
Temperature
Membranes
Recovery
Proteins

ASJC Scopus subject areas

  • Biochemistry

Cite this

Thermodynamics and energy coupling in the bacteriorhodopsin photocycle. / Váró, G.; Lanyi, Janos K.

In: Biochemistry, Vol. 30, No. 20, 1991, p. 5016-5022.

Research output: Contribution to journalArticle

Váró, G. ; Lanyi, Janos K. / Thermodynamics and energy coupling in the bacteriorhodopsin photocycle. In: Biochemistry. 1991 ; Vol. 30, No. 20. pp. 5016-5022.
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