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Biophysical Journal 84:1202-1207 (2003)
© 2003 The Biophysical Society

Characterization of the Photochemical Reaction Cycle of Proteorhodopsin

György Váró*, Leonid S. Brown{dagger}, Melinda Lakatos* and Janos K. Lanyi{dagger}

* Institute of Biophysics, Biological Research Center of the Hungarian Academy of Sciences, Szeged, H-6701, Hungary; and {dagger} Department of Physiology and Biophysics, University of California, Irvine, California 92697 USA

Correspondence: Address reprint requests to Janos K. Lanyi, Tel.: 949-824-7150; Fax: 949-824-8540; E-mail: jlanyi{at}orion.oac.uci.edu.

Absorption changes in the photocycle of the recently described retinal protein, proteorhodopsin, are analyzed. The transient spectra at pH 9.5, where it acts as a light-driven proton pump, reveal the existence of three spectrally different intermediates, K, M, and N, named in analogy with the photointermediates of bacteriorhodopsin. Model analysis based on time-dependent absorption kinetic signals at four wavelengths suggested the existence of two more spectrally silent intermediates and lead to a sequential reaction scheme with five intermediates, K, M1, M2, N, and PR', before decay to the initial state PR. An L-like intermediate was not observed, probably for kinetic reasons. By measuring the light-generated electric signal of an oriented sample, the electrogenicity of each intermediate could be determined. The electrogenicities of the first three intermediates (K, M1, and M2) have small negative value, but the last three components, corresponding to the N and PR' intermediates and PR, are positive and two-orders-of-magnitude larger. These states give the major contributions to the proton translocation across the membrane. The energetic scheme of the photocycle was calculated from the temperature-dependence of the absorption kinetic signals.




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