Occupancy of Two Primary Chloride-Binding Sites in Natronobacterium pharaonis Halorhodopsin Is a Necessary Condition for Active Anion Transport

I. V. Kalaidzidis and Ya. L. Kalaidzidis^*

Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: yannis@phtbio.genebee.msu.su

^* To whom correspondence should be addressed.

Received December 6, 2002

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The existence of two primary chloride-binding sites was found on the basis of the study of halorhodopsin spectra at different chloride concentrations. SVD analysis of the spectra revealed two chloride-dependent components at low chloride concentration (0.1-10 mM). Global fitting of SVD components found K_D values of 0.47 and 5.2 mM with unitity Hill coefficients. The second K_D coincides with the apparent K_D of the photovoltage response of halorhodopsin.

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KEY WORDS: halorhodopsin, chloride-binding site, halorhodopsin electrogenesis, Natronobacterium pharaonis, SVD analysis

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