Studies of cation binding in ZnCl2-regenerated bacteriorhodopsin by x-ray absorption fine structures: effects of removing water molecules and adding Cl- ions.

TitleStudies of cation binding in ZnCl2-regenerated bacteriorhodopsin by x-ray absorption fine structures: effects of removing water molecules and adding Cl- ions.
Publication TypeJournal Article
Year of Publication1997
AuthorsZhang, K, Song, L, Dong, J, EL-Sayed, MA
JournalBiophysical journal
Volume73
Issue4
Pagination2097-105
Date Published1997 Oct
ISSN0006-3495
KeywordsBacteriorhodopsins, Binding Sites, Biophysical Phenomena, Biophysics, Cations, Chlorides, Kinetics, Ligands, Spectrum Analysis, Water, X-Rays, Zinc, Zinc Compounds
Abstract

The binding of Zn2+ in Zn2+-regenerated bacteriorhodopsin (bR) was studied under various conditions by x-ray absorption fine structures (XAFS). The 0.9:1 and 2:1 Zn2+:bR samples gave similar XAFS spectra, suggesting that Zn2+ might have only one strong binding site in bR. It was found that in aqueous bR solution, Zn2+ has an average of six oxygen or nitrogen ligands. Upon drying, two ligands are lost, suggesting the existence of two weakly bound water ligands near the cation-binding site in bacteriorhodopsin. When excess Cl- ions were present before drying in the Zn2+-regenerated bR samples, it was found that two of the ligands were replaced by Cl- ions in the dried film, whereas two remain unchanged. The above observations suggest that Zn2+ has three types of ligands in regenerated bR (referred to as types I, II, and III). Type I ligands are strongly bound. These ligands cannot be removed by drying or by exchanging with Cl- ions. Type II ligands cannot be removed by drying, but can be replaced by Cl- ligands. Type III ligands are weakly bound to the metal cation and are most likely water molecules that can be removed by evaporation under vacuum or by drying with anhydrous CaSO4. The results are discussed in terms of the possible structure of the strongly binding site of Zn2+ in bR.

DOI10.1016/S0006-3495(97)78240-7
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http://www.ncbi.nlm.nih.gov/pubmed/9336205?dopt=Abstract

Alternate JournalBiophys. J.
PubMed ID9336205