Binding of calcium ions to bacteriorhodopsin

György Váró, Leonid S. Brown, Richard Needleman, Janos K. Lanyi

Research output: Contribution to journalArticle

36 Citations (Scopus)

Abstract

Adding Ca2+ or other cations to deionized bacteriorhodopsin causes a blue to purple color shift, a result of deprotonation of Asp85. It has been proposed by different groups that the protonation state of Asp85 responds to the binding of Ca2+ either 1) directly at a specific site in the protein or 2) indirectly through the rise of the surface pH. We tested the idea of specific binding of Ca2+ and found that the surface pH, as determined from the ionization state of eosin covalently linked to engineered cysteine residues, rises about equally at both extracellular and cytoplasmic surfaces when only one Ca2+ is added. This precludes binding to a specific site and suggests that rather than decreasing the pK(a) of Asp85 by direct interaction, Ca2+ increases the surface pH by binding to anionic lipid groups. As Ca2+ is added the surface pH rises, but deprotonation of Asp85 occurs only when the surface pH approaches its PK(a). The nonlinear relationship between Ca2+ binding and deprotonation of Asp85 from this effect is different in the wild-type protein and in various mutants and explains the observed complex and varied spectral titration curves.

Original languageEnglish
Pages (from-to)3219-3226
Number of pages8
JournalBiophysical journal
Volume76
Issue number6
DOIs
Publication statusPublished - Jun 1999

ASJC Scopus subject areas

  • Biophysics

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