Extensive counter-ion interactions seen at the surface of subtilisin in an aqueous medium

Michele Cianci, Jacopo Negroni, John R. Helliwell, Peter J. Halling

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2 Citations (Scopus)
80 Downloads (Pure)


The extent of protein and counter-ion interactions in solution is still far from being fully described and understood. In low dielectric media there is documented evidence that counter-ions do bind and affect enzymatic activity. However, published crystal structures of macromolecules of biological interest in aqueous solution often do not report the presence of any counter-ions on the surface. The extent of counter-ion interactions within subtilisin in an aqueous medium has been investigated crystallographically using CsCl soak and X-ray wavelength optimised anomalous diffraction at the Cs K-edge. Ten Cs+, as well as six Cl- sites, have been clearly identified, revealing that in aqueous salt solutions ions can bind at defined points around the protein surface. The counter-ions do not generally interact with formal charges on the protein; formally neutral oxygens, mostly backbone carbonyls, mostly coordinate the Cs+ ions. The Cl- ion sites are also found likely to be near positive charges on the protein surface. The presence of counter-ions substantially changes the protein surface electrical charge. The surface charge distribution on a protein is commonly discussed in relation to enzyme function. The correct identification of counter-ions associated with a protein surface is necessary for a proper understanding of an enzyme's function.

Original languageEnglish
Pages (from-to)36771-36776
Number of pages6
JournalRSC Advances
Issue number69
Early online date18 Jul 2014
Publication statusPublished - 2014


  • counter-ion interactions
  • crystal structure
  • crystallography
  • proteins

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