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The Role of Electrostatic Interactions in Calmodulin-Peptide Complex Formation

Ingemar André ^*, Tõnu Kesvatera ^* , Bo Jönsson , Karin S. Åkerfeldt  and Sara Linse ^*

^* Department of Biophysical Chemistry, Lund University, Chemical Center, SE-22100 Lund, Sweden; Laboratory of Bioorganic Chemistry, National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia; Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041 USA; and Department of Theoretical Chemistry, Lund University, Chemical Center, SE-22100 Lund, Sweden

Correspondence: Address reprint requests to Ingemar André, Tel: 46-46-222 8238; Fax: 46-46-2224543; E-mail: ingemar.andre{at}bpc.lu.se.

The complex between calmodulin and the calmodulin-binding portion of smMLCKp has been studied. Electrostatic interactions have been anticipated to be important in this system where a strongly negative protein binds a peptide with high positive charge. Electrostatic interactions were probed by varying the pH in the range from 4 to 11 and by charge deletions in CaM and smMLCKp. The change in net charge of CaM from –5 at pH 4.5 to –15 at pH 7.5 leaves the binding constant virtually unchanged. The affinity was also unaffected by mutations in CaM and charge substitutions in the peptide. The insensitivity of the binding constant to pH may seem surprising, but it is a consequence of the high charge on both protein and peptide. At low pH it is further attenuated by a charge regulation mechanism. That is, the protein releases a number of protons when binding the positively charged peptide. We speculate that the role of electrostatic interactions is to discriminate against unbound proteins rather than to increase the affinity for any particular target protein.

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