Structural and energetic parameters of Ca2+ binding to peptides and proteins

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Abstract
The characteristics of Ca2+‐binding sites and of the structural reorganization induced by Ca2+‐binding in storage proteins and ion carriers are being studied as models for molecular mechanisms in Ca2+ channels and in Ca2+‐dependent modulatory proteins. A first step in the study was the development of energy parameters for Ca2+ compatible with those in the CHARMM package of computer simulation software. Such parameters were obtained from an analytical fit to the potential surface for [(Ca)(OCH2)4]2+ calculated with an ab initio molecular orbital method. The resulting parametrization was tested for the hexapeptide cyclo‐(Pro‐Gly)3, and a 75 residue long calcium binding protein from bovine intestine (ICaBP). The geometrical parameters calculated for the hexapeptide and its 2:1 complex with Ca2+ were in good agreement with experimental data from crystallography and nmr. Similarly, the structure of ICaBP optimized with CHARMM using the new Ca2+ parameters showed good agreement with the x‐ray structure both in the local structures of the calcium‐binding sites and in the overall shape of the protein.