Two trifluoperazine‐binding sites on calmodulin predicted from comparative molecular modeling with troponin‐C

Among the known regulatory proteins that are conformationally sensitive to the binding of calcium ions, calmodulin and troponin‐C have the greatest primary sequence homology. This observation has led to the conclusion that the most accurate predicted molecular model of calmodulin would be based on the X‐ray crystallographic coordinates of the highly refined structure of turkey skeletal troponin‐C. This paper describes the structure of calmodulin built from such a premise. The resulting molecular model was subjected to conjugate gradient energy minimization to remove unacceptable intramolecular non‐bonded contacts. In the analysis of the resulting structure, many features of calmodulin, including the detailed conformation of the Ca 2+ ‐binding loops, the amino‐ and carboxy‐terminal hydrophobic patches of the Ca 2+ ‐bound form, and the several clusters of acidic residues can be reconciled with much of the previously published solution data. Calmodulin in missing the N‐terminal helix characteristic of troponin‐C. The deletion of three residues from the central helical linker (denoted D/E in troponin‐D) shortens the molecule and changes the orientation of the two domains of calmodulin by 60° relative to those in troponin‐C. The molecular model has been used to derive two possible binding sites for the antipsychotic drug trifluoperazine, a potent competitive inhibitor of calmodulin activity.