Studies on the peptide corresponding to residues 34-47 of bovine factor X

Calcium binding studies of a 14-residue peptide corresponding to the 37-46 sequence of bovine factor X were performed using calcium ion selective electrode titrations and equilibrium dialysis. The presence of gamma-carboxyglutamic acid residues at positions 36 and 40 coupled with the assumption that the peptide would bind calcium ions also prompted an investigation of possibly secondary conformational changes in the peptide by use of circular dichroism spectroscopy. Equilibrium dialysis revealed a single relatively weak calcium binding site (log Ka = 2.39); an ion selective electrode experiment confirmed this result (log Ka = 2.17). The peptide maintained a random coil conformation throughout the calcium ion titrations as measured by circular dichroism.