CALCIUM ION-PROTEIN INTERACTIONS IN PROTHROMBIN ACTIVATION

1. The protein concentration dependence observed in the calcium binding to fragment 1 indicates that calcium-mediated dimerization is responsible for the cooperative calcium binding behavior usually observed. "Unusual" fragment 1, which exhibits negative cooperativity (the type of binding behavior expected for ions interacting with a charged protein) at high concentration, also exhibit altered self-association behavior. 2. The calcium-induced spectral perturbations that are observed by fluorescence and ultraviolet difference spectroscopy are influenced by calcium-mediated dimerization. Similar spectral perturbations may also be induced by other divalent, trivalent, and monovalent ions, as well as changes in pH. Because this is a multi-site system, only limited interpretation of the spectral data is possible without calcium binding data. 3. Although strong side chain CD signals make estimation of fragment 1 secondary structure ambiguous, the CD data do indicate small changes in structure during calcium binding. Similar changes are observed upon addition of monovalent ions at high concentration or after lowering the pH. No coupling between changes in conformation and the cooperative calcium binding behavior has yet been observed to exist.