Characterization of cation-binding sites on Panulirus interruptus hemocyanin by 43Ca and 23Na NMR

The binding of Na+ and Ca2+ ions to Panulirus interruptus hemocyanin has been studied by nuclear magnetic resonance. The combined use of 43Ca and 23Na NMR has allowed two different classes of sites to be distinguished. The sites are characterized by a relatively fast chemical exchange, local mobility and binding constants of the order of 10(3)-10(4) M-1 for Ca2+ and Mg2+ and approximately equal to 10(2) M-1 for Na+. These features and the observed pH dependence of the Ca2+ binding are consistent with the protein ligands being carboxylate groups, some of which may occur in clusters.