Ionization behavior of acidic residues in calbindin D9k

The ionization state of seven glutamate residues, one aspartate, and the C-terminal alpha-COOH group in bovine apo calbindin D-9k has been studied by measurement and modeling of the pH titration curves and apparent pK(a) values. The observed pK(a) ranged from 3.0 to 6.5. Most of the observed acidic groups were half-ionized at lower pH values than those in unstructured proteins. As a rule, the ionization equilibria extended over a wider pH range than in the case of unperturbed single titrations, indicating a complex influence of protein charges on the charge state of each individual residue. Glu17, which is a backbone Ca2+-ligand in the N-terminal binding loop of calbindin D-9k, was half-protonated at pH 3.6 but manifested biphasic titration with apparent pK(a) values of 3.2 and 6.5. Complementary Monte Carlo simulations of the titration process and pK(a) values of the acidic groups in calbindin D-9k reproduce the experimentally observed titration features, except for the pronounced double titration of Glu17, Discrepancies between the results from direct measurement and from modeling may be partly caused by changes in the protein structure when the net charge changes from -8 to +11 over the isoelectric point at pH 5. (C) 1999 Wiley-Liss, Inc.