Symmetrical stabilization of bound Ca2+ ions in a cooperative pair of EF-hands through hydrogen bonding of coordinating water molecules in calbindin D-9k

Water molecules are found to complete the Ca2+ coordination sphere when a p rotein fails to provide enough ligating oxygens. Hydrogen bonding of these water molecules to the protein backbone or side chains may contribute favor ably to the Ca2+ affinity, as suggested in an earlier study of two calbindi n D-9k mutants [E60D and E60Q; Linse et al. (1994) Biochemistry 33, 12478-1 2486]. To investigate the generality of this conclusion, another side chain , Gln 22, which hydrogen bonds to a Ca2+-coordinating water molecule in cal bindin DR, was mutated. Two calbindin D9k mutants, (Q22E+P43M) and (Q22N+P4 3M), were constructed to examine the interaction between Gln 22 and the wat er molecule in the C-terminal calcium binding site II. Shortening of the si de chain, as in (Q22N+P43M), reduces the affinity of binding two calcium io ns by a factor of 18 at low ionic strength, whereas introduction of a negat ive charge, as in (Q22E+P43M), leads to a 12-fold reduction. In 0.15 M KCl, a 7-fold reduction in affinity was observed for both mutants. The cooperat ivity of Ca2+ binding increases for (Q22E+P43M), while it decreases for (Q2 2N+P43M). The rates of Ca2+ dissociation are 5.5-fold higher for the double mutants than for P43M at low ionic strength. For both mutants, reduced str ength of hydrogen bonding to calcium-coordinating water molecules is a like ly explanation for the observed effects on Ca2+ affinity and dissociation. In the apo forms, the (Q22E+P43M) mutant has lower stability toward urea de naturation than (Q22N+P43M) and P43M. 2D H-1 NMR and crystallographic exper iments suggest that the structure of (Q22E+P43M) and (Q22N+P43M) is unchang ed relative to P43M, except for local perturbations in the loop regions.