15 N NMR assignments of (Cd 2+ ,)2‐calbindin D 9k and comparison with (Ca 2+ ) 2 ‐calbindin D 9k . Cadmium as a substitute for calcium in calcium‐binding proteins

The amide 15 N and 1 H chemical shifts of (Cd 2+ ) 2 ‐calbindin D 9k are presented. A detailed comparison is made with those of (Ca 2+ ) 2 ‐calbindin D 9k , showing that the (Cd 2+ ) 2 and (Ca 2+ ) 2 states of the protein are very similar, with differences predominantly located in ion‐binding loop I. The present studies were carried out on the P43G mutant calbindin D 9k , and corroborate previous conclusions based on a comparative analysis of the 1 H chemical shifts of the (Cd 2+ ) 2 and (Ca 2+ ) 2 states of wild‐type calbindin D 9k . A value for the rate constant for the dissociation of cadmium from ion‐binding loop I in calbindin D 9k of k off ≈ 0.6 × 10 3 ± 0.2 × 10 3 was estimated by analyzing line broadening in protein samples containing equal amounts of (Cd 2+ ) 1 ‐ and (Cd 2+ ) 2 ‐calbindin D 9k .