N-15 NMR ASSIGNMENTS OF (CD2+)(2)-CALBINDIN D-9K AND COMPARISON WITH (CA2+)(2)-CALBINDIN D-9K - CADMIUM AS A SUBSTITUTE FOR CALCIUM IN CALCIUM-BINDING PROTEINS

The amide N-15 and H-1 chemical shifts of (Cd2+)(2)-calbindin D-9k are presented. A detailed comparison is made with those of (Ca2+)(2)-calbindin D-9k, showing that the (Cd2+)(2) and (Ca2+)(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 H-1 chemical shifts of the (Cd2+)(2) and (Ca2+)(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) approximate to 0.6 x 10(3) +/- 0.2 x 103 was estimated by analyzing line broadening in protein samples containing equal amounts of(Cd2+)(1)- and (Cd2+)(2)-calbindin D-9k.