Metal-ion binding to parvalbumin. A 113Cd-n.m.r. study of the binding of different lanthanide ions

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Abstract
113Cd-NMR studies were used to investigate the binding of the La3+, Gd3+, Tb3+, Yb3+ and Lu3+ to carp parvalbumins. Lanthanide ions with a smaller ionic radius bind sequentially to Cd2+-saturated parvalbumin, whereas those with a larger ionic radius bind with similar affinity to both the CD site and the EF site. The smallest ion, Lu3+, does in fact not compete significantly with Cd2+ for the CD site in carp parvalbumin, but appears to bind only to the EF site. This preference of the smaller lanthanide ions for the EF site was used to assign the NMR signals for protein-bound 113Cd. By using Cd NMR and Tb3+ fluorescence it was also shown for .alpha.-lineage parvalbumin from pike that these proteins possess a third site that can bind lanthanide ions. This site is much weaker than in the .beta.-lineage parvalbumins. It was used to assign the 113Cd resonances from protein-bound Cd2+ ions in the spectrum of pike pI [isoelectric point] 5.0 parvalbumin.