Lanthanide-induced peptide folding: variations in lanthanide affinity and induced peptide conformation

Abstract

The utility of the diamagnetic lanthanides Lu and La as metal binding probes for a synthetic 13-residue fragment representing Ca binding site 3 of rabbit skeletal troponin C (residues 103-115) were demonstrated. The peptide conformation induced by these metals was monitored by the PMR at 270 MHz. The peptide affinity for these rare earths is 50-400 times higher than that for Ca (KLu3+, 1.3 .times. 104 M-1; KLa3+, 1.1 .times. 105 M-1; KCa2+, 3 .times. 102 M-1) which is related to the change in cation charge from 2+ to 3+. The peptide conformation induced by the presence of La3+ generates a different 1H NMR spectrum than the one observed for the Lu-saturated peptide. Thus, it appears that these metals do not fold the peptide into exactly the same conformation. The resonance shifts observed during the Lu3+ titration are much smaller than those seen in the case of La3+ addition. The fact that Lu binds less tightly than La to the peptide may be linked directly or indirectly to the difference in ionic radius between these metals (Lu3+, 0.86 .ANG.; La3+, 1.03 .ANG.). This may in turn indicate that the peptide primary sequence encodes for some aspects of metal ion specificity. The 1H NMR results also demonstrate that glycine-108 adopts a restricted geometry in the absence of metal such that its 2 .alpha.-C protons are in different environments which are further affected by the addition of either metal. Geometric constraints arising from the particular peptide folding pattern near this residue probably correlate with the highly conserved nature of this site of the EF hand. This position remains occupied by glycine in most EF hand domains with the exception of known distorted Ca binding sites present in intestine Ca binding proteins and S-100.