α-Helix nucleation by a calcium-binding peptide loop

A 12-residue peptide AcDKDGDGYISAAENH(2) analogous to the third calcium-binding loop of calmodulin strongly coordinates lanthanide ions (K = 10(5) M(−1)). When metal saturated, the peptide adopts a very rigid structure, the same as in the native protein, with three last residues AAE fixed in the α-helical conformation. Therefore, the peptide provides an ideal helix nucleation site for peptide segments attached to its C terminus. NMR and CD investigations of peptide AcDKDGDGYISAAEAAAQNH(2) presented in this paper show that residues A13-Q16 form an α-helix of very high stability when the La(3+) ion is bound to the D1-E12 loop. In fact, the lowest estimates of the helix content in this segment give values of at least 80% at 1°C and 70% at 25°C. This finding is not compatible with existing helix-coil transition theories and helix propagation parameters, s, reported in the literature. We conclude, therefore, that the initial steps of helix propagation are characterized by much larger s values, whereas helix nucleation is even more unfavorable than is believed. In light of our findings, thermodynamics of the nascent α-helices is discussed. The problem of CD spectra of very short α-helices is also addressed.