Ca(2+)-induced conformational transitions of phosphorylated peptides

CD spectroscopic studies on protected peptides containing lysine and serine, or phosphoserine, and on serine-containing fragments of the neurofilament protein midsized subunit, both in the unphosphorylated and phosphorylated form, are reported. The introduction of the phosphoryl group was not found to have a significant spectral effect in aqueous solution. In trifluoroethanol (TFE), spectral shifts toward unordered (type U) spectra or the appearance of distorted spectra likely reflect the adoption of aperiodic polypeptide conformations due to salt bridge(s) between negatively charged phosphoserine and positive lysine side-chain groups. A turn-stabilizing effect of phosphorylation was also observed. CD-monitored titration experiments in TFE revealed a high conformational sensitivity of phosphopeptides toward Ca2+ ions. The appearance of the unordered spectra or spectral shifts were the sign of a bulk disordering effect of Ca2+ ions. Spectra with specific spectroscopic features reflect the formation of Ca2+ complexes and the adoption of ordered unique backbone conformations. When ordered structures were obtained on addition of Ca2+ ions, the observed CD curves showed a resemblance to the spectrum of beta-pleated sheets. This may originate from chain extension and the formation of beta-pleated sheet segments fixed by Ca2+ bridges between PO3H-1 groups of adjacent peptide chains. The data clearly show that the effect of the Ca2+ ions is highly specific: the sequence, chain length, presence and distribution of charged side-chain groups, degree and site of phosphorylation, and environmental factors appear to be determining in the process of chain extension or beta-sheet formation.