PEPTIDE MODELS OF PROTEIN-FOLDING INITIATION SITES .2. THE G-H TURN REGION OF MYOGLOBIN ACTS AS A HELIX STOP SIGNAL

A series of peptide fragments of sperm whale myoglobin, corresponding to segments of the region between the G- and H-helices of the protein, have been synthesized and their conformational preferences investigated using circular dichroism and nuclear magnetic resonance spectroscopy in aqueous solution and in solvent mixtures containing water and trifluoroethanol. The smallest fragment, Mb-GH5, a five-residue peptide with the sequence HPGDF corresponding to the connecting loop between the two helices in the folded protein, adopts highly populated turn conformations in aqueous solution. A 25-residue peptide, Mb-GH25, containing the same sequence flanked by contiguous segments of the G- and H-helix sequences, was also found to contain a high proportion of conformers with a turn in this region. No helix formation was observed in the flanking sequences in water solution, either in Mb-GH25 or in control 10-residue peptides (Mb-G10 and Mb-H10) with sequences corresponding to the G- and H-helix segments. No additional helicity above that of the sum of the components was observed for Mb-GH25, indicating that a helical hairpin structure is not formed in the monomeric peptide in aqueous solution. In the presence of TFE, ordered helix is formed in Mb-GH25 according to the CD spectrum, and NMR spectra indicate that this is localized in the N-terminal portion of the peptide. NOESY spectra clearly show that the turn conformation is retained under these conditions. A peptide, Mb-AA25, in which the central Pro-Gly sequence of the turn are replaced with Ala-Ala was found to contain significantly more helix by CD in the presence of TFE, indicating that the turn sequence acts as a helix stop signal at the C-terminus of the G-helix under these conditions. The effect of TFE on the conformational preferences of these peptides confirms that helix is induced only in sequences which have a high propensity for helix formation-there is no effect on the turn conformation. These results suggest that the turn sequence maybe important for helix termination during the initial stages of folding as well as in the native protein.