PEPTIDE MODELS OF PROTEIN-FOLDING INITIATION SITES .3. THE G-H HELICAL HAIRPIN OF MYOGLOBIN

As part of an extensive dissection of the folding pathway of myoglobin , a series of peptides corresponding to fragments of sperm whale myogl obin have been synthesized, and their conformational preferences inves tigated using circular dichroism and nuclear magnetic resonance spectr oscopy in aqueous solution and in solvent mixtures containing water an d trifluoroethanol. The behavior of short fragments corresponding to t he sequences of the G- and H-helices of myoglobin and to the turn regi on between these helices has been described in accompanying papers. At the next level of complexity, peptide model compounds have been synth esized to explore the longer-range interactions which may take place i n protein folding after initial secondary structure formation has occu rred. A series of disulfide-bridged dimeric peptides containing the co mplete sequences of the G- and H-helices of myoglobin were synthesized and their conformational preferences examined. CD spectra indicate th at disulfide-bridged peptides consisting of two H-helix sequences (Mb- HssH) and of one G- and one H-helix (Mb-GssH) are highly helical in wa ter solution, as a result of intermolecular association. A 51-residue peptide, Mb-GH51, encompassing the entire G-H helical hairpin of myogl obin, including the turn sequence between the two helices, has been su ccessfully synthesized by standard methods. This peptide was designed to be monomeric in aqueous solution. Mb-GH51 does not appear from CD s pectra to contain any additional helix in water solution above what wo uld be expected from an equimolar mixture of the G- and H-helix peptid es. NMR spectra indicate that the turn conformation observed in shorte r peptide fragments is retained in Mb-GH51 in high population. The add ition of TFE results in the formation of some helix, though not as muc h as might be expected even from a simple combination of the elliptici ties of the component helical peptides in TFE. Current experimental an d theoretical studies of myoglobin folding implicate the G-H helical h airpin in the earliest stages of folding: the present results imply th at other parts of the polypeptide chain may be participating in these early events to a greater extent than heretofore imagined.