Hc. Shin et al., PEPTIDE MODELS OF PROTEIN-FOLDING INITIATION SITES .3. THE G-H HELICAL HAIRPIN OF MYOGLOBIN, Biochemistry, 32(25), 1993, pp. 6356-6364
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.