FOLDING PROPENSITIES OF PEPTIDE-FRAGMENTS OF MYOGLOBIN

Myoglobin has been studied extensively as a paradigm for protein foldi ng. As part of an ongoing study of potential folding initiation sites in myoglobin, we have synthesized a series of peptides covering the en tire sequence of sperm whale myoglobin. We report here on the conforma tional preferences of a series of peptides that cover the region from the A helix to the FG turn. Structural propensities were determined us ing circular dichroism and nuclear magnetic resonance spectroscopy in aqueous solution, trifluoroethanol, and methanol. Peptides correspondi ng to helical regions in the native protein, namely the B, C, D, and E helices, populate the alpha region of (phi, psi) space in water solut ion but show no measurable helix formation except in the presence of t rifluoroethanol. The F-helix sequence has a much lower propensity to p opulate helical conformations even in TFE. Despite several attempts, w e were not successful in synthesizing a peptide corresponding to the A -helix region that was soluble in water. A peptide termed the AB domai n was constructed spanning the A- and B-helix sequences. The AB domain is not soluble in water, but shows extensive helix formation througho ut the peptide when dissolved in methanol, with a break in the helix a t a site close to the A-B helix junction in the intact folded myoglobi n protein. With the exception of one local preference for a turn confo rmation stabilized by hydrophobic interactions, the peptides correspon ding to turns in the folded protein do not measurably populate p-turn conformations in water, and the addition of trifluoroethanol does not enhance the formation of either helical or turn structure. In contrast to the series of peptides described here, earlier studies of peptides from the GH region of myoglobin show a marked tendency to populate he lical structures (H), nascent helical structures (G), or turn conforma tions (GH peptide) in water solution. This region, together with the A -helix and part of the B-helix, has been shown to participate in an ea rly folding intermediate. The complete analysis of conformational prop erties of isolated myoglobin peptides supports the hypothesis that spo ntaneous secondary structure formation in local regions of the polypep tide may play an important role in the initiation of protein folding.