ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
We have determined the ultraviolet optical rotatory properties of the cyanogen bromide peptides of myoglobin and reduced, S-carboxymethylated α-lactalbumin in both aqueous and aqueous alcohol solutions. Similar measurements were also made on the tryptic digests of apomyoglobin. In aqueous solutions the α-helicity of the various peptides was between 5 and 15%, while in concentrated ethanol solutions the helicity could be increased significantly, but never to more than about 55%. The maximum helicity attained by the various peptides at high ethanol concentrations, as well as the cooperativity of the coil-to-helix transition (reflected in the slope at the steep portion of the helicity-alcohol concentration curves), does not depend on the number of residues in the peptide in the manner expected. We have used a model which treats proline residues as absolute helix breakers, thus introducing the concept of effective chain length. By applying available theories of helix-coil transitions of short-chain polypeptides to this model, one can satisfactorily describe most of the data on the myoglobin peptides. Significantly, it was possible to predict the helicity of acid-denatured apomyoglobin from the behavior of the shorter fragments. By using the model, the peptides were found to have an equal intrinsic helix-forming tendency which, with only two exceptions, was not raised by the formation of tertiary structure. The exceptions were apomyoglobin and peptide 56-131, which show, respectively, a considerable and a very small helicity attributable to tertiary structure formation in water at neutral pH. These results agree with the demonstrated absence of stable intermediates in protein unfolding equilibria. The results offer a further correlation between helical structure in the native molecule and the tendency to helix formation in isolated peptides. The results do not support the hypothesis that small folded regions are responsible for initiating the folding of the molecule, and an alternate description is proposed which envisages approximately half-folded structures at the rate limiting step in the folding reaction. Helix formation in the 33-residue C-terminal peptide of α-lactalbumin was found to be as easy as in the myoglobin peptides. If the proposed structural analogy between lysozyme and α-lactalbumin is correct, then this is a case where helix formation occurs in a peptide which is not helical in the native protein. On the other hand, an α-lactalbumin peptide corresponding to a region which has β-structure in lysozyme did not lend to form α-helices.
Additional Material:
8 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.360100512
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