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Amino acid sequences of alpha-helical segments from S-carboxymethylkerateine-A. Complete sequence of a type-II segment.

ABSTRACT: 1. The helical fragments obtained by partial chymotryptic digestion of S-carboxymethylkeratine-A, the low-sulphur fraction from wool, were fractionated into type-I and type-II helical segments in aqueous urea under conditions limiting carbamoylation. 2. The amino acid sequence of a 109-residue type-II segment was completed by using the sequenator. 3. When the data were incorporated into a helical model of 3.6 residues per turn the hydrophobic residues generated a band aligned at a slight angle to the helical axis. This result is in accord with the postulated coiled-coil structure of the crystalline regions of alpha-keratin.

SUBMITTER: Crewther WG

PROVIDER: S-EPMC1185788 | biostudies-other | 1978 Aug

REPOSITORIES: biostudies-other

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Project description:BackgroundA large number of studies have been carried out to obtain amino acid propensities for α-helices and β-sheets. The obtained propensities for α-helices are consistent with each other, and the pair-wise correlation coefficient is frequently high. On the other hand, the β-sheet propensities obtained by several studies differed significantly, indicating that the context significantly affects β-sheet propensity.ResultsWe calculated amino acid propensities for α-helices and β-sheets for 39 and 24 protein folds, respectively, and addressed whether they correlate with the fold. The propensities were also calculated for exposed and buried sites, respectively. Results showed that α-helix propensities do not differ significantly by fold, but β-sheet propensities are diverse and depend on the fold. The propensities calculated for exposed sites and buried sites are similar for α-helix, but such is not the case for the β-sheet propensities. We also found some fold dependence on amino acid frequency in β-strands. Folds with a high Ser, Thr and Asn content at exposed sites in β-strands tend to have a low Leu, Ile, Glu, Lys and Arg content (correlation coefficient = -0.90) and to have flat β-sheets. At buried sites in β-strands, the content of Tyr, Trp, Gln and Ser correlates negatively with the content of Val, Ile and Leu (correlation coefficient = -0.93). "All-β" proteins tend to have a higher content of Tyr, Trp, Gln and Ser, whereas "α/β" proteins tend to have a higher content of Val, Ile and Leu.ConclusionsThe α-helix propensities are similar for all folds and for exposed and buried residues. However, β-sheet propensities calculated for exposed residues differ from those for buried residues, indicating that the exposed-residue fraction is one of the major factors governing amino acid composition in β-strands. Furthermore, the correlations we detected suggest that amino acid composition is related to folding properties such as the twist of a β-strand or association between two β sheets.

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Amino acid sequences of alpha-helical segments from S-carboxymethylkerateine-A. Complete sequence of a type-II segment.

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