Question 11 of 23 Incorrect Map fran Sapling Learning earning this question has

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Question 11 of 23 Incorrect Map fran Sapling Learning earning this question has been customized by Peter van der Geer at San Diego State University a-Keratin is an intermediate filament with a basic structural unit of two a helices in a coiled coil. Each helix has a seven residue repeating unit (heptad repeat). A representation looking down the a helices of a coiled coil dimer is shown below. Each letter represents a different amino acid residue. Identify the three true statements about the structure of keratin. Click here to view a table of the amino acids. a-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein. The a helix of the coiled coil is wound less tightly than predicted for an a helix The pitch of these helices is 5.1 angstroms instead of the normal 5.4 angstroms The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent. Gly-Asp-Glu-Ala-Cys-His-Ser is a likely repeat in the a helix of keratin. His-Ala-Ser-Cys-Ser-Asp-Thr is a likely repeat in the a helix of keratin. Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains.

Explanation / Answer

Following three statements are true regarding keratin molecule

(1) -Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein.

(2) Gly-Asp-Glu-Ala-Cys-His-Ser is a likely repeat in the helix of keratin.

(3) The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent

Cys residues are abundant in alpha-keratin, but they are not in such numbers as the hydrophobic residues found in alpha-keratin. Cys residues allow for covalent disulfide bonds for strength, but the close packing between two-chain coils that make them insoluble and hard is due to the intermolecular hydrogen bonding, not covalent disulfide bonds, but they are responsible for covalent cross-links between peptide chains, which increase the strength of the protein.

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