Question in picture alpha-Keratin is an intermediate filament with a basic struc

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alpha-Keratin is an intermediate filament with a basic structural unit of two alpha 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. Each polypeptide in the dimer has 3.6 residues per turn, and a nonpolar group occurs every 3.5 residues, resulting in a slight winding, or twist, around the other polypeptide, forming a coiled coil. Leu-Asp-Glu-Val-Thr-His-Lys is a likely repeat in the alpha helix of keratin. The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent. His-Val-Lys-Thr-Lys-Asp-Ser is a likely repeat in the alpha helix of keratin. Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains. The alpha helix of the coiled coil is wound less tightly than predicted for an alpha helix. alpha-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein.

Explanation / Answer

Keratin is the protein that is found in hair, feather, scales, nails, or hooves of animals. Keratin is mechanically strong, unreactive, and resistant to stress. It exists majorly in two forms as alpha and beta. The arrangement of keratin is coiled-coil of two alpha-helices. The stability of alpha helix is enhanced in coiled-coil conformation rather than straight rods. In coiled-coil conformation, two or more alpha helices are intertwined to form a stable structure. The weak interactions between the helices are Van der Waals forces and ionic interactions.

The alpha helix conformation is formed by burying their hydrophobic residues away from solvent interface. In contrast, the charged chains are in contact with solvent. The hydrophobic or non-polar residues occur at regular intervals throughout the chain. The helix strands are held together by hydrophobic interactions. The helix is less tightly wound than normal alpha helix and has less number of residues per turn, which is 3.5 and pitch of the helix is 5.1 Ao.

Thus, the correct options are as follows:

Each polypeptide in the dimer has 3.5 residues per turn, resulting in the slight winding or twist, around the other polypeptide, that helps from the coiled coil.

Leu-His-Asp-Val-Arg-Arg-Asn is a likely repeat in the alpha-helix of keratin.

Alpha keratin rich in Cys residues enabling the formation of covalent cross links between peptide chains and increasing the strength of the proteins.

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