cx-Keratin is an intermediate filament with a basic structural unit of two helic
ID: 197406 • Letter: C
Question
cx-Keratin is an intermediate filament with a basic structural unit of two helices in a coiled coil. Each helix has a seven residue repeating unit (heptad repeat). A representation looking down the 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. Arg-Ala-Cys-Gln-Cys-Thr-Asn is a likely repeat in the helix of keratin The helix of the coiled coil is wound less tightly than predicted for an helix. a-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein. Val-Thr-Asp-Ala-Gln-Arg-Cys is a likely repeat in the helix of keratin. 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 Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent.Explanation / Answer
We will go through all the options one by one.
Option E is correct. In the alpha helix, all the polar residue are on one side and all the non-polar residues are on the other side such that only polar residues from H-bond with each other and non-polar residues from H-bond with each other.
B- In keratin, the coiled-coil is wound more tightly than the predicted alpha helix. So option B is wrong
C- Alpha helix of Keratin is rich in cysteine residue which forms the Disulphide linkage and provides stability to the alpha helix. So option C is correct.
If we look at the given sequence we will find that first and fourth residues are polar but second and the fifth residue is not same. As a result of it they alpha-helix will disrupt. So option A is wrong.
If we look at the given sequence we will find that first and fourth residues are non-polar but second and the fifth residues are polar. As we have seen in the Statement E that all the polar residue is on one side and all the non-polar residues are on the other side such that only polar residues from H-bond with each other and non-polar residues from H-bond with each other. So option D is correct.
Keratin molecules impart its strength due to disulphide linkage. So statement F is wrong.
B and C residues should be polar as they are in contact with solvent molecules. So option G is wrong.
So the correct options as D, C and E
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