Mapc Sapling Learning macmillan learning α-Keratin is an intermediate filament with a basic structural unit of two α helices in a coiled coil. Each helix has a seven residue repeating unit (heptad repeat). A representation looking down the α helices of a coiled coil dimer is shown below. Each letter represents a different amino acid residue. 0 Identify the three true statements about the structure of keratin. Click here to view a table of the amino acids. The α helix of the coiled coil is wound less tightly than predicted for an α helix. The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent. □ Glu-Leu-Thr-Asn-Thr-Lys-Cys is a likely repeat in the α helix of keratin. Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains. o-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein. Ile-Lys-Asn-Leu-Asn-Glu-Thr is a likely repeat in the α 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 □