Chemistry (Biochem)
posted by Robert .
I'm given two pictures looking down the alpha helices of a coiled coil dimer of alphakeratin. I'm asked to identify three statements out of the following that are true. My guesses are 2, 3, and 6.
1. LeuGluHisValCysLysSer is a likely repeat in the α helix of keratin.
2. Each polypeptide in the dimer has 3.5 residues per turn, resulting in the slight winding, or twist, around the other polypeptide, that helps form the coiled coil.
3. The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent.
4. Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains.
5. LysValSerCysSerGluThr is a likely repeat in the α helix of keratin.
6. The α helix of the coiled coil is wound less tightly than predicted for an α helix.
7. αKeratin is rich in Cys residues, enabling the formation of covalent crosslinks between peptide chains and increasing the strength of the protein.

1. True. Residues such as Glu, Cys, Thr, Ser, and Asn can cause destabilization of alphahelixes, so the spacing of the residues are critical. The spacing and the intermolecular interaction caused by the spacing seem correct, so this could be a likely repeat in the alphahelix of keratin. However, I am not 100% positive.
2. False. This could be true, but I believe that it is false. From my understanding, each polypeptide in a dimer has 3.6 residues/turn not 3.5.
3. True. If they are in contact with the solvent, which assuming is a polar solvent then they are likely to be polar residues.
4. False. The strength is due to hydrogen bonding and hydrophobic interactions
5. False. Look at 1 for an explanation.
6. False. Studies have shown that it is wound more tightly then predicted.
7. True. Cys residues are abundant in alphakeratin, but they are not in such numbers as the hydrophobic residues found in alphakeratin. Cys residues allow for covalent disulfide bonds for the formation of profilaments for strength, but the close packing between twochain coils that make them insoluble and hard is due to the intermolecular hydrogen bonding, not covalent disulfide bonds, but they are responsible for covalent crosslinks between peptide chains, which increase the strength of the protein.
I hope this helps, but I can not be 100% positive about my choices. 
The true answers are 1,2 and 7. If the residues are polar then it only makes sense for them to come into contact with the solvent, as they are the hydrophilic amino acids.
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