I'm given two pictures looking down the alpha helices of a coiled coil dimer of alpha-keratin. I'm asked to identify three statements out of the following that are true. My guesses are 2, 3, and 6.
1. Leu-Glu-His-Val-Cys-Lys-Ser 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. Lys-Val-Ser-Cys-Ser-Glu-Thr 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. α-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein.
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.
1. True. Residues such as Glu, Cys, Thr, Ser, and Asn can cause destabilization of alpha-helixes, 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 alpha-helix 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 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 the formation of profilaments 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.
I hope this helps, but I can not be 100% positive about my choices.
The correct statements out of the given options are 2, 3, and 7.
Statement 1 is incorrect because Leu-Glu-His-Val-Cys-Lys-Ser is not a likely repeat in the α helix of keratin.
Statement 2 is correct. Each polypeptide in the dimer of alpha-keratin has 3.5 residues per turn, resulting in a slight twist around the other polypeptide, which contributes to the formation of the coiled coil.
Statement 3 is correct. Residues at positions b and c in the α helix of the coiled coil are less likely to be polar or charged because they are in contact with the solvent.
Statement 4 is incorrect. Keratin molecules are not primarily stabilized by hydrophilic interactions between charged amino acid side chains. Instead, keratin is rich in hydrophobic amino acids that contribute to its strength.
Statement 5 is incorrect. Lys-Val-Ser-Cys-Ser-Glu-Thr is not a likely repeat in the α helix of keratin.
Statement 6 is incorrect. The α helix of the coiled coil is actually wound more tightly than predicted for a typical α helix.
Statement 7 is correct. α-Keratin is indeed rich in Cys residues, which can form covalent cross-links between peptide chains and increase the strength of the protein.
To identify the true statements, let's analyze the options and the given information:
1. Leu-Glu-His-Val-Cys-Lys-Ser is a likely repeat in the α helix of keratin.
This statement cannot be determined based on the given information. The sequence Leu-Glu-His-Val-Cys-Lys-Ser is mentioned but no information is provided to determine if it 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.
This statement is likely true. The fact that each polypeptide in the dimer has 3.5 residues per turn suggests a twisted conformation that helps form the coiled coil structure.
3. The residues at positions b and c are less likely to be polar or charged because they are in contact with the solvent.
This statement is likely true. The fact that the residues at positions b and c are in contact with the solvent suggests that they are exposed to the aqueous environment and are, therefore, less likely to be polar or charged.
4. Keratin molecules are very strong due to hydrophilic interactions between charged amino acid side chains.
This statement is false. Keratin molecules are primarily strengthened by the formation of covalent cross-links between peptide chains, not hydrophilic interactions between charged amino acid side chains.
5. Lys-Val-Ser-Cys-Ser-Glu-Thr is a likely repeat in the α helix of keratin.
This statement cannot be determined based on the given information. The sequence Lys-Val-Ser-Cys-Ser-Glu-Thr is mentioned but no information is provided to determine if it 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.
This statement is likely true. The information provided suggests that the α helix in the coiled coil is wound less tightly than the typical α helix conformation.
7. α-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein.
This statement is true. The presence of Cys residues in α-keratin enables the formation of covalent cross-links between peptide chains, thereby increasing the strength of the protein.
Based on the analysis, the true statements are 2, 3, and 7. They are:
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.
7. α-Keratin is rich in Cys residues, enabling the formation of covalent cross-links between peptide chains and increasing the strength of the protein.