Protein structure. Consider whether the following statements are reasonable (R)

Question

Protein structure. Consider whether the following statements are reasonable (R) or unreasonable (U) in terms of what you know of the stuctures and properties of proteins. Indicate either R or U for each statement and write a brief justification for your answer for those statements you judge as unreasonable. No more than three sentences should be required. Credit will not be given without written justification.

(A) Only the hydrogen bonds that are surface-exposed are important in stabilizing the three dimensional structure of a globular protein.

(B) Polylysine (a polymer of lysine monomer units linked by alpha-peptide bonds) is likely to fold into an alpha helical structure at pH 7.0

(C) A coiled-coil protein possesses alpha helices that contain only hydrophobic amino acid residues.

(D). Interactions between hemoglobin subunits are stabilized in high salt concentrations.

(E) Alpha helices are commonly found in the interior of globular proteins

Explanation / Answer

Ans. Unreasonable: The hydrogen bonds are the contributing factors in providing stability to the structure, but the major contribution is of non covalent bonds like vanderwaal interaction, ionic interaction etc.

Ans. Unreasonable: Poly (L-lysine) appear in a random-coil formation at a low pH, alpha-helix at a pH above 10.6, and transforms into beta-sheet when the alpha-helix polylysine is heated

Ans. Reasonable: The maximum amino acids in the coiled coil structures are hydrophobic. In such proteins of a, d, a’ and d’ residues in the helices are usually hydrophobic. So, two strands in a coiled-coil are held together by hydrophobic interaction as well as ionic interactions and disulfide bonds.

Ans. Unreasonable: The affinity of the hemoglobin for oxygen decreased with the increased conc. of the salt. It is because the interaction between the hemoglobin decreased.

Ans. Reasonable. This provide the globular more coiled and compact structure.

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