Below are the sequences of three peptides (X, Y and Z) Peptide X: Met-Trp-Gly-Gl

Question

Below are the sequences of three peptides (X, Y and Z)

Peptide X: Met-Trp-Gly-Glu-Pro-Cys-Ser-Gly-Pro-Ala-His-Val-Arg-Cys-Asp-Gly

Peptide Y: Glu-Ile-Arg-Leu-Ile-Ile-Cys-Leu-Val-Val-Ala-Ala-Glu-Val-Ile-Gln-leu

Peptide Z: Lys–Arg–Arg–Lys–Lys-Lys-Arg–Arg–Arg–Lys–Lys-Lys- Arg–Arg–Arg

A. Which polypeptide would be most likely to form an helical structure? Give one reason why it is better than each of the others?

(for example if you say X is best….give one reason why it is better than Y and one reason why it is better than Z)

B. Which peptide(s) could have a disulfide bond?

Explanation / Answer

A.

Peptide Y would most likely to form an helical structure.

Peptide Y is better than peptide X because peptide X contains amino acid residues namely proline and glycine. These amino acid residues are known for breaking helical structures. In peptides, proline can't participate in H-bonding through amide hydrogen because it lacks amide hydrogen and the higher conformational flexibility of glycine entropically disfavors its participation in constrained helical structure.

Peptide Y is better than peptide Z because peptide Z contains two adjacent amino acids lysine and arginine. They are too large to make 3.6 turn and also the protonated side chain electrostatically repel each other and thus destabilizes peptides.

B.

Peptide X could have a disulfide bond because it has two cystine residues. Cystine has sulfhydryl or -SH group. These -SH groups from two cystine residues can oxidize to form a disulfide bond.

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