Answer the following questions concerning the polypeptides whose amino acid sequ

Question

Answer the following questions concerning the polypeptides whose amino acid sequences are shown below. Explain your reasoning for all.

A) Gly-Ile-Trp-Leu-Ile-Ile-Phe-Gly-Val-Val-Ala-Gly-Val-Ile-Gly-Trp-Ile-Leu-Leu-Leu

B) Gly-Pro-Asp-Gly-Pro-Met-Gly-Pro-Ser-Gly-Pro-Arg-Gly-Pro-Glu-Gly-Pro-Asp-Gly

C) Gly-Met-Trp-Pro-Glu-Met-Cys-Gly-Glu-Pro-Ala-His-Val-Arg-Asp-Tyr-Pro-Leu-Leu

D) Gly-Met-Trp-Pro-Glu-Met-Cys-Gly-Glu-Pro-Ala-His-Val-Arg-Asp-Tyr-Cys-Leu-Leu

1. Which would be most likely to form an a-helical structure?

2. Which could have at least one disulfide bond?

3. Suppose that the first amino acid residue in peptide (1) were altered from Gly to Pro. Would you expect this change to have an effect on the secondary structure of this peptide? Why or why not?

Explanation / Answer

1. Proline plays a role as alpha helix breaker. Only in sequence mentioned in A, there is no proline residue present. So, A will form alpha helical structure.

2. For the formation of atleast one disulfide bond, two Cys residues must be present in the amino acid sequence. Each Cys residue contains one -SH group. Oxidative cleavage of two -SH groups creates a disulfide bond. The amino acid sequence mentioned in D contains two Cys residues. So, D could have atleast one disulfide bond.

3. Presence of Pro residues in alpha helices after the first turn causes a kink in the helix structure. As proline is unable to complete the H-bonding chain of the helix, this kink is formed. Besides, steric hindrance caused by proline keep it away from adapting the prefered helical geometry. If Gly is replaced with Pro, kink will be formed as it is present after the first turn. So, the alpha helix structure will not be maintained anymore.

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