can somebody explain how this residue is left hand and have 4 turn in question 3

Question

can somebody explain how this residue is left hand and have 4 turn in question 35 and question 36 how do you know the ala 5 will have hydrogen bond with backbone amide of Val 9

The following amino acid sequence forms a 15 residue -helix: Ser-Asp-Ile-Arg-Ala-Ile-Trp-Ala-Val-Leu-Lys-Glu-Ser-Ala-Ala

36. This helix is most probably ________ and contains ________ turns.

a. left-handed; 4

b. left-handed; 5

c. right-handed; 4

d. right-handed; 5

37. For this -helix, the backbone carbonyl of Ala-5 will:

a. need to be capped

b. hydrogen bond with the backbone amide of Val-9

c. hydrogen bond with the backbone amide of Ser-1

d. hydrogen bond with solvent water

Explanation / Answer

36.

An helix has the following features:

Since there are 15 amino acid residues therefore the number of turns will be 15/3.6 = 4.1.

So 4 turns will be present.

37. For Ala-5, i = 5 and so the carbonyl of Ala-5 will form hydrogen bond with amide of 5+ 4 or the 9th residue i.e. Val-9

Regarding the left or right handedness of the helix, due to steric hindrance L- amino acids tend to form a right handed helix while D- amino acids tend to form a left handed helix. Since it is not mentioned whether the amino acids are in D or L form, it will be difficult to say whether it will be a right handed helix or a left handed. The amino acid sequence provided can only tell us if alpha helix is more favoured or beta sheets and the sequence provided does favour an alpha helical structure.

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