Question

A small generic section of the primary structure of an alpha helix is given below

-amino acid1-amino acid2-amino acid3-amino acid4-amino acid5-amino acid-6-amino acid7-

a.) which amino acid residue's backbone forms a hydrogen bond witht he backbone of the seventh(7th) residue?

6, 1, 3, 2, 5, OR 7?

b.) which of the following peptide segments is most likely to be part of a stable alpha helix at physiological pH?

a.) -Lys-Lys-Ala-Arg-Ser-

b.) -Gly-Arg-Lys-His-Gly-

c.) -Pro-Leu-Thr-Pro-Trp-

d.) -Gly-Gly-Gly-Ala-Gly-

e.) -Glu-Glu-Glu-Glu-Glu-

f.) -Glu-Leu-Ala-Lys-Phe-

g.) -Tyr-Trp-Phe-Val-IIe-

Answer 1

a)

The common secondary structure of proteins is alpha helix. The N-H group of an amino acid forms hydrogen bond with C=O group of other amino acid that exists four residues.

According to the rule i+4→i.

Thus, based on this rule the 7^th amino acid forms hydrogen bond with 3^rd amino acid.

Hence, the correct amino acid is 3.

b)

Sequential positive and negative charges lead to repulsions, which further results in unstable structure of protein. Thus, to prevent this problem positively charged residues are usually located three amino acids away from negatively charged amino acids. The bulkiness and flexible nature of proline and glycine amino acids respectively do not favor alpha helices. Similarly, the aromatic amino acids also do not favor alpha helices.

Hence, the correct option among the following is f.