Question

Which of the following polypeptides is most likely to form an α helix? Briefly explain your answer.

(a) CRAGNRKIVLETY
(b) SEDNFGAPKSILW
(c) QKASVEMAVRNSG

Answer 1

Polypeptide (a) CRAGNRKIVLETY is most likely to form an α-helix due to the absence of structure-breaking amino acids and the presence of residues that can stabilize the helix.

Step-by-step explanation:

The polypeptide most likely to form an α-helix is (a) CRAGNRKIVLETY. An α-helix is a type of secondary protein structure characterized by a helical coil stabilized by hydrogen bonds between the carbonyl groups and amine groups in the backbone, with the R groups protruding outward. Factors that favor the formation of an α-helix include the presence of amino acids that do not disrupt the helical structure, such as alanine, and the absence of amino acids that tend to break α-helices like proline or glycine. The presence of multiple charged residues, like lysine (K), arginine (R), glutamate (E), or aspartate (D), which can form salt bridges and stabilize the helix, is also favorable.

Answer 2

The polypeptide most likely to form an α-helix is sequence (c) QKASVEMAVRNSG, because it lacks helix-disrupting amino acids and includes several that favor or are neutral towards forming an α-helix.

Step-by-step explanation:

The question asks which of the following polypeptides is most likely to form an α-helix. An α-helix is a secondary structure of proteins, where the polypeptide chain coils into a helix stabilized by hydrogen bonds between the backbone atoms. Certain amino acids like proline can disrupt an α-helix, whereas others like alanine, leucine, and methionine often help form an α-helix due to their helix-forming tendencies.

Comparing the given sequences:(a) CRAGNRKIVLETY(b) SEDNFGAPKSILW

(c) QKASVEMAVRNSG

Sequence (a) has a mixture of amino acids with both helix-forming and hindering tendencies, which makes its potential to form an α-helix unpredictable without assessing the influence of the exact positions of these residues. Sequence (b) contains proline (P), which is a well-known helix break due to its rigid structure that cannot fit easily into an α-helix. Sequence (c), while not obviously dominated by helix-forming residues, lacks proline and also has several residues that do not strongly disrupt helices.

Therefore, sequence (c) QKASVEMAVRNSG is most likely to form an α-helix because it lacks helix-breaking amino acids such as proline. Instead, it combines residues that tend to form α-helices or are at least neutral with respect to this secondary structure formation.