\"(2) What amino acid, if included in the middle of an alpha helix, would cause
ID: 18274 • Letter: #
Question
"(2) What amino acid, if included in the middle of an alpha helix, would cause a bend in the alpha helix? (Hint: There is only one amino acid that would automatically lead to this effect.) What feature of this amino acid leads to the bend? Would including this amino acid destabilize the helix? Be sure to include non-covalent interactions in your answer.On Internet I found the following: Proline residues--proline is the one amino acid without a primary alpha-amine (it is actually an immino acid). Because there is not free rotation about an amide formed with the proline immino group, it puts a kink (called a beta-turn) in the alpha-helix and disrupts it.
Not sure... I need more explanations...""
Explanation / Answer
A common motif in the secondary structure of proteins, the alpha helix (a-helix) is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier ( hydrogen bonding). This secondary structure is also sometimes called a classic Pauling–Corey–Branson alpha helix (see below). Among types of local structure in proteins, the a-helix is the most regular and the most predictable from sequence, as well as the most prevalent. Different amino-acid sequences have different propensities for forming a-helical structure. Methionine, alanine, leucine, uncharged glutamate, and lysine ("MALEK" in the amino-acid 1-letter codes) all have especially high helix-forming propensities, whereas proline and glycine have poor helix-forming propensities.[20] Proline either breaks or kinks a helix, both because it cannot donate an amide hydrogen bond (having no amide hydrogen), and also because its sidechain interferes sterically with the backbone of the preceding turn - inside a helix, this forces a bend of about 30° in the helix axis.[10] However, proline is often seen as the first residue of a helix, presumably due to its structural rigidity. At the other extreme, glycine also tends to disrupt helices because its high conformational flexibility makes it entropically expensive to adopt the relatively constrained a-helical structure.
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