The carbon-nitrogen peptide bond is rigid, but rotation can take place about the

Question

The carbon-nitrogen peptide bond is rigid, but rotation can take place about the N-Ca and the CC bonds in a protein (Ca is the carbon atom). These bond angles define the conformation of the peptide chain. Answer questions (a) and (b). A Ramachandran plot is given for reference a) Which of the following most likely describes part of 180 an observed helix? 120 =-60°; --47"; many Ala residues -+60°; +40°; many Gly residues =-59"; = +150°; many Ser residues =-140°; = +130°; many Val residues =-57"; --47"; many Pro residues 0 a 60 -120 -180 b) Which of the following would not likely occur in a sheet? Choose all that apply -180 180 (degrees) =-51"; = + 153°; many Gly and Pro residues --1 39" +135°; many Val residues = _ 57%--49"; many Ala residues = +60°; +60°; many Gly residues --1 20"; + 120°; many Tyr residues

Explanation / Answer

A. Phi -60 psi -47, many ala residues. The range of phi and psi falls under lower left quadrant of ramachandran plot. Where right handed alpha helices can be found. More alanine residues give the characteristic feature of alpha helix, can makes it more stable.

B. Phi -57 psi -49. Many ala residues. This phi and psi bond angles give a polypeptide chain helical confirmation. Many alanine residues also gives a polypeptide helical confirmation. So it will not likely occur in a beta sheet

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