Question

pH has a major influence on protein structure by altering electrostatic interactions. In order to illustrate this point, let us think about polylysine. At pH 10 and above, polylysine forms an α-helix. At a pH of 7 and below, however, this same polypeptide chain assume an unfolded conformation

1) Can you explain why this transition occurs at pHs below the pKa of Lys?

2) What other residue(s) might you expect to show a similar pattern of pH-dependent folding and unfolding?

3) The residue(s) you might expect based on charge to be capable of forming α-helices do not do so in water when they alone make up a polypeptide chain. Can you come up with a reasonable explanation for why this might be?

4) Speculate on what you think might happen to the pH-dependence of α-helix formation if you had a polypeptide chain consisting of both glutamate and lysine residues.

Answer 1

1.It happens because high PH does not allow them to fold.It happens because of the repulsive forces that are active at PH prevents them to form an alpha helix.

2.While alanine, glutamate, and leucine have a propensity for being in α helices, valine and isoleucine apparently favor β strands, and glycine, asparagine, and proline tend to be present in beta-turns. They also tend to unfold at higher PH and folds at lower PH.

3.It is because of the hydrophobic nature of water. the water molecules just repel to unfold.

4.When the O (δ-) and N (δ+) of a peptide backbone hydrogen bond with one another, that removes the polar aspects of the backbone from further interactions, thus making the peptide behave in a largely hyrdophobic manner.