Question

I have a few questions about alpha-helices and beta-sheets.

1)Is there a tendency for the polypeptide chains to become one form of the secondary structure over the other?

2)Are there specific advantages and/or disadvantages to being in the alpha-helix or beta-sheet state?

3)What causes the polypeptide chain to fold in a particular way?

Answer 1

1.Answer:- Normally protein doesn't change it's conformation from alpha helix to beta pleated or vice versa but if the conversion arise from any of these secondary structure to another then it seems some pathological or disease condition. So normally there is no such tendency of polypeptide chain to become one form of secondary structure over the other .

2.Answer:- alpha helix structure has advantage as it is more stable than beta pleated structure due to the hydrogen bond formation between the amino and carbonyl group of every four peptide bond residue and these hydrogen bonds are parallel to helix axis which confer it's stability. Alpha helix structure is stable in both polar and non polar environment but beta pleated sheet is not stable in polar environment hence protein in polar environment hardly form betasheet however in haltolerant condition the beta pleated sheet is more stable . The tightly wound alpha helix structure also provide potential functional group for the space it occupies which also confer toward it's stability. The net dipole moment formed due to allignment of backbone dipole of alpha helix also help in helix packing and in protein function. Hence alpha helix has more advantage over beta pleated sheet.

3) Answer:- The sequence present in polypeptide chain of primary structure of a protein determine which amino acid will form alpha helix and which will form beta pleated structure . So it is the sequence of polypeptide chain which directs the folding of polypeptide chain in particular way to form the particular structure.