a What amino acid shows a larger range of possible phi and p

a) What amino acid shows a larger range of possible phi and psi angles than those shown by a Ramachandran plot? Which amino acid shows a smaller range of possible phi and psi angles than those shown by a Ramachandran plot?

Solution

In Ramachandran plot, we plot the phi and psi dihedral angels for residues in protein of X and Y axes. Therefore, every residue in Ramachandran plot signifies a specific combination of phi and psi angles. Phi angle denotes rotation around Calpha-N bond while psi angle signify rotation around calpha-C bond.

Proline has a cyclic side chain therefore rotation around bond is constrained by its inclusion in pyrrolidine ring; Phi values are restricted to angles around -60. Hence, proline is most conformationally restricted amino acid residue.

Glycine has no beta carbon atoms i.e no side chain. Therefore, it is least sterically hindered as compared to other amino acids. This fact permits it to cover a large range of area in the plot. Therefore, glycine residues polypeptide chain often assumes conformation that are forbidden to other residues. So, glycine frequently occurs in turn regions of proteins where any other residue would be sterically hindered.