Question

A small generic section of the primary structure of an alpha helix is given below.

A small generic section of the primary structure of an a helix is given below. -amino acid1 -amino acid2 -amino acid3 -amino acid4 -amino acid5 -amino acid6 -amino acid7 - (a) Which amino acid residue?s backbone forms a hydrogen bond with the backbone of the seventh (7th) residue? Choose the correct residue from the drop-down menu. amino acid Select answer (b) Which of the following peptide segments is most likely to be part of a stable a helix at physiological pH?

Answer 1

Concepts and reason

The coiled helical arrangement of a polypeptide chain like spring is known as the helix. In the helix of secondary structure of a protein a hydrogen bond links the carbonyl group to the N-H group of a residue in the same polypeptide chain with a gap of four residues. This arrangement makes it appear as a cylinder.

Fundamentals

Pauling and Corey proposed the secondary structure of a protein. Secondary structure of proteins has local structural conformations. There are different forms of a secondary structure of a protein. Secondary structure of a protein is formed by the repeating forms known as helix and beta sheets.

The structure of alpha helix is:

Part a

The third amino acid forms a bonding with the seventh amino acid. It follows the (i + 4) th rule of the hydrogen bonding of amino acids.

Part b

Protein secondary structures are of two types – alpha helix and beta sheets. The bonding between the amino acids is based on the charge present on them. The side chain attached to each amino acid gives the polarity to the molecule. Based on this property amino acids are classified as polar and non-polar amino acids. The non-polar amino acids do not form hydrogen bonding. The polar amino acids are charged and are hydrophilic in nature and participate in hydrogen bonding.

Presence of positive and negatively charged amino acids in continuous sequence does not favor the existence and stability of alpha-helix. Glycine and proline cannot form the alpha helix as they are flexible and large in structures respectively.

Ans: Part a

Part b