Question

The α-helix and the β-pleated sheet are part of which

protein structure?

a. primary

b. secondary

c. tertiary

d. quaternary

Short answer

The α-helix and the β-pleated sheet are part of the secondary structure of proteins.

Step-by-step solution

Introduction

Proteins are macromolecules that play an important role in tissue repair, hormone production, immune function and participate in key enzymatic reactions. Every protein is made up of amino acids joined together by peptide bonds in a particular order to form a polypeptide chain. The polypeptide chain folds to give the protein its shape to function efficiently. A single amino acid change can render the protein dysfunctional.

Explanation

The primary structure of a protein is the unique linear order of amino acids linked together by peptide bonds to form the polypeptide chain. Hence, option a is incorrect.

The tertiary structure is formed when the polypeptide chain folds to give a protein its unique three-dimensional shape for optimum function. Several forces hold the tertiary structure of the protein. Hydrophobic interactions play a major role in the folding of the protein. The hydrophilic R groups are exposed to the aqueous environment whereas the hydrophobic R groups position themselves at the center of the protein. Hydrogen bonding, ionic bonding, and disulfide bridges also contribute to the folding and stabilization of the protein structure. Hence, option c is incorrect.

A quaternary structure is formed when the three-dimensional structures of multiple polypeptide chains interact with one another. Each polypeptide chain is referred to as a subunit that arranges itself to form the multi-subunit protein. Hence, option d is incorrect.

The secondary structure of a protein refers to the folding of the polypeptide backbone. The two most well-known secondary structures are α-helix and the β-pleated sheet. Both these structures are held in shape by hydrogen bonding between the carbonyl group of one amino acid and amino groups of another amino acid. In an α-helix, the carbonyl group of one amino acid forms a hydrogen bond with an amino acid that is four down the polypeptide chain. This gives the curled ribbon appearance. In a β-pleated sheet, polypeptide units lying adjacent to each other form hydrogen bonds giving it a folded or pleated appearance. Hence, option b is correct.