Question

A $beta $-pleated sheet organization in a polypeptide chain is an example of
A. $1^\circ $ structure
B. $2^\circ $ structure
C. $3^\circ $ structure
D. $4^\circ $ structure

Answer 1

Hint: Polypeptides or proteins are long chains of amino acids. The amino acids are the small molecular weight compounds that have an amino group, a carboxyl group, and a place where different groups are substituted forming the different amino acids.

Complete answer:
$1^\circ $ structure –
It is also known as the primary structure of the amino acids. It is the sequence of amino acids in the polypeptide chain concerning the disulfide bonds that are present in the chains. It is usually denoted with the help of the standard three-letter notation or abbreviations for the amino acids.
$2^\circ $ structure –
It is also known as the secondary structure of amino acids. It is the folding of the primary chains that are stabilized by hydrogen bonding. It consists of two main structures, the $alpha $-helix and the $beta $-pleated sheets. The $alpha$-helix is right-handed or clockwise spirals. The $beta $-pleated sheets are an extension of the polypeptide chains with the neighboring chains that are extending anti-parallel to each other.
$3^\circ $ structure –
It is also known as the tertiary structure of amino acids. It is a three-dimensional arrangement of the polypeptide chains. Disulfide bridges are very commonly observed in them.
$4^\circ $ structure –
It is also known as the quaternary structure of amino acids. It consists of several monomers arranged together in a three-dimensional space.
Thus, based on the above information we can conclude that a $beta $-pleated sheet organization in a polypeptide chain is an example of $2^\circ $ structure.

Hence, the correct answer is option (B).

Note:
The hemoglobin molecule has a quaternary structure. The hemoglobin is made up of four monomeric units or four different polypeptide chains. The structure of hemoglobin consists of two alpha chains and two beta chains. Due to this reason, it is heteroquaternary in structure.