Identify the essential amino acid from the following.

Assertion : Carboxypeptidase is an exopeptidase.
Reason : It cleaves the $N$-terminal bond.

In the following compound,the favourable site/s for protonation is/are:
(Image description: Adenine structure with labels $a$ on the exocyclic $-NH_2$ group,$b$ on the pyrimidine ring nitrogen,$c$ on the other pyrimidine ring nitrogen,$d$ on the imidazole ring nitrogen,and $e$ on the imidazole ring $-NH-$ group.)

Which of the following structures of proteins represents its constitution?

What is the number of amino acids present in a single turn of an $\alpha$-helix of a protein?

What is the secondary structure of proteins composed of?