Which of the following is not a function of proteins?

Which of the following is not an essential amino acid?

Nitrogen is an essential constituent of all

Hydrolysis of proteins gives

The number of amino acids and number of peptide bonds in a linear tetrapeptide (made of different amino acids) are respectively

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.)