Which of the following is not an essential amino acid?

Choose the correct option for structures of $A$ and $B,$ respectively.

The correct statement in respect of the protein haemoglobin is that it

Which of the following is a Zwitter ion?

Which statement is correct for the protein hemoglobin?

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.)