Which of the following amino acids contains a heterocyclic ring in its side chain $(R)$ group?

$A$ tetrapeptide has a $-COOH$ group on alanine. This produces glycine $(Gly)$,valine $(Val)$,phenylalanine $(Phe)$,and alanine $(Ala)$ on complete hydrolysis. For this tetrapeptide,the number of possible sequences (primary structures) with the $-NH_2$ group attached to a chiral center is:

Identify the protein present in nail.

Which one of the following is an essential amino acid?

Which compound can exist in a dipolar (zwitter ion) state?

What do you mean by the sodium-potassium pump?