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An amino acid is
A. Leucine
B. Phenylalanine
C. Lysine
D. Proline

Last updated date: 14th Jun 2024
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Hint: Amino acids are organic that contain carbon and hydrogen compounds that have amino groups and the amino acids containing secondary amine groups are known as amino acids. It consists of the $NH$ group and in the case of amino acid $\mathop {NH}\nolimits_2 $ group. Imino acid found in protein structure.

Complete answer:
Amino acid is an organic ( the compounds that contain carbon and hydrogen) compound with one amino and one carboxyl group attached to it but in case of imino acid there is a secondary amine group and a carboxyl group. Proline has a second amino group whereas cysteine has only a primary group and there is no secondary group.
Now let us match this with given options :-
Leucine :- it is a type of amino acid that contains one amino group and one carboxylic group. As amino groups are basic in nature and carboxylic groups are acidic in nature and they are present in equal amounts they nullify the effect of each other and make leucine a neutral amino acid.
Phenylalanine :- it is a kind of amino acid that possesses cyclic structure. Due to its cyclic structure it is known as the aromatic amino acid. It has a side chain that bears the carboxylic acid and amino group that is $\mathop {NH}\nolimits_2 $.
Lysine :- the amino acids are compounds that have amino and carboxyl groups. It is an acid that has an extra amino group. As amines are basic in nature they increase their basicity and make it a basic amino acid.
Proline :- It has a secondary amino group that confers presence of $NH$ group attached to it and thus it is called as imino acid. It is important for the formation of healthy cartilage that provides elasticity and collagen in our body and it also affects flexibility of both muscles and joints.

Our required option is D that is proline.

Note: Proline is a well known imino acid that is used for various processes, it is required for proper functioning of joints and tendons, the structure that connects muscles to bones , it also helps in maintaining strength of heart muscles and keeping the heart healthy. It is important for maintaining reparative function of muscles as well as youthful skin is maintained by it. It plays its role in the immune system also.