Question

Which is the 21st amino acid?
A. Pyrrolysine
B. Selenocysteine
C. Cystine
D. Histidine

Answer 1

Proteins are the polymers of amino acids. There are approximately 300 amino acids that are known so far although only 20 amino acids are used in the formation of proteins. Amino acids are basically the substituted methanes. The 21st Amino acid is the analog of Neutral amino acid Cysteine.

Complete answer:
Amino acids are amphoteric compounds as it contains one acidic –COOH group and an alkaline –NH2 group. Out of 20 amino acids that are responsible for protein formation, 10 amino acids are known as Essential amino acids as they are not synthesized inside our body but the uptake is required from dietary substances. The amino acids which are synthesized inside the body are Non-Essential amino acids. These amino acids are classified on the basis of the number of carboxyl groups and amino groups. The 21st amino acid is named Selenocysteine. The name Selenocysteine is given because of the presence of selenium in its structure which resembles the structure of Cysteine which is a sulfur-containing compound.

Therefore the correct option is (B) Selenocysteine

Note: Amino acids are acidic, basic, and neutral in nature. Glycine is the simplest amino acid where else the most complex amino acid is Tryptophan. The amino acids which do not participate in the formation of protein synthesis are known as non-protein amino acids. Ornithine and Citrulline are the two examples of non-protein amino acids. Protein and hydroxyproline amino acids consist of the imino group instead of the usual amino group and hence, they are also known as ‘imino’ acids.