Question

Which is the ${21^{{\text{st}}}}$ Amino acid
A. Pyrrolysine
B. Selenocysteine
C. Cystine
D. Histidine

Answer 1

A human body consists of various organic compounds. The amino acid is a type of biomolecules present in the human body which is the main component for the formation of protein. They performed various biological and chemical functions in the human body.

Complete answer:
Amino acids are a type of organic compounds that are very necessary ingredients for the growth and development of human beings. Amino acids are the main component combined to build proteins as well as they can be determined as the building components of proteins. These biomolecules are involved in performing several biological and chemical functions in the human body.

Amino acids are composed of the basic amino groups i.e. –${\text{N}}{{\text{H}}_2}$ and carboxyl groups i.e. ${\text{COOH}}$ . The ingredients found in proteins are amino acids. In amino acid structure, an amino group, a carboxyl group, and a hydrogen atom is attached to a carbon atom. The long chains of amino acids form the peptides and the proteins. There are twenty amino acids, which are responsible for the construction of proteins.

The amino acid has a very high melting and boiling point and they are white crystalline solid substances. Amino acids are sweet, tasteless, and bitter. These acids are soluble in water and are insoluble in organic solvents.

The ${21^{{\text{st}}}}$amino acid is selenocysteine which is an analogue of cysteine. This amino acid holds selenium (Se) in place of a sulfur atom in the cysteine. Selenocysteine is found in the active sites of enzymes and involved in oxidation-reduction reactions which includes glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinase. Into the growing peptide chain, the incorporation of selenocysteine is caused due to a unique suppressor tRNA and a stop codon.

Pyrrolysine is the ${22^{{\text{nd}}}}$ amino acid, Cystine is the ${6^{{\text{th}}}}$ amino acid, and Histidine is the ${9^{{\text{th}}}}$ amino acid.

Hence, the correct answer is option (B).

Note: Amino acids are a type of organic compounds that are made up of an amine group i.e. ${\text{N}}{{\text{H}}_2}$and a carboxyl group ${\text{COOH}}$. These are the functional groups that present along with a side chain that is specific to each amino acid. The main components of an amino acid are carbon, hydrogen, oxygen, and nitrogen. There are also some other elements found in the side chains of certain amino acids. These are the main components of protein which are responsible for the growth and development of the human body. Selenocysteine is the ${21^{{\text{st}}}}$amino acid.