Question

In the above general formula for an amino acid, the letter R stands for

a. Amino group
b. A carboxyl group
c. A variable group
d. A hydroxyl group

Answer 1

Amino acids have four substituent groups occupying four valency positions namely hydrogen, carboxyl group, amino group, and a variable group. Amino acids are organic compounds. They are called $\alpha$ -amino acids because they contain an amino group and an acidic group as substituents on the same carbon which is called the $\alpha$ -carbon.

Complete answer:
The variable group is designated as R. The R group in these amino groups could be hydrogen (then the amino acid is called Glycine), a methyl group (alanine), a hydroxymethyl group (serine), etc. The amino, carboxyl, and the R group determine the physical and chemical properties of amino acids.

Amino acids can be classified into seven types:
• Neutral amino acids have one amino group and one carboxylic group with a noncyclic hydrocarbon chain. For example Glycine
• Acidic amino acids have a carboxyl group as their functional group. For example Glutamic acid
• Basic amino acids have an extra amino group. For example Lysine.
• Sulphur containing amino acids contain sulphur. For example Cysteine
• Alcoholic amino acids have a hydroxyl group. For example Serine
• Aromatic amino acids have cyclic structures with a straight side chain. For example: Tryptophan
• Heterocyclic amino acids have nitrogen in the ring structure. For example: Histidine

Hence, the correct answer is option (C).

Additional information:
• Proline is the only $\beta$ - amino acid.
• There are many non-protein amino acids like $\gamma$ -amino butyric acid, ornithine.

Note: Peptides are formed by the condensation of amino acids. There are $20$ amino acids of which $10$ are essential amino acids and $10$ are non-essential amino acids. Amino acids are the building blocks of proteins and enzymes. Amino acids are left behind when proteins are digested or left behind.