Question

Which one is diaminodicarboxylic amino acid?
A. Cystine
B. Lysine
C. Cysteine
D. Aspartic Acid

Answer 1

Proteins are polymers of amino acids, which are building blocks and structural units of proteins. They are the monomers of proteins. They contain an amino group, carboxyl group, hydrogen atom and distinctive side chain. They all bond to the alpha carbon atom. The side chain varies for different amino acids. Amino Acids are essential for growth of the body, repair of body tissue and breaking down of food matter. They are also involved in protein synthesis and nutrient reabsorption.

Complete answer:
Option A Cystine: Cystine is a sulfur containing amino acid. It has two units of cysteine amino acids. It has two amino groups, hence the name diamino is correct. Also, two carboxylic groups are present, so the name dicarboxylic is again correct. Thus, cysteine is diaminodicarboxylic amino acid.
Option A is correct.
Option B Lysine: Lysine is a basic amino acid. There are two aminoacids present, so it is a diamino amino acid. But there is only one carboxyl group present. So, it is not dicarboxylic.
Hence, Option B is not correct.
Option C Cysteine: Cysteine is present in human hair. It contains a single amino group. So, it is diamino amino acid.
So, Option C is not correct.
Option D Aspartic Acid: Aspartic acid is an acidic amino acid. It contains two carboxylic groups, so it is dicarboxylic. But there is a single amino group only. So, it is not diamino amino acid.
Option D is not correct.

Therefore, Option A) is the correct answer.

Note:
Amino Acids are amphoteric compounds, since they contain both amino group and carboxylic group. They have the capability to either accept or donate a proton. They act as zwitterion at specific pH called isoelectric point. The $21$ amino acids, which involve bodybuilding, are standard amino acids.