Question

Which one is diamino dicarboxylic amino acid?

• A. Cystine
• B. Lysine
• C. Valine
  
• D. Aspartic Acid

Answer 1

The correct option is A Cystine

Amino acids are amphoteric compounds having a carboxylic group, amino group, hydrogen and a substituent group bonded to same carbon atom called the $\alpha$ carbon atom. Lysine is a basic amino acid having two amino groups. Hence, it is a diamino amino acid. Aspartic acid is an acidic amino acid having two carboxylic groups. Hence, it is a dicarboxylic amino acid. Cysteine is a sulphur containing amino acid having one carboxylic and one amino group. Cystine is the amino acid which is formed by bonding together of two units of cysteine through disulfide bond. Carefully, note the different spelling of these two amino acids, i.e., Cystine and Cysteine. Cystine contains two carboxylic groups and two amino groups because it is formed by two units of amino acids. Hence, cystine is diamino dicarboxylic amino acid while cysteine is monoamino monocarboxylic amino acid.