Isoleucine is an $\alpha$-amino acid that is used in the biosynthesis of proteins. It contains an $\alpha$ amino group, an $\alpha$-carboxylic acid group, and a hydrocarbon side chain with a branch. It is classified as a non-polar, uncharged, branchedchain, aliphatic amino acid.

Lysine is an $\alpha$-amino acid that is a precursor to many proteins. It contains an $\alpha$-amino group, an alpha-carboxylic acid group, and a side chain lysyl, classifying it as a basic, charged, aliphatic amino acid. It is encoded by the codons AAA and AAG. Glutamic acid is an amino acid used to form proteins.

Cysteine and methionine are sulphur containing amino acids. Amino acids get linked to one another by peptide bond formation and form a polypeptide chain of proteins. Hence cysteine and methionine are found in several proteins.

(a) isoleucine


(b) cysteine


(c) lysine


(d) methionine


(e) glutamic acid