(a) Peptide linkage
Definition : The amide bond $-CO-NH-$ formed between the carboxyl group $(-COOH)$ of one amino acid and the amino group $(-NH_2)$ of another amino acid, with the elimination of a water molecule, is called a peptide linkage.
Formation :
$$H_2N-\underset{|}{\overset{R}{C}}H-COOH + H_2N-\underset{|}{\overset{R'}{C}}H-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
Key points :
- The product of two amino acids joining is a dipeptide; many units give a polypeptide, and a long polypeptide is a protein.
- The peptide bond has partial double bond character, which makes it planar and rigid — an important factor in protein folding.
- It is this linkage that is broken during the hydrolysis of proteins into $\alpha$-amino acids.
(b) Essential amino acids
Definition : Amino acids that cannot be synthesised in the human body and must therefore be obtained from the diet are called essential amino acids.
Key points :
- Ten of the twenty amino acids found in proteins are essential.
- Examples : valine, leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine, arginine.
- The remaining ten, which the body can make itself, are the non-essential amino acids — for example glycine, alanine, aspartic acid and glutamic acid.
