(i) Peptide linkage versus Glycosidic linkage
| Peptide linkage | Glycosidic linkage |
|---|
| The amide bond $-CO-NH-$ formed between the $-COOH$ group of one amino acid and the $-NH_2$ group of another, with loss of water | The ether bridge $-O-$ formed between two monosaccharide units through their hydroxyl groups, with loss of water |
| Found in proteins and polypeptides | Found in disaccharides and polysaccharides |
| Links amino acid units | Links monosaccharide units |
| Has partial double bond character, making it planar and rigid | An ordinary ether linkage, freely rotating |
Formation of a peptide linkage :
$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
Formation of a glycosidic linkage :
$$\text{Monosaccharide} + \text{Monosaccharide} \rightarrow \text{Disaccharide} + H_2O$$
Example : in maltose an $\alpha$-1,4-glycosidic linkage joins two glucose units.
(ii) Essential versus Non-essential amino acids
| Essential amino acids | Non-essential amino acids |
|---|
| Cannot be synthesised by the human body | Can be synthesised within the body |
| Must be supplied through the diet | Need not be supplied in the diet |
| Ten of the twenty amino acids | The remaining ten |
| Examples : valine, leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine, arginine | Examples : glycine, alanine, aspartic acid, glutamic acid, serine |
A deficiency of any essential amino acid in the diet leads to poor growth and protein deficiency disorders such as kwashiorkor.
