(i) Fibrous protein versus Globular protein
| Fibrous protein | Globular protein |
|---|
| The polypeptide chains lie parallel to one another and are held by hydrogen and disulphide bonds, giving a thread-like (fibre) shape | The chains coil around to give a spherical (globular) shape |
| Generally insoluble in water | Generally soluble in water |
| Serve a structural role in the body | Serve a functional role — enzymes, hormones, transport |
| Comparatively stable to changes in temperature and pH | Easily denatured by heat or change in pH |
| Examples : keratin (hair, nails, wool), myosin (muscles), collagen | Examples : insulin, haemoglobin, albumin |
(ii) Peptide linkage versus Phosphodiester linkage
| Peptide linkage | Phosphodiester linkage |
|---|
| The amide bond $-CO-NH-$ formed between the $-COOH$ of one amino acid and the $-NH_2$ of another, with loss of water | The bridge formed when a phosphate group esterifies the $-OH$ groups of two sugar units |
| Joins amino acid units | Joins nucleotide units |
| Found in proteins and polypeptides | Found in nucleic acids — DNA and RNA |
| Links the C-1 carboxyl of one residue to the N of the next | Links C-5 of one sugar to C-3 of the next |
| Has partial double bond character, making it planar and rigid | An ordinary ester linkage on each side of the phosphorus |
Formation of a peptide linkage :
$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
Role of the phosphodiester linkage : it forms the sugar-phosphate backbone that runs along each strand of the DNA double helix, with the bases projecting inwards to pair with those of the opposite strand.
