Classification of proteins
Proteins are grouped by shape (fibrous, globular), by composition (simple, conjugated, derived), by function, and by nutritional quality (complete, incomplete).
By shape
Shape is judged by the axial ratio (length ÷ width): high in fibrous proteins, low in globular ones.
By composition
| Class | What it is | Examples |
|---|---|---|
| Simple: albumins | Only amino acids on hydrolysis; soluble in water, coagulate on heating | Serum albumin, egg albumin (ovalbumin), lactalbumin (milk) |
| Simple: globulins | Insoluble in pure water, soluble in dilute salt solution | Serum globulins, ovoglobulin (egg), legumin (peas), arachin (groundnut) |
| Simple: glutelins | Soluble in dilute acid or alkali | Glutenin (wheat), oryzenin (rice) |
| Simple: prolamines | Soluble in dilute (70–80%) alcohol, not in water | Gliadin (wheat), zein (maize), hordein (barley) |
| Simple: scleroproteins (albuminoids) | Insoluble, fibrous, structural | Collagen, keratin, elastin |
| Simple: histones, protamines | Basic proteins that bind DNA | Histones in chromosomes; protamines in fish sperm |
| Conjugated: glycoproteins | Protein + carbohydrate (the non-protein part is called the prosthetic group) | Mucin (saliva), immunoglobulins |
| Conjugated: lipoproteins | Protein + lipid | Chylomicrons, VLDL, LDL, HDL |
| Conjugated: phosphoproteins | Protein + phosphate | Casein (milk), vitellin (egg yolk) |
| Conjugated: chromoproteins | Protein + coloured group | Haemoglobin (haem), rhodopsin, flavoproteins |
| Conjugated: metalloproteins | Protein + metal ion | Ferritin (iron), ceruloplasmin (copper), carbonic anhydrase (zinc) |
| Conjugated: nucleoproteins | Protein + nucleic acid | Nucleohistones, ribosomes |
| Derived: primary | Slightly changed (denatured) proteins | Coagulated proteins (boiled egg white), proteans, metaproteins |
| Derived: secondary | Products of step-by-step hydrolysis | Proteoses → peptones → polypeptides → peptides |
By function
| Function | Examples |
|---|---|
| Enzymes (catalytic) | Amylase, pepsin, trypsin |
| Structural | Collagen, keratin, elastin |
| Transport | Haemoglobin (oxygen), albumin (fatty acids, bilirubin), transferrin (iron) |
| Hormones (regulatory) | Insulin, glucagon, growth hormone |
| Defence (protective) | Antibodies (immunoglobulins); fibrinogen and thrombin for clotting |
| Storage | Ferritin (iron); casein and ovalbumin (nutrient reserve) |
| Contractile | Actin, myosin |
By nutritional quality
| Class | Meaning | Examples |
|---|---|---|
| Complete (first class) | Has all essential amino acids in the right amounts; supports growth | Egg, milk (casein), meat, fish |
| Partially incomplete | Low in one or more essential amino acids; maintains life but supports growth poorly | Rice and wheat (low lysine), pulses (low methionine) |
| Incomplete | Completely lacks one or more essential amino acids; cannot support growth | Gelatin (lacks tryptophan), zein of maize (lacks tryptophan and lysine) |
Most Indian diets get protein from cereals and pulses, which are partially incomplete. Combining them (rice with dal, idli, dosa, khichdi, chapati with dal) gives a near-complete mix: this is mutual supplementation. Adding a little milk, curd or egg raises quality further. Protein quality is measured by biological value (BV), net protein utilisation (NPU) and PDCAAS.
2 marks: “What are conjugated proteins? Give two examples.” Proteins joined to a non-protein prosthetic group, e.g., haemoglobin (haem) and casein (phosphate).
5 marks: “Classify proteins based on nutritional value.” Complete, partially incomplete and incomplete, each with its meaning and two food examples; add cereal–pulse supplementation.
10 marks: “Classify proteins with suitable examples.” Tables for shape, composition (simple, conjugated, derived), function and nutritional quality, with two examples in each class.
Simple, Conjugated, Derived = Solo, Couple, Damaged: a simple protein is amino acids alone, a conjugated one has a partner group, a derived one is broken down or changed.