Importance of Proteins: MDCAT Chemistry notes
Importance of Proteins for MDCAT: structural, transport, storage and hormonal proteins, ceruloplasmin, ferritin, casein uses and digestion.
Why proteins matter
Proteins make up most of the dry mass of cells and do almost every job in the body: they build tissues, carry substances, store nutrients, catalyse reactions, send signals and defend the body. MDCAT questions usually give a protein and ask its function, or give a function and ask the protein.
Functional classification
| Class | Function | Examples |
|---|---|---|
| Structural | Build and support tissues | Collagen (connective tissue: tendons, ligaments, skin, bone, cartilage); keratin (hair, nails, feathers) |
| Transport | Carry substances in blood or across membranes | Haemoglobin (oxygen in red cells); ceruloplasmin (copper in plasma); transferrin (iron in plasma); lipoproteins (lipids) |
| Storage | Store nutrients or ions | Ferritin (iron, in liver, spleen and bone marrow); casein (milk); ovalbumin (egg white) |
| Hormonal | Carry chemical messages | Insulin, growth hormone; thyroxine is also placed in this group in the textbook |
| Enzymes | Catalyse reactions | Pepsin, trypsin, amylase |
| Defensive | Protect the body | Antibodies (immunoglobulins); fibrinogen and thrombin in clotting |
| Contractile | Movement | Actin, myosin |
| Genetic | Associated with nucleic acids | Nucleoproteins, histones |
Haemoglobin is a large conjugated protein (globin + haem) found in red blood cells; it is a transport protein, not a structural one.
Hydrogen bonding in biomolecules
Hydrogen bonding is the strongest of the intermolecular forces. It holds together:
- the $\alpha$-helix and $\beta$-pleated sheet of proteins (between C=O and N–H of peptide groups);
- the two strands of the DNA double helix (Watson–Crick model), where bases pair A=T with two H-bonds and G≡C with three.
Digestion of proteins
Proteins cannot be absorbed intact. In the stomach pepsin begins breaking peptide bonds; in the small intestine trypsin, chymotrypsin and peptidases continue. The final products available for absorption are amino acids and small peptides (di- and tripeptides), which are absorbed through the intestinal wall. They are then used to build new proteins, or deaminated for energy.
Industrial uses
- Casein (milk protein) is used to make buttons, buckles and other plastics, as well as glues and paints.
- Gelatine, from collagen, is used in foods, jellies, medicinal capsules and photographic film.
- Leather is produced by tanning the collagen of animal hides.
- Wool and silk are natural protein fibres.
Nutritional importance
Proteins supply the essential amino acids that the body cannot synthesize. A protein-deficient diet in children leads to kwashiorkor. Meat, eggs, milk, fish and pulses are good sources.
Common MDCAT traps
- Ceruloplasmin carries copper; ferritin stores iron. Do not swap them, and do not confuse storage (ferritin) with transport (transferrin).
- Collagen is the connective-tissue protein; lecithin is a phospholipid and cholesterol a steroid.
- Plasma is not a macromolecule; haemoglobin is.
- Protein digestion ends in amino acids and small peptides, not polysaccharides.
- DNA strands are held by hydrogen bonds, not ionic bonds.
Quick revision
- Haemoglobin: transport of $\mathrm{O_2}$.
- Ceruloplasmin: copper carrier.
- Ferritin: iron store.
- Casein: buttons and buckles.
- Thyroxine: classed as hormonal.