Proteins - 2

The roles of proteins depend on their molecular shape.

Fibrous proteins:

• Fibrous proteins are made of long, thin molecules arranged to form fibres.
• Their shape make them insoluble in water, so they have structural functions.
• Polypeptides are in parallel chains or sheets, with many cross-linkages forming long fibres.
• Fibrous proteins are tough and strong.
• Collagen is another fibrous protein, consisting of three polypeptide chains coiled round each other in a triple helix. The 3 chains are linked by hydrogen bonds, making it very stable.
• We are largely held together by collagen as it is found in bones, cartilage, tendons and ligaments.

Globular proteins:

• Globular proteins are compact and folded into spherical molecules.
• They are soluble in water and have many functions, including enzymes, antibodies, plasma proteins and hormones.
• Folds are highly specific and a particular protein will always be folded in the same way.
• If the structure is disrupted, the protein ceases to function properly and is said to be denatured.

A globular protein based mostly on an α-helix is haemoglobin.

A globular protein based mostly on a β-pleated sheet is the immunoglobulin antibody molecule.

Functions of proteins

1. Virtually all enzymes are proteins.
2. Structural: e.g. collagen and elastin in connective tissue, keratin in skin, hair and nails.
3. Contractile proteins: actin and myosin in muscles allow contraction and therefore movement.
4. Hormones: many hormones have a protein structure (e.g. insulin, glucagon, growth hormone).
5. Transport: for example, haemoglobin facilitates the transport of oxygen around the body, a type of albumin in the blood transports fatty acids.
6. Transport into and out of cells: carrier and channel proteins in the cell membrane regulate movement across it.
7. Defence: immunoglobulins (antibodies) protect the body against foreign invaders; fibrinogen in the blood is vital for the clotting process.

Biochemical test:

The reagent used to test for proteins is called biuret reagent. It can be used as two separate solutions of copper sulphate and potassium or sodium hydroxide or as a ready-made biuret solution. In either case, a purple colour indicates a positive result.