Proteins and enzymes

Proteins are long chains of α-amino acids: acids with an –NH₂ group on the carbon next to the –COOH. About 20 different amino acids make up all our proteins. They join when the –COOH of one reacts with the –NH₂ of the next, losing water and forming a peptide bond, –CO–NH–. A long chain of them is a polypeptide.

Amino acids as zwitterions. In water, the acidic –COOH gives its H⁺ to the basic –NH₂ in the same molecule, giving an ion with both a positive and a negative charge: a zwitterion. That explains their properties: they are crystalline solids with high melting points, they dissolve in water, and they can react with both acids and bases (they are amphoteric).

Some amino acids are made in our bodies: the non-essential ones. The rest, the essential amino acids, cannot be made by the body and must come from our food.

Proteins are described at four levels:

• Primary structure: the sequence of amino acids in the chain. Change even one, and the protein can stop working.
• Secondary structure: the way the chain is shaped locally, held by hydrogen bonds between –CO– and –NH– groups. Either an α-helix, a spiral like a telephone cord, or a β-pleated sheet, chains lying side by side like a folded fan.
• Tertiary structure: how the whole chain folds in space, held by hydrogen bonds, disulphide links, and other attractions.
• Quaternary structure: how two or more chains fit together, as in haemoglobin.

By overall shape, proteins are fibrous (long, thread-like, insoluble in water, found in hair, silk and muscle) or globular (curled into a ball, usually soluble, like insulin and albumin).

Denaturation. Heat, or a change in pH, breaks the hydrogen bonds that hold the shape. The helices uncoil and the globules unfold, and the protein loses its activity: the secondary and tertiary structures are destroyed, but the primary structure (the sequence) is unchanged. Boiling an egg is a familiar example: the clear albumin turns white and solid. So is milk curdling into curd.

Enzymes are proteins that act as catalysts in living things. They are very specific: each works on one reaction or one kind of molecule, like a key in a lock. They work fast at body temperature and lower the activation energy of the reactions they catalyse.