EnzymesAQA A-Level Chemistry: Revision notes

Enzymes are proteins that act as biological catalysts. Like all catalysts they speed up a reaction by providing an alternative route with a lower activation energy, and are not used up.

Their shape comes from the protein's tertiary structure, held by hydrogen bonds and S–S bonds.

The active site is a region of the enzyme with a specific three-dimensional shape and arrangement of groups. The substrate has a complementary shape, binds to the active site by intermolecular forces and forms an enzyme–substrate complex.

The substrate is converted into products, which leave the active site. The enzyme is then free to bind another substrate molecule.

Many biological molecules are chiral and exist as two enantiomers, which are non-superimposable mirror images. The active site is stereospecific: it has a specific 3D shape that fits only one enantiomer.

The other enantiomer has the same formula but its groups are arranged differently in space, so it cannot bind in the same way. This applies to substrates and drugs.

An enzyme inhibitor is a molecule that binds to the active site and blocks it. A drug whose shape is complementary to the active site (often similar to the substrate) fits into it, so the substrate cannot bind and the reaction slows or stops.

This is the principle behind many drugs, such as antibiotics that block enzymes bacteria need.

Computer modelling shows the 3D shape of the active site and predicts how well candidate molecules fit and bind. Many molecules can be screened before any are made, saving time and cost and reducing laboratory work.

Because the active site is stereospecific, a drug is often made as a single enantiomer: the other is wasted, may need a larger dose, and can cause side effects.