Enzymes and chemical reactions

Almost all enzymes are proteins. Some nucleic acids also act as enzymes; these are ribozymes.

Like any protein, an enzyme has primary, secondary and tertiary structure. As the chain folds in the tertiary structure it crosses over itself and leaves crevices or pockets. The active site is the pocket into which the substrate fits, and through it the enzyme catalyses reactions at a high rate.

Inorganic catalysts work best at high temperature and pressure, whereas enzymes are damaged at high temperatures (above about 40°C). Enzymes from organisms of hot vents and sulphur springs (thermophiles) stay stable and active up to 80–90°C.

A physical change alters shape or state without breaking bonds, such as ice melting or water turning to vapour. A chemical reaction breaks bonds and forms new ones, for example Ba(OH)2 + H2SO4 → BaSO4 + 2H2O, or the hydrolysis of starch into glucose.

The rate of a process is the amount of product formed per unit time (δP/δt); with a direction it is called velocity. As a rule of thumb, the rate doubles or halves for every 10°C rise or fall in temperature.

Catalysed reactions run far faster than uncatalysed ones. CO2 + H2O → H2CO3 forms about 200 molecules of carbonic acid an hour without an enzyme, but about 600,000 every second with carbonic anhydrase, roughly a 10 million-fold speed-up.

A multistep reaction whose steps are catalysed by one enzyme complex or several enzymes is a metabolic pathway. Glucose becomes pyruvic acid through ten enzyme-catalysed reactions.

With one or two extra reactions the same pathway gives different end products: lactic acid in skeletal muscle under anaerobic conditions, pyruvic acid under normal aerobic conditions, and ethanol in yeast during fermentation.