Cells and movement across membranes – WJECEnzymes

An enzyme is a protein that functions as a biological catalyst – a substance that speeds up a chemical reaction without being changed by the reaction.

Structure of enzymes – Higher tier only

Different enzymes contain up to 20 different amino acids linked together to form a chain which then folds into the globular enzyme shape. Enzymes have active sites which only match specific substrates.

Lock and key model

Enzymes are folded into complex shapes that allow smaller molecules to fit into them. The place where these molecules fit is called the active site.

In the lock and key model, the shape of the active site matches the shape of its substrate molecules. This makes enzymes highly specific – each type of enzyme can catalyse only one type of reaction (or just a few types of reactions).

The diagram shows how this works. In this example, the enzyme splits one molecule into two smaller ones, but other enzymes join small molecules together to make a larger one.

If the shape of the enzyme changes, its active site may no longer work. We say that the enzyme has been denatured. Enzymes can be denatured by high temperatures or extremes of pH.

Effect of temperature

As with ordinary chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases. However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function as a biological catalyst.

1. At low temperatures the enzymes and substrates have low kinetic energy. This results in the particles colliding less often, which means there will be fewer successful collisions between the substrate and the enzyme’s active site.
2. As the temperature increases, the kinetic energy increases, leading to more collisions and enzyme substrate complexes formed per unit time. This increases the rate of reaction.
3. At the optimum temperature the maximum number of enzyme-substrate complexes form per unit time.
4. If the temperature continues to increase past the optimum, the increased kinetic energy breaks the weak hydrogen bonds holding the enzyme’s unique active site shape. Enzyme–substrate complexes can no longer form as the substrates no longer fit into the active site. The enzyme is denatured.

Effect of pH

Changes in pH alter the shape of an enzyme’s active site. Different enzymes work best at different pH values.

The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5, but stomach enzymes have an optimum pH of about 2.