Explain how enzymes catalyse reactions via the active site and how temperature, pH, concentration and inhibitors affect their activity

What this dot point is asking

You need to explain how enzymes work using the active-site model and explain how the main factors change the rate of an enzyme-controlled reaction. This dot point builds directly on protein structure: an enzyme's function depends on its 3D shape.

What an enzyme is

An enzyme is a biological catalyst, usually a protein, that speeds up a chemical reaction without being used up. Enzymes work by lowering the activation energy (the energy needed to start a reaction), so reactions proceed faster at the relatively low temperatures inside cells.

Each enzyme is specific: it catalyses only one reaction or type of reaction. This specificity comes from the active site, a region whose shape is complementary to a particular substrate.

How enzymes catalyse reactions

1. The substrate binds to the enzyme's active site, forming an enzyme-substrate complex.
2. The active site strains or positions the substrate so the reaction occurs more easily, lowering activation energy.
3. The product(s) are released, and the unchanged enzyme is free to bind another substrate.

Factors affecting enzyme activity

Temperature

As temperature rises, molecules move faster and collide more often, so the reaction rate increases up to an optimum temperature (around 37 degrees Celsius for many human enzymes). Above the optimum, heat breaks the bonds maintaining the enzyme's tertiary structure, so the enzyme denatures, the active site changes shape, and activity falls sharply.

pH

Each enzyme has an optimum pH at which its active site is the correct shape. For example, pepsin in the stomach works best around pH 2, while many cellular enzymes prefer about pH 7. Moving away from the optimum disrupts ionic and hydrogen bonds, changing the active site and reducing activity; extreme pH denatures the enzyme.

Substrate concentration

Increasing substrate concentration increases the reaction rate because more substrate molecules collide with active sites. Eventually all active sites are occupied (saturation), so the rate levels off at a maximum; adding more substrate then has no effect.

Enzyme concentration

If substrate is plentiful, increasing enzyme concentration increases the rate because more active sites are available to bind substrate.

Inhibitors

Inhibitors reduce enzyme activity. Competitive inhibitors resemble the substrate and bind to the active site, blocking it; their effect can be reduced by adding more substrate. Non-competitive inhibitors bind elsewhere on the enzyme, changing the active site's shape so the substrate no longer fits.