We would not survive without enzymes, proteins which are nature's catalysts. They are extremely efficient and highly selective. Click on the animation to see how the active site of an enzyme can accept only one out of a variety of substrates.

Molecular model of an enzyme showing the active site, and the active site occupied by substrate.

The stages of an enzyme catalysed reaction are summarised as

As the reaction proceeds there are subtle changes in shape.

Enzymes are fragile, and very sensitive to changes in pH and temperature which cause changes in shape. Click on the image to see the animated effect of pH changes.

Co-factors may assist with catalysis. These are small organic molecules which can interact with the substrate. Sometimes bound metal atoms play either a structural or catalytic role.

Is there a limit to the increase in rate achievable by enzymes? Yes, the rate at which the products can diffuse away from the active site. In reality few enzymes reach this limit.