Enzymes are biological catalysts, they are large protein molecules. They are many times larger than the molecules involved in the reaction they catalyse. Many enzymes require the presence of an additional molecule a non-protein cofactor. Some of these are metal ions such as Zn²⁺ and Cu²⁺. Some cofactors are small organic molecules called coenzymes. The B-group vitamins thiamine and riboflavin are precursors of coenzymes.

Enzymes bind temporarily to one or more of the reactants of the reaction they catalyse. In doing so, they lower the amount of activation energy needed for the reaction to proceed and this speeds up the reaction. All metabolic reactions are catalysed by enzymes

Figure 1

G = the change in energy

The following two specific examples of enzymes shows the amazing speed at which enzyme molecules can catalyse reactions.

1. Catalase catalyses the decomposition of hydrogen peroxide into water and oxygen.

H₂O₂ H₂O + O₂

In this reaction H₂O₂ is the substrate and H₂O + O₂ are the products. This reaction is important as hydrogen peroxide is a powerful oxidising agent and therefore needs to be detoxified rapidly. A single molecule of catalase can break 5.6 million molecules of hydrogen peroxide each minute. The number of substrate molecules that a single enzyme molecule can catalyse in a given period of time is called the turnover number. In mammals catalase is found in high concentrations in liver cells (hepatocytes).

2. Carbonic anhydrase is found in red blood cells. It catalyses the reaction between carbon dioxide and water, which leads to the formation of hydrogencarbonate ions, which are important in the transport of carbon dioxide by blood.

Enzymes work by lowering the activation energy required for a reaction to proceed. In order to do so, an enzyme molecule must unite - even if very briefly - with the substrate molecule (or molecules). Enzymes are able to do this because an area of their tertiary structure is complementary to the substrate. This area is called the active site and is complementary in shape, charge and hydrophobic / hydrophilic areas. The substrate attaches to the active site by a random collision but is held briefly by noncovalent forces (an assortment of hydrogen bonds, ionic interactions and hydrophobic interactions). Because the substrate must fit into the active site the mode of enzyme action is analogous to a lock and key. Figure 2 below shows the lock and key model of enzyme action   

Figure 2

When the substrate attaches to the active site it triggers a conformational shape change in the enzyme molecule which improves the fit further and causes the reaction to occur. This is referred to as induced fit.

The progress of an enzyme catalysed reaction is shown in figure 3 below.   

Figure 3

This requirement for complementarity in the configuration of substrate and enzyme explains the remarkable specificity of most enzymes. Generally, a given enzyme is able to catalyse only a single chemical reaction or, at most, a few reactions involving substrates with very similar molecular structures.