One type of globular protein is the enzyme. Enzymes are biological catalysts which means that they’re able to speed up reactions by lowering the activation energy required. Nearly all biological reactions require enzymes.

Each enzyme has a specific tertiary shape which makes them, in general, specific to one particular reaction. Every enzyme contains an active site where the reaction takes place. The rest of the enzyme maintains its shape.

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Enzyme action

The active site has a specific shape in which its substrate fits. The substrate can be made up of one or more molecules. Once joined, the two form an enzyme-substrate (ES) complex. The reaction then takes place and, once complete, an enzyme-product (EP) complex is formed. The product is then released and the enzyme is left unchanged.

There are two hypothesises about how the substrate and enzyme fit together:

• the lock-and-key hypothesis
• the induced fit hypothesis

The lock-and-key hypothesis

In the lock-and-key hypothesis the shape of the enzyme’s active site exactly complements the shape of the substrate. In other words, the substrate fits into the active site like a key in a lock. Within this model it’s possible for an enzyme to catalyse a reverse reaction too.

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The induced-fit hypothesis

The induced-fit hypothesis suggests that the active site and substrate don’t complement each other exactly but, instead, the active site changes shape. When the correct substrate collides with the active site the enzyme recognises it and changes shape, thereby allowing it to fit. This model is regarded as a better explanation than the lock-and-key hypothesis as it allows the substrate to fit more closely with the enzyme.

The properties of an enzyme depend on its tertiary structure. The speed at which a reaction occurs will depend on:

• temperature
• pH
• substrate concentration.