Enzyme Action

Seite 1

Falling apart and staying together

Covalently bonded molecules do not just assemble or break up - they are far too stable.

Maltose is made up of two glucose molecules joined by a glycosidic bond. In order to split maltose into two glucose molecules, the glycosidic bond needs to be broken. At the same time, a water molecule must split. The parts of the split water molecule must then bond back onto the split parts of the maltose molecule to re-form glucose molecules.

In order to make this reaction happen, maltose can be boiled in acid. This provides the right conditions for maltose molecules to collide with water molecules to achieve hydrolysis. The extra energy that is needed to enable the reaction to take place is called the activation energy.

Reducing activation energy

Enzymes work by reducing the amount of activation energy required. This means that reactions can proceed quickly at temperatures much lower than the boiling point. They can do this because of the way the active site is shaped to fit the substrate molecule.

An enzyme has a specifically shaped active site. The shape of the active site is complementary to the shape of the substrate molecules.

Because the substrate fits into the enzymes, the term lock-and-key is sometimes used to descrive how enzymes work.

We now think that changes in the shape of the enzyme occur as the substrate binds to the active site. We also believe that the charges on the amino acids in the active site also contribute to holding the substrate so that the reaction can occur. This is called the induced-fit hypothesis.

Induced-Fit Hypothesis

As a substrate molecule collides with an enzyme's active site, the enzyme molecule changes shape slightly.

This makes the active site fit more closely around the substrate. The substrate fits into place and is also held because oppositely charged groups on the substrate and the active site are found near to each other. This is called an enzyme-substrate complex.

This change in enzyme shape also places a strain on the substrate molecule. This destabilises the substrate molecule, so the reaction occurs more easily.

This produces a product, and is now referred to as an enzyme-product complex.

The products formed are a different shape from a substrate.

Products no longer fit into the active sit and move away.

The enzyme is now able to catalyse the same reaction with another substrate molecule.

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