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Intramolecular bonds

Forces between molecules or between parts of a molecule e.g. hydrogen bonds.

Amino acid

The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body.

Active site

The specially shaped site on an enzyme where the substrates of the reaction bind. It is formed by the folding of the amino acid chains which make up the protein.


A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.


A list of often difficult or specialised words with their definitions.


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.

Enzymes – biological catalysts that control the reactions of life

Inside every cell hundreds of chemical reactions take place. Enzymes help control the rate of all these reactions and make sure that they take place at the right place and in the right order for the cell to survive.

How do the right chemicals react together in your cells? Why do reactions take place so fast inside a cell? Why is it so dangerous if you have a fever that goes over 40°C? How can organisms survive in hot springs at temperatures above the boiling point of water?

Enzymes are globular proteins, built up of chains of amino acids. The shape of the molecule is held together by intramolecular bonds. The shape of an enzyme is key to the way it functions. Anything that affects the shape of the active site affects the function of the enzyme itself.

Enzymes control the rate of all the reactions in the body. Specialised feedback control systems enable the rate of complex chains of reactions to be matched to the demands of the body.

Enzymes are increasingly being used in industry, harvesting their power of catalysis to enable industrial process to take place in conditions of low pressure and temperature - making them much more economically viable.

Enzyme substrate complex

Photos by Anthony Short unless credited otherwise. Animations and diagrams by Edward Fullick throughout.


Enzymes work by forming structures called enzyme-substrate complexes. The structure of an enzyme is closely related to its function in the cell. The model on the left shows COX-2, an enzyme inhibited by aspirin (Jeff Dahl, public domain).

How to use this site

There are a number of interactive features in this e-source:

  • A glossary of terms: any word with a glossary entry is highlighted like this. Moving the mouse over the highlighted word will show a definition of that word.
  • Quick questions: at the end of most pages or sections there is a question or set of quick questions to test your understanding.
  • Animations: most of the animations can be expanded to full screen size, ideal for showing on an interactive whiteboard. The animations will play all the way through or can be viewed one section at a time.
  • Downloads: Teachers can download individual diagrams, animations and other content from the Download Library area of the website. Terms and Conditions apply.