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Human Biology (Year 11) - Cell Metabolism

Gemma Tueno

What is an Enzyme?

Enzymes are proteins that act as biological catalysts, meaning they speed up chemical reactions. Without enzymes, the same reactions would either be too slow or wouldn’t happen. This is because chemical reactions require a certain amount of energy (activation energy) to occur. Enzymes provide an alternate chemical pathway that has a lower activation energy, allowing the reactions to occur at body temperature. This concept is often graphed (see below) to show how there is a separate pathway with lower activation energy compared with the original reaction pathway.

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Enzymes work by binding to a target molecule called a substrate at their active site. Enzymes can only bind to specific substrates because the enzyme and the substrate have complementary properties (such as their shape/structure) that allow them to fit together like how a key fits into a lock. As such, the enzyme-substrate relationship is described as a lock-and-key model.

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Enzymes are produced by ribosomes and have a specific 3D shape that allows them to bind with their target molecules. This specific 3D shape also means they only have one specific target molecules (aka substrates).

What factors affect the functioning of an Enzyme?

Temperature and pH

Enzymes are pH (the acidity of an environment) and temperature sensitive and work within a small, optimal range. Temperatures and pH levels outside of the optimum range will cause the enzyme to change in shape. If you change their shape, they won’t work properly because they will no longer be able to bind with their target molecule and therefore become denatured. It is important to remember that each enzyme has its own specific optimum range of temperature and pH levels (just like how they are each specific to a substrate)

Concentration of Enzyme and Substrate

The higher the concentration of the enzyme, the faster the rate of reaction because more substrate molecules will be able to bind at one time. However, this rate can only increase while there are still available substrate molecules and if they all get used up the rate of reaction will plateau. Increasing the concentration of substrate will also increase the rate of reactions but only until all the available enzymes have been used up. When this occurs, the rate of reaction can no longer increase and will plateau. The human body can regulate which reactions occur and their rate by regulating type and amount of enzymes present.


Enzymes also require the presence of smaller molecules called cofactors (sometimes also called co-enzymes) that increase the ability of the enzyme to bind with the substrate. These molecules can be ions or vitamins or minerals. Sometimes, enzymes will naturally exist in a state where their active site is not configured to bind with their substrate. The binding of these co-factors can change the shape of the active site so that the enzyme can combine with its substrate.


Enzyme inhibitors are molecules that slow or stop the enzyme’s activity by binding to it and changing its shape so that it can no longer bind to its substrate. Competitive inhibitors bind directly to the active site, physically blocking the enzyme and substrate from binding. Non-competitive inhibitors bind to a different part of the enzyme, causing the enzymes shape to change, preventing it from binding with the substrate and decreasing the rate of reaction.


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