What are Enzymes?

Enzymes are proteins made up of long chains of amino acids. They control chemical reactions that take place in all living things such as digestion, respiration and photosynthesis. They are known as Biological catalysts as they speed up chemical reactions allowing them to occur at a faster rate. During this process, however, the enzymes themselves do not get used up and so can continue catalysing the reaction.

Each enzyme has its own specific 3-dimensional structure and is folded into a unique shape which allows other molecules to fit into it. The area where these molecules fit into is known as the enzyme’s active site. The molecules which fit into the active site are known as the substrate. The enzyme and the substrate fit together using a lock and key mechanism.

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How Enzymes Work

Enzymes work by intervening in chemical reactions by locking onto the substrate at their active site and speeding up the reaction. Enzymes can work in different ways, some enzymes help to break down large molecules into smaller ones, others build up large molecules from smaller ones whilst there are some that help change one molecule into another.

One of the fastest enzymes known is catalase. Catalase is found in many living cells and breaks down hydrogen peroxide into water and oxygen. Hydrogen peroxide is often formed as a product of reactions in cells and can be poisonous if it builds up it is the job of catalase to remove the harmful hydrogen peroxide.

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How Enzymes Work - 2


The substrate (hydrogen peroxide) and the enzyme (catalase) are continuously on the move. Every so often they collide so that the substrate molecule fits into the enzyme’s active site. Once in the active site the reaction takes place between the substrate and enzyme, the substrate is broken down into the two products (water and oxygen). When the required product has been produced, the enzyme releases itself and moves on. The enzyme is not used up in the reaction and so is ready to work again! No matter what job an enzyme does it will work in this similar way.

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Optimum conditions for enzymes

Enzymes are sensitive and work best under specific conditions. Enzyme activity is affected by several things and each type of enzyme has its own specific optimum conditions under which it works best.

1. Substrate concentration, Enzymes work best when there is a high enough substrate concentration for the reaction they catalyse. If too little substrate is available the rate of the reaction is slowed and cannot increase any further. Sometimes, if there is too much product accumulating, the reaction can also be slowed down. Therefore it is important that the product is removed regularly.

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Optimum conditions for enzymes - 2

2. Temperature

Enzymes are affected a great deal by temperature. If the temperature is too cold the enzymes move around too slowly to meet the substrate and for a reaction to occur. As the temperature increases though, so does the rate of reaction. This is because heat energy causes more collisions between the enzyme and the substrate. However, as you will remember all enzymes are proteins and at too high temperatures the proteins break down. The active site of the enzyme becomes distorted and so the substrate no longer fits and hence the reaction does not occur. We say that the enzyme has been denatured.

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Optimum conditions for enzymes - 3

3. pH

The pH must be correct for each enzyme to work at its best. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on its site of action. For example, enzymes in the stomach have an optimum pH of about 2 because the stomach is acidic, but intestinal enzymes have an optimum pH of about 7.5. If conditions are too alkaline or acidic for that particular enzyme then its activity is affected. This occurs because the enzyme’s shape, especially the active site, is changed so can no longer bind to the substrate molecule. We say that the enzyme is denatured.

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