Enzymes are biological catalysts that speed up the rate of chemical reactions within organisms. They do this by lowering the activation energy of a reaction, meaning less energy is required to start the reaction. Without enzymes many of the reactions within an organism would occur to slowly to sustain life.
Structure of Enzymes
Enzymes are globular proteins, with a 3D structure, made up of many amino acids. The sequence of amino acids that make up an enzyme are encoded for by DNA.
Enzymes have an active site, which is the point at which the reaction occurs. The active site has a specific shape which can bind to a substrate. This unique shape is determined by the sequence of amino acids in the enzyme’s primary structure.
Substrate – a molecule that an enzyme acts on.
Due to the specificity of the active sites, enzymes are selective to a particular substrate.
The Induced Fit Model
The induced fit model provides a simple explanation of how an enzyme work. The diagram below shows the steps involved:
Step 1 – the substrate enters the active site of the enzyme.
Step 2 – the substrate binds to the active site, as this happens the active site changes shape slightly. This forms the enzyme-substrate complex.
Step 3 – the reaction occurs and the substrate is converted into the products of the reaction. This forms the enzyme-product complex.
Step 4 – the products are released from the enzyme.
Increasing the Rate of Reaction
As mentioned above, enzymes work by lowering the activation energy of a reaction. This can be seen in the graph below:
EA – the activation energy of a reaction with an enzyme.
EA – the activation energy of a reaction without an enzyme.
The enzymes themselves are unchanged once the reaction is complete.
The Effect of Temperature and pH on Enzymes
Generally, increasing temperature increases the rate of a reaction. However, if the temperature reaches a certain point it can cause bonds in the active site of the enzyme to break. The enzyme can denature, changing the shape of the active site.
This prevents the enzyme from functioning and so the rate of reaction dramatically reduces.
Enzymes can also be affected by pH. All enzymes have an optimum pH at which they work best. However, if the pH is altered too much the enzyme will denature (the active site changes shape). If the enzyme can no longer function, the rate of reaction will fall sharply.
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