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An enzyme is a globular protein molecule with a specific tertiary structure that acts as a biological catalyst.



Enzymes consist of hundreds of amino acids and their shape is determined by their tertiary structure. They are held together by hydrogen bonds and ionic forces.


Biological Catalysts

Enzymes speed up chemical reactions by lowering the activation energy, so they are often referred to as biological catalysts. Unlike inorganic catalysts, enzymes are specific to one type of substrate. The active site of the enzyme is the area where the catalytic activity occurs and each enzyme has its own specific active site shape. They are formed by a small number of proteins in a tertiary structure. This structure has to be maintained for the enzyme to work properly. The catalytic activity is affected by temperature and pH.

Different reactions in the body using enzymes:

·      ->Metabolism

·      ->Respiration

·      ->Digestion


Examples of reactions that are catalysed in the body:

·     ->Breakdown of milk sugar, lactose into glucose and galactose monomers by lactase

·     ->Breakdown of hydrogen peroxide to water and oxygen gas by catalase

·     ->Breakdown of ATP to produce ADP and a phosphate group; This reaction releases a small amount of energy that is used to drive energy requiring processes such as active transport by ATP-ase

       ->Extracellular enzymes catalyse reactions outside the cell and intracellular enzymes catalyse reactions inside the cell.


The lock-and-key theory describes how enzymes bind to substrates. According to the theory, the shape of the enzyme’s active site is complementary to the substrate molecule.

However, we now think that this occurs a little bit differently.

Induced-Fit Hypothesis

 According to the induced-fit hypothesis, the enzyme’s shape changes slightly when the substrate molecule collides with the enzyme’s active site. The active site then unites with the substrate (this is called enzyme-substrate complex). The shape changes of the enzyme place a strain on the substrate molecule, which destabilises it and makes the reactin occur more easily. A product is then produced. This is called the enzyme-product compley. As the product(s) no longer fit into the active site, they move away.


Effect of temperature, pH and concentration


Increasing the temperature increases the rate of reaction of an enzyme-controlled reaction at first but, as the temperature increases further, the rate of reaction decreases until the enzyme stops working.

This is because kinetic energy, which makes the molecules move around contuinually, is increased by heat. As the molecules move around faster and with more force when a gas or liquid is heated, there


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