BY1- Enzymes

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  • Created by: georgia s
  • Created on: 20-01-13 19:08

Enzyme Structure

Enzymes are globular proteins that act as biological catalysts. They speed up the rate of metabolic reactions. Some enzymes catalyse reactions where larger molecules are broken down into smaller ones. Other catalyse reactions where small molecules are built up into larger, more complex molecules. 

Enzymes are tertiary globular proteins where the protein chain is folded back on itself into a spherical or globular shape with hydrophillic R groups on the outside of the molecule,making enzymes soluble. Each enzyme has its own sequence of amino acids and is held in its tertiary form by hydrogen bonds, disulphie bridges and ionic bonds. This complex and precise 3-D shape gives the enzyme many of its properties. Although the enzyme molecule is large, only a small region, called the active site is functional. 

Enzymes react with another molecule called a substrate. Each enzyme as its own special shape, with an area, the active site, onto which the substrate molecules bind. Only three to twelve amino acids make up the active site but its shape is an exact fit for the substrate. The combination of enzyme and substrate in place. When the reaction is complete, more products are released, leaving the enzyme unchanged and ready to receive another substrate molecule.

Properties of Enzymes

  • Each enzyme will act on only one type of substrate molecule. This is becuse the shape of the active site allows only one shape of molecule to fit with it. The particular enzyme is said to be specific for that particular substrate.
  • Enzymes are very fast acting and are said to have a high turnover number. This means that they can converrt many molecules of substrate per unit time. 
  • Reactions need energy to start them off otherwise the reactants will not be converted to products. This energy is called activation energy and is the energy needed to break existing chemical bonds inside molecules. One way of making chemicals react togehter is to increase the energy in the form of heat. Living organisms cannot do this as temperature above 40 degree celsius will cause irreversble damge to the structural and functional proteins in their bodies. 


For a reaction to occur the reactants must collide with sufficient energy to alter the arrangment of the atoms in the molecules, meaning- an initial boost of energy is needed, that is, and activation energy level which must be overcome before the reaction can proceed.

In chemical reactions this may be carried out by heating. Enzymes work by lowering the activation energy. When a substrate enters the active site of an enzyme, the shape of the molecule is altered slightly. This enables metabolic processes to take place at temperatures relatively lower that chemical reactions not catalysed by enzymes. 

Models of Enzyme Action

The lock and key theory proposes that enzymes work in the same way as a key operates a lock. Each key has a specific shape that fits and operates a particular lock. In a…


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