Enzymes (AQA A Level Chemistry)

Enzymes

• Enzymes are biological catalysts

  • ‘Biological’ because they function in living systems

  • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or changed

• Enzymes are also globular proteins

• Critical to the enzyme’s function is the active site where the substrate binds

• Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions

  • Virtually every metabolic reaction within living organisms is catalysed by an enzyme – enzymes are therefore essential for life to exist

• Enzymes have an active site where specific substrates bind forming an enzyme-substrate complex

• The active site of an enzyme has a specific shape to fit a specific substrate

• Extremes of heat or pH can change the shape of the active site, preventing substrate binding – this is called denaturation

• Substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur

The active site of an enzyme has a specific shape to fit a specific substrate (when the substrate binds an enzyme-substrate complex is formed)

Enzyme Specificity

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

• The shape of the active site (and therefore the specificity of the enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme:

  • Proteins are formed from chains of amino acids held together by peptide bonds

  • The order of amino acids determines the shape of an enzyme

  • If the order is altered, the resulting three-dimensional shape changes

An example of enzyme specificity – the enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

Drug-receptor Interactions

• Receptors are proteins found on enzymes, cell membranes or DNA

• Most drugs work by their ability to bind to receptors stopping their normal biological activity and interrupting the development of disease

• Drugs bind to receptors generally using intermolecular forces or ionic bonds

• The stronger the interaction the more effective the drug activity

• Drug-receptor interaction has become very important in drug design

• Computers are widely used to model drug-receptor interactions

Only the correct orientation of one enantiomer enable the drug to bind to the biomolecule making it stereoselective

• Many naturally occurring organic molecules consist of enantiomers, in which only one enantiomer is biologically active

• Similarly, many drugs have enantiomeric forms in which only one form of the drug is active

• The site where the drug binds to the biomolecule can only accept one orientation; it is said to be stereoselective