Enzymes

• Enzyme inhibitors can be competitive or non-competitive.
• • Enzymes are globular proteins with complex tertiary structures
  • Some are formed from a single polypeptide, whilst others are made up of two or more polypeptides and therefore have a quaternary structure
• • Enzymes are globular proteins
  • This means their shape (as well as the shape of the active site of an enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme and is therefore highly specific
• Although it is very similar to the lock and key hypothesis, in this model the enzyme and substrate interact with each other:
• • The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme
• • These changes in shape are known as conformational changes
• • This maximises the ability of the enzyme to catalyse the reaction
• Activation energy is the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
• • Enzymes speed up chemical reactions because they reduce the stability of bonds in the reactants
• • The destabilisation of bonds in the substrate makes it more reactive
• • Rather than lowering the overall energy change of the reaction, enzymes work by providing an alternative energy pathway with a lower activation energy
• Larger organic (carbon-containing) cofactors are known as coenzymes
• • Some coenzymes are permanently bound to the enzyme they assist, often in or near the active site
• • Some coenzymes only bind temporarily during the reaction
• • Coenzymes are involved in carrying electrons or chemical groups between enzymes, aiding in catalysis 
• • Coenzymes link different enzyme-catalysed reactions into a sequence during metabolic processes, such as photosynthesisand respiration
• Particular inorganic ions may help to stabilise the structure of the enzyme or may actually take part in the reaction at the active site

  • For example, chloride ions act as a cofactor for amylase
• • This means that in order for amylase to be able to digest starch into maltose, chloride ions must be present
• • Some cofactors are actually a permanent part of the structure of the enzyme they assist
  • These cofactors are known as prosthetic groups
• Prosthetic groups are essential to the enzyme functioning properly, as they help to form the final 3D shape of the enzyme, the substrate is the molecule that the enzyme is catalysing.
• • For example, by forming part of the active site of the enzyme, a zinc ion acts as the prosthetic group for carbonic anhydrase (an enzyme found in red blood cells that converts CO₂ and H₂O into carbonic acid, H₂CO₃)
• • Cofactors are non-protein substances (i.e. not made from amino acids) that enzymes require in order to function properly. Cofactors can be a temporary part of the enzyme or a permanent part (known as a prosthetic group)
• • Coenzymes are organic non-protein cofactors. Coenzymes contribute to enzyme-catalysed reactions by accepting or donating hydrogen ions or chemical groups (e.g. phosphate groups)