Enzymes

• Enzymes are globe shaped proteins that act as biological catalysts to speed up chemical reactions
• Enzymes speed up reactions without actually being used up themselves because they are able to reduce the activation energy needed for a reaction to take place.
• Almost all essential biological reactions are catalysed by enzymes so that they can occur quickly enough to meet the body’s metabolic needs.
• Enzymes have a very specific structure which is essential to how/if they work.
• Active site – A specific region on an enzyme which is small indentation in the structure.
• Substrate – The molecule on which the enzyme is acting.
• The active site and the substrate fit closely together to form enzyme-substrate complexes.
• The substrate is held temporarily within the active site by bonds that form between the enzyme’s amino acids and parts of the substrate molecule. This reduces the activation energy.
• Induced fit model – Suggests that the enzyme modifies it’s shape slightly in order to more accurately fit the shape of the substrate.
• Lock and key model - Suggests that the enzyme and the substrate have exactly complementary shapes.
• In order for an enzyme to work, it must:

  Come in to contact with the substrate

  Have an active site with a complimentary shape to the substrate
• In order to measure the rate of reaction, we can measure the rate of appearance/disappearance of products. We can measure the rate of reaction by dividing the amount of product formed by the time it takes for this product to be formed. We can also do this by measuring the tangent of a graph showing the amount of product formed.
• Increasing temperature increases kinetic energy of molecules. This means the enzyme and substrate collide more frequently, resulting in more successful collisions which forms more enzyme-substrate complexes.
• As the temperature increases bonds in the enzyme start to break so the enzyme structure changes. This process begins around 45'C and at 60'C denatures the enzyme, so it no longer works.
• pH can affect enzymes in different ways. A change in pH changes the charges on the amino acids that make up the active site. This means that the substrate can no longer become attached to the active site. A change is pH may also cause the bonds holding the tertiary structure of an enzyme in place to break. This can change the shape of the enzyme including that of the active site and denature the enzyme.
• If there is excess substrate, an increase in enzyme concentration leads to a proportionate increase in the rate of reaction. However, if there is not enough substrate to use up all of the enzymes’ active sites, then any increase in enzyme concentration will not lead to an increase in the rate of reaction.