Enzymes

• • Enzymes speed up chemical reactions by acting as biological catalysts.
  • They catalyse metabolic reactions — both at a cellular level (e.g. respiration) and for the organism as a whole (e.g. digestion in mammals).
  • Enzymes can affect structures in an organism (e.g. enzymes are involved in the production of collagen, an important protein in the connective tissuesof animals) as well as functions (like respiration).
  • Enzyme action can be intracellular — within cells, or extracellular — outside cells.
• • Enzymes are proteins
  • Enzymes have an active site, which has a specific shape
  • The active site is the part of the enzyme where the substrate molecules bind to
  • Enzymes are highly specific due to their tertiary structure
• • In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start
  • This is called the activation energy - it is often provided as heat
  • Enzymes lower the amount of activation energy that's needed, often making reactions happen at a lower temperature than they could without an enzyme
  • This speeds up the rate of reaction
• When a substrate fits into the enzymes active site it forms an enzyme-substrate complex - its this that lowers the activation energy. Here's two reasons why

  • If two substrate molecules need to be joined being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily
  • If the enzyme is catalysing a breakdown reaction, fitting into he active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily
• • Enzyme properties are related to its tertiary structure.
  • Enzymes are very specific - they usually only catalyse one reaction e.g maltase only breaks down maltose
  • This is because only one complementary substrate will fit into the active site
  • The active sites shape is determined by the enzymes tertiary structure
  • Each different enzyme has a different tertiary structure and so a different shaped active site
  • If the substrate shape doesn't match the active site, an enzyme-substrate complex won't be formed and the reaction won't be catalysed
• • If the tertiary structure of a protein is altered in any way, the shape of the active site will change
  • This means the substrate won't fit into the active site, an enzyme-substrate complex won't be formed and the enzyme will no longer be able to carry out its function
  • The tertiary structure of an enzyme may be altered by changes in pH or temperature
  • The primary structure of a protein is determined by a gene. If mutation occurs in that gene, it could change the tertiary structure of the enzyme produced
• Measuring the rate of reaction can be done in two ways

  • How fast the product is made - There are different molecules present at the end of a chemical reaction than there are at the beginning. By measuring the amount of end product present at different times during the experiment the reaction rate can be calculated
  • How fast the substrate is broken down - To produce the end products in a reaction, substrate molecules have to be used up. By measuring the amount of substrate molecules left at different times during the experiment the reaction rate can be calculated
• Temperature

  • Like any chemical reaction, the rate of an enzyme-controlled reaction increases when the temperature’s increased.
  • More heat means more kinetic energy, so molecules move faster.
  • This makes the substrate molecules more likely to collide with the enzymes’ active sites.
  • The energy of these collisions also increases, which means each collision is more likely to result in a reaction.
  • But, if the temperature gets too high, the reaction stops.
  • The rise in temperature makes the enzyme’s molecules vibrate more.
  • If the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape.
  • The active site changes shape and the enzyme and substrate no longer fit together. At this point, the enzyme is denatured — it no longer functions as a catalyst
• pH

  • All enzymes have an optimum pH value. Most human enzymes work best at pH 7 (neutral), but there are exceptions. Pepsin, for example, works best at pH 2 (acidic), which is useful because it’s found in the stomach.
  • Above and below the optimum pH, the H+ and OH– ions found in acids and alkalis can disrupt the ionic bonds and hydrogen bonds that hold the enzyme’s tertiary structure in place.
  • The enzyme becomes denatured, and the active site changes shape.
• Substrate concentration

  • The higher the substrate concentration, the faster the reaction — more substrate molecules means a collision between substrate and enzyme is more likely and so more active sites will be occupied.
  • This is only true up until a ‘saturation’ point though.
  • After that, there are so many substrate molecules that the enzymes have about as much as they can cope with (all the active sites are full), and adding more makes no difference
• Enzyme concentration

  • The more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme- substrate complex.
  • So increasing the concentration of the enzyme increases the rate of reaction.
  • But, if the amount of substrate is limited, there comes a point when there’s more than enough enzyme molecules to deal with all the available substrate, so adding more enzyme has no further effect.
• Competitive inhibitors:

  • Competitive inhibitor molecules have a similar shape to that of substrate molecules.
  • They compete with the substrate molecules to bind to the active site, but no reaction takes place.
  • Instead they block the active site, so no substrate molecules can fit in it.
  • How much the enzyme is inhibited depends on the relative concentrations of the inhibitor and substrate.
  • If there’s a high concentration of the inhibitor, it’ll take up nearly all the active sites and hardly any of the substrate will get to the enzyme.
  • But if there’s a higher concentration of substrate, then the substrate’s chances of getting to an active site before the inhibitor increase.
  • So increasing the concentration of substrate will increase the rate of reaction (up to a point).
• Non-competitive inhibitors

  • Non-competitive inhibitor molecules bind to the enzyme away from its active site. This causes the active site to change shape so the substrate molecules can no longer bind to it
  • Non-competitive inhibitor molecules don’t compete with the substrate molecules to bind to the active site because they are a different shape. Increasing the concentration of substrate won’t make any difference — enzyme activity will still be inhibited.