• Enzyme catalysed reactions can be affected by changes in?pH, temperature or substrate concentration
• The?rate of reaction?can be determined by measuring the?rate of disappearance of a substrate?or the?rate of product accumulated?in a?given time period
• This may be shown as a change in quantity (usually volume or mass) of substrate or product over a measured time period:

• Or,?if we cannot collect quantitative data on the amount of substrate or product, we can calculate the rate of reaction?based on the time measured?using the following equation:

• • 1 ÷ time taken (seconds)?and should include the units s?1
• A?high?rate of reaction is when the reaction happens in?less time?i.e. it is faster
• A?low?rate of reaction is when the reaction happens in?more time?i.e. it is slower
• The rate of a reaction is likely to change throughout a reaction as the?substrate concentration will decrease?as the reaction proceeds
  • This leads to a graph that starts out as a?directly proportional?straight line (the value on the X increases at the same rate as the value on the Y) but then?plateaus as the reaction slows down
• The?steeper the line the faster the rate of reaction
• The rate of reaction can be calculated from a graph plotted where the reaction?time?is shown on the X-axis and the?quantity of product or substrate?is shown on the Y-axis

Graph produced when plotting the volume of a product produced against time

• The?gradient?is calculated from a point on the graph and used as a measure of the rate of reaction at that point in time
• A?tangent?must be drawn to calculate the change in x and y so the rate of reaction can be calculated
  • E.g. if calculating the initial rate of reaction
    • Place a ruler on the point of?origin?and draw a line that corresponds to the curve during the early part of the reaction
    • Extend the line?as far as is convenient to perform the calculations e.g. to 60 seconds

Drawing a tangent to the graph to calculate the initial rate of reaction

Calculating the rate of reaction

• Once the tangent is drawn you can calculate the?gradient?of the line which is equal to the rate of the reaction
  • Initial rate = a?÷?b
  • Where
    • a = change in volume and
    • b = change in time
  • The units will be?cm3 sec?1?(this means volume per sec)

• The effect of competitive and non-competitive inhibitors on enzyme controlled reactions can be represented graphically
• Both types of inhibitors?slow down?or?stop?enzyme activity, decreasing the rate of reaction
• Increasing?the?concentration?of an inhibitor?reduces?the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will?stop completely
  • For?competitive inhibitors?countering the increase in inhibitor concentration, by increasing the substrate concentration,?can?increase?the rate of reaction but the substrate needs to reach a high enough concentration in order to displace the inhibitor (more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes)
  • For?non-competitive inhibitors?increasing the substrate concentration?cannot?increase?the rate of reaction, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form

• A graph can be used to distinguish between the two different types of inhibitors and their effect on the rate of reaction
• The patterns shown are?notably different for each type of inhibitor?and also for an?uninhibited enzyme

• A competitive inhibitor will lower the initial rate of reaction (by occupying some of the available active sites), whilst the maximal rate is not affected
  • Eventually,?the same amount of product will be produced as would have been produced without the competitive inhibitor
• Non-competitive inhibitors lower the initial rate of reaction?and the maximal?rate of reaction
  • A lower amount of product is produced than would normally be produced