8.1.4 Skills: Rates of Reaction & Types of Inhibition

• 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