Determining Enzyme Reaction Rates
- 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
Volume of a product produced against time graph
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
- E.g. if calculating the initial rate of reaction
Drawing a tangent to calculate initial rate of reaction diagram
Drawing a tangent against the line through the origin 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)