The above graph shows how the rates of the different enzyme reactions change with different substrate concentrations. This type of graph is most useful for understanding the effects on reaction rate that result when the reaction includes a competitive inhibitor or when the concentration of the enzyme has been lowered. Important points to note are:

• The reaction rate (v) is measured in absorbance units per second, absorbance being directly proportional to the amount of product formed.
• There is a maximal rate of the reaction, Vmax, that occurs when substrate concentration is high enough to saturate the enzyme. The enzyme is beginning to be saturated when the rate changes very little in response to increasing amounts of substrate (where the line levels off).
• When a competitive inhibitor is added, a much higher concentration of substrate is required to achieve the same reaction rate. This means that there is a lower relative affinity of enzyme and substrate, which is indicated by a higher Km value. Another way of thinking about affinity is that it describes the likelihood that enzyme will be bound to substrate for a particular concentration of enzyme.
• The effect of a competitive inhibitor can be overcome with the addition of more substrate molecules. This means that for the reaction with competitive inhibitor, Vmax should be the same as it is in the standard reaction.
• A lower level of enzyme results in a lower Vmax, since fewer enzyme molecules are available to catalyze the reaction. A similar effect would be seen if a non-competitive inhibitor had been added to the reaction. Because a non-competitive inhibitor does not bind to the catalytic site of the enzyme, there is no effect on the relative affinity between enzyme and substrate. Thus, Km should be the same between the reaction with lower enzyme concentration and the standard reaction.

Now, view the double-reciprocal plot.

Home page