Michaelis-Menton Constant (Km)

One needs measuring tools in order to find out how enzymes work and how they are controlled.
The Michealis-Menton contant is one such tool.
It allows you to measure the affinity of enzyme for substrate, to measure the effect of various inhibitors and activators, and in addition, is a parameter that can be relatively easily measured
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How to determine the Km of a reaction

In order to determine the Km of a reaction, one has to measure the rate of reaction .
The rate of reaction can be determine from the following kind of curve.

In this graph the amount of product formed is plotted versus the time of reaction.

There are two curves in this figure.

In one experiment one unit of enzyme is used along with 10 units of substrate.

In the other experiment, one unit of enzyme is used and 100 units of substrate.

The rate of reaction is measured as dp/dt.

You can see that the rate of reaction with 100 units of substrate is greater than with 10 units of substrate.
If similar experiments are done, one can plot the results in the following way.

In this graph the rates of reaction at different substrate concentrations, and the same enzyme concentration, is plotted.

As the substrate concentration is increased the rate of reaction increases.

However the rate of reaction will eventually reach a maximum (Vmax).

At the Vmax, as the substrate concentration is increased, the rate does not increase.


 Michaelis and Menton derived equations that showed the value Vmax/2 can be used to compute the Km of the reaction as shown in the above figure.

How can the Km be used?

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