The Michaelis–Menten model


Enzymes are biological molecules that catalyze (speed up) chemical reactions in living cells. They are essential for many processes such as metabolism, digestion, and DNA replication. Enzymes work by binding to specific molecules called substrates and converting them into products.

The rate of an enzyme-catalyzed reaction is often called its velocity. It measures how fast the enzyme can transform the substrate into the product. Enzyme velocity can be affected by various factors such as temperature, pH, inhibitors, and the concentration of the enzyme and the substrate.

To measure enzyme velocity, we need to monitor the amount of product formed or substrate consumed over time. This can be done by using various methods such as spectrophotometry, chromatography, or radioisotope labeling. The most common way is to use a spectrophotometer, which measures the absorbance of light by a solution at a specific wavelength. By knowing the relationship between absorbance and concentration, we can calculate the amount of product or substrate in the solution.

However, enzyme velocity is not constant throughout the reaction. It changes depending on how much substrate is available and how much product is accumulated. Therefore, we need to measure the velocity at a specific point in time, usually at the beginning of the reaction when no product is yet present. This is called the initial velocity (symbol V0; μmol min-1).

To obtain the initial velocity, we plot the amount of product formed or substrate consumed against time for an enzyme-catalyzed reaction. We then draw a straight line through the linear part of the curve, starting at the zero time-point. The slope of this line is equal to V0.

The following figure shows an example of a plot of product formed against time for an enzyme-catalyzed reaction and how to obtain V0 from it.

Figure 1: Plot of product formed against time for an enzyme-catalyzed reaction