Biomolecules:
Enzymes

In this module:

In a mathematical description of enzyme action developed by Leonor Michaelis and Maud Menten in 1913, two constants, V_max and K_m, play an important role. These constants are important to know, both to understand enzyme activity on the macroscale and to understand the effects of different types of enzyme inhibitors.

Maximal Velocity (V_max): Increasing the substrate concentration indefinitely does not increase the rate of an enzyme-catalyzed reaction beyond a certain point. This point is reached when there are enough substrate molecules to completely fill (saturate) the enzyme's active sites. The maximal velocity, or V_max, is the rate of the reaction under these conditions. V_max reflects how fast the enzyme can catalyze the reaction. Click on the image at right to see how high V_max and low V_max enzymes compare. V_max is given by the asymptote to the velocity curve as the substrate concentration is extrapolated to infinity. Notice that K_m stays constant for the two enzymes described here.

Michaelis Constant (K_m): Enzymes have varying tendencies to bind their substrates (affinities). An enzyme's K_m describes the substrate concentration at which half the enzyme's active sites are occupied by substrate. A high K_m means a lot of substrate must be present to saturate the enzyme, meaning the enzyme has low affinity for the substrate. On the other hand, a low K_m means only a small amount of substrate is needed to saturate the enzyme, indicating a high affinity for substrate. Click on the image at right to see how high K_m and low K_m enzymes compare. Graphically, the K_m is the substrate concentration that gives the enzyme one-half of its V_max. Although it may look like the V_max drops, if the graph is extended along the x-axis, the V_max stays constant for the two enzymes described here.

The table shows the K_m and V_max of some common enzymes.