Table of Contents
Biomolecules Gateway Page
Jmol Tutorial

In this module:

Enzymes' Capabilities
Models of Enzyme Action
Enzyme Kinetics
Competitive Inhibition
Noncompetitive Inhibition
An Enzyme at Work
Protease Inhibitors

In a mathematical description of enzyme action developed by Leonor Michaelis and Maud Menten in 1913, two constants, Vmax and Km, 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 (Vmax): 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 Vmax, is the rate of the reaction under these conditions. Vmax reflects how fast the enzyme can catalyze the reaction. Click on the image at right to see how high Vmax and low Vmax enzymes compare. Vmax is given by the asymptote to the velocity curve as the substrate concentration is extrapolated to infinity. Notice that Km stays constant for the two enzymes described here.

Michaelis Constant (Km): Enzymes have varying tendencies to bind their substrates (affinities). An enzyme's Km describes the substrate concentration at which half the enzyme's active sites are occupied by substrate. A high Km 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 Km 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 Km and low Km enzymes compare. Graphically, the Km is the substrate concentration that gives the enzyme one-half of its Vmax. Although it may look like the Vmax drops, if the graph is extended along the x-axis, the Vmax stays constant for the two enzymes described here.

The table shows the Km and Vmax of some common enzymes.

Enzyme Km Vmax
Carbonic Anhydrase 8000 600,000
Chymotrypsin 5000 100
Penicillinase 50 2000
Lysozyme 6 0.5

Enzyme Kinetics