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

How do enzymes accelerate chemical reactions? How do enzymes achieve their specificity? The answer to both questions lies in how enzymes interact with their substrates.

In 1890 the chemist Emil Fischer proposed that the substrate of an enzyme fits into the enzyme's active site , the physical location on an enzyme where the reaction takes place, to form an enzyme-substrate complex. The analogy he used was of a lock and key. The key (substrate) has a specific shape (arrangement of functional groups and other atoms) that allows it and no other key to fit into the lock (the enzyme).

Click on the numbers below to see how the lock-and-key model of enzyme action works. Click on the mouse at left to clear the images and text.


The substrate and enzyme complement each other.


Therefore, they can fit together, like a lock and key.


Different molecules do not complement the enzyme's active site.

In 1958, Daniel E. Koshland Jr. modified the lock-and-key model by proposing that binding of the substrate to the enzyme alters the configuration of both, providing a better fit.

Click on the numbers below to see how the induced fit model of enzyme action works. Click on the mouse at left to clear the text and images.


Before binding, the substrate and enzyme do not exactly fit each other.


Binding of the substrate to the enzyme changes the configuration of both so that they fit together.


Different molecules cannot induce a fit with the enzyme.

The induced fit model explains both of the questions asked at the top of the page:
  • The substrate is distorted (atoms are shifted, bonds are stretched, and reactive groups are brought close together) to resemble the transition state of the reaction. This stabilizes the transition state, accelerating the reaction like any catalyst.

  • Only molecules with the correct functional groups in the correct configurations are able to be induced to fit the active site of the enzyme.

Models of Enzyme Action