Cells and Energy - Part 4

Induced Fit Model

The interaction of an enzyme with its substrate.


  • The substrate is held in the active site by a combination of weak forces (hydrogen or ionic bonds) creating an enzyme-substrate complex.
  • The structure of substrates almost, but not quite, match the structure of the enzyme's active site.
  • Binding to the active site alters the structure of the substrate, straining some of the bonds.
  • Strained bonds are easier to break, or rearrange so the enzyme greatly enhances the rate of the reaction.

Note: the lock and key model once in favor has been abandoned since it did not describe how the substrates bonds could be broken.

Factors Affecting Enzyme Activity


The ability of an enzyme to catalyze a reaction depends on its three-dimensional shape. The shape of the protein in turn depends on a variety of environmental factors including temperature and pH.


Enzymes have an optimal temperature. The rate of an enzyme-catalyzed reaction increases with rising temperature up to the point at which thermal vibrations break the hydrogen, hydrophobic and even the ionic bonds critical in maintaining its tertiary or quaternary structures. Temperatures within the acceptable range can double the rate of a reaction for every 10 degrees of increase.


pH can denature (not necessarily destroy) an enzyme by disrupting the hydrogen bonding within the molecule. Enzymes have an optimum pH which can be seen in the activity curves below.

Many enzymes require another molecule other than the substrate to be fully active. These may be cofactors or coenzymes

  • Cofactors are small inorganic molecules or atoms (often metals)
  • Coenzymes are larger organic molecules such as vitamins.


Inhibitors. Enzyme inhibitors selectively prevent enzymatic action by combining:

  1. reversibly using weak forces similar to the normal substrate molecule
  2. irreversibly by attaching to the active site with covalent bonds


Inhibitors can be classified as competitive or noncompetive

  1. competitive inhibitors are substances which can fit into the enzymes active site. If both molecules have equal access to the enzyme its function will be diminished.
  2. noncompetitive inhibitors change the enzymes 3D conformation while leaving the active site available but unable to effectively catalyze the reaction.


Enzymes can also be activated (and regulated) at an allosteric site which is generally part of the quatenary structure of the protein. This site explains the process of allosteric activation.

An enzyme that is involved in a catabolic pathway such as respiration can be inhibited by the production of ATP and activated by an increase in ADP. The temporary attachment of these metabolites at the allosteric site of the enzyme strongly depends upon their concentrations. The competition between ATP (inhibition) and ADP (activation) for the allosteric site is a classic example of homeostasis.

Modified July 10, 2005