Theories of Enzyme Action

A. Lock and key hypothesis:

One theory of how enzymes work is that the active site in the enzyme is like a lock into which only one substrate molecule can fit like a key. The substrate must be complementary to the active site of the enzyme so that it can bind to it.

The active site is a fixed shape so a substrate has to collide in the correct orientation with the active site in order that bonds can form and produce an enzyme-substrate complex.

While the substrate binds to the enzyme chemical changes can take place and the substrate molecules are either digested (broken down) or combined to form new products.

The enzyme is not affected by the reaction and can be reused. In the first diagram, two substrate molecules are combined to form a single product molecule – anabolism. However, many enzymes (e.g. digestive enzymes) are involved in breaking down complex substrate molecules into two or more product molecules – catabolism.

B. Induced fit hypothesis:

Another theory of how enzymes work is the induced fit theory.

As the substrate molecule enters the active site forces of attraction between the substrate and the R groups/polar atoms of the amino acids in the active site are formed.

This causes the shape of the active site to change and stronger bonds are then formed with the substrate. This weakens/strains the bonds in the substrate, which lowers the activation energy of the reaction. When the products are released from the substrate, the active site returns to its original shape.

This has been shown to occur with the enzyme lysozyme.

Again, the enzyme is not affected by the reaction and can be reused.