The active site model describes the part of an enzyme to which substrates bind and where a reaction is catalyzed.

The enzyme’s active site binds to the substrate. Since enzymes are proteins, this site is composed of a unique combination of amino acid residues (side chains or R groups). Each amino acid residue can be large or small; weakly acidic or basic; hydrophilic or hydrophobic; and positively-charged, negatively-charged, or neutral. The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site, including changes in local pH. A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate.

Environmental conditions can affect an enzyme’s active site and, therefore, the rate at which a chemical reaction can proceed. Increasing the environmental temperature generally increases reaction rates because the molecules are moving more quickly and are more likely to come into contact with each other.

However, increasing or decreasing the temperature outside of an optimal range can affect chemical bonds within the enzyme and change its shape. If the enzyme changes shape, the active site may no longer bind to the appropriate substrate and the rate of reaction will decrease. Dramatic changes to the temperature and pH will eventually cause enzymes to denature.

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Practice Exam 1 B/B Section Question 31

Practice Exam 1 B/B Section Passage 6 Question 33

Key Points

• The enzyme‘s active site binds to the substrate

• When an enzyme binds its substrate it forms an enzyme-substrate complex.

• Enzymes promote chemical reactions by bringing substrates together in an optimal orientation, thus creating an ideal chemical environment for the reaction to occur.

Key Terms

enzyme: A substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.

substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.

active site: The active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed.