An enzyme is a biological catalyst: it speeds up a chemical reaction without being consumed by that reaction. Most enzymes are proteins. Some RNA molecules also act as catalysts.
Enzymes help cells carry out reactions fast enough to support life. Different enzymes act on different molecules, so one enzyme cannot do every job.
The words used to describe a reaction
A substrate is a molecule an enzyme acts on. The active site is the region where a substrate binds and the reaction takes place. The molecules formed by the reaction are its products.
For example, lactase helps break down lactose into glucose and galactose. Lactose is the substrate. Glucose and galactose are products.
The National Human Genome Research Institute's enzyme glossary explains the catalyst definition and the distinction between protein and RNA enzymes.
How an enzyme speeds up a reaction
A reaction needs enough energy to reach a transition state. This barrier is called the activation energy. An enzyme lowers the barrier, which increases the reaction rate under the same conditions.
An enzyme does not create energy or change the overall energy difference between the starting materials and products. It provides a different route for the reaction.
A useful diagram needs to show both points. The catalyzed route has a lower peak, while the starting and ending energy levels stay the same.
Enzymes and substrates are not rigid puzzle pieces
The shape and chemical properties of an active site help determine which substrates it binds. Binding can also change an enzyme's shape. The induced-fit model describes this flexibility more accurately than a completely rigid lock and key.
After products leave, an enzyme can catalyze another reaction. Cells can also regulate enzymes, so reaction rates depend on more than whether the enzyme is present.
OpenStax Biology explains activation energy, active sites, and induced fit.
Examples in the body
Amylase starts breaking down starch in the mouth. Chewing and enzyme action have different roles: chewing breaks food into smaller pieces, while the enzyme acts on molecules.
Lactase acts on lactose in the small intestine. Its name resembles the substrate's name, but enzyme names do not all follow that pattern.
Lipases act on fats. Different enzymes contribute at different stages of digestion, and the products depend on the substrate and the particular reaction.
DNA polymerases help synthesize DNA using an existing strand as a template. This is a reminder that enzymes do more than break molecules apart.
OpenStax's digestive-enzyme overview lists the substrates and products for these digestive enzymes. The NIDDK digestive-system guide explains where digestive juices act. The NHGRI DNA-replication glossary describes the DNA example.
Why temperature and pH matter
An enzyme's activity depends on its environment. Temperature can change the reaction rate. High temperatures can disrupt a protein's structure and reduce its activity.
Changes in pH can affect the active site and its interactions with a substrate. Different enzymes work in different ranges. There is no single temperature or pH that is best for every enzyme.
A decrease in measured activity does not by itself prove that an enzyme was permanently denatured. A study needs to distinguish changes in reaction rate from changes in structure.
Read an enzyme experiment carefully
Suppose a class measures product formation at several temperatures. To interpret the results, first identify what the experiment kept constant. Substrate concentration, enzyme amount, and measurement duration can all affect the comparison.
A graph of product formed over time answers a different question from a graph of temperature against initial reaction rate. Label the measured outcome, its unit, and the time interval.
Use the research-questions guide to turn an observation into a question. The methods guide explains how to describe the comparison so another student can repeat it.
