Stated simply, a substrate is the name for the molecule upon which an enzyme acts. The bodies of living things, from plants and mammals to tiny unicellular microbes, contain enzymes that act upon substrates to drive the very biochemical processes that make life possible. The substrate and enzyme fit together like a key into a lock.

What is an Enzyme?

An enzyme is essentially a biochemical catalyst. A catalyst hastens the rate of a chemical reaction without being consumed by the reaction itself. An enzyme catalyzes reactions, often by producing an intermediate molecule with a structure that reacts more readily than the original molecule would have in the absence of the catalyst. Some reactions would proceed so slowly without the enzyme that the presence of enzymes in living systems is absolutely crucial.

Enzyme and Substrate Kinetics

Enzyme kinetics is the study of how enzymes and substrates affect the rate of reactions. An equation called the Michaelis-Menten equation is often used to describe the way enzymes and their substrates interact. The equation states that the substrate concentration is directly related to how fast the reaction goes. According to Michaelis-Menten kinetics, enzymes can reach a saturation point, when all available enzyme is bound to a substrate. At this point, the rate of the reaction is at a maximum.

Lock and Key

The lock and key model was first proposed by a scientist named Emil Fischer in 1894. It supposes that the substrate is the key and the enzyme is the lock. The keyhole in this analogy is a part of the enzyme called the active site, where only a certain substrate will fit. When the substrate is bound to the active site, the reaction will proceed. If the wrong substrate binds, the reaction will not occur.

Substrate Example

An example of a substrate in biochemistry is glucose. Glucose is acted upon in the body by an enzyme called hexokinase. Hexokinase will catalyze a chemical reaction that adds a phosphate to the glucose molecule, producing a product called glucose-6-phosphate. This is the first step of a very important biochemical pathway called glycolysis, which is how the human body metabolizes glucose from the food we eat. Glucose-6-phosphate then becomes the substrate for the next step in the pathway, to be acted upon by a different enzyme, producing a new product. That product will become the substrate for the next reaction, and the process repeats itself continuously.