Enzyme-Substrate Complex: Overview

Brittany W., Joanne Abramson
  • Author
    Brittany W.

    Brittany has a bachelor's degree in biological sciences and biochemistry.

  • Instructor
    Joanne Abramson

    Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Understand what enzyme-substrate complex is, how it is formed, and what factors affect its formation. See examples of enzymes, their substrates, and products. Updated: 11/19/2021

Table of Contents


What Is The Substrate Of An Enzyme?

Enzymes are proteins that catalyze (initiate and accelerate) biochemical reactions. A catalyst is a substance that lowers the activation energy of chemical reactions. Activation energy is the energy required for a reaction to take place. Enzymes lower the activation energy by:

  • Bringing molecules together in the right orientation
  • Creating the appropriate environment for the reaction to take place
  • Bending molecules in a way that makes it easy for their bonds to be broken
  • Taking part in the chemical reaction themselves by forming temporary covalent (shared electrons) bonds other molecules

Enzymes are important in a biological system because they enable important reactions to occur at a rate that supports the life of the organism. Without them, life-sustaining reactions would cease to exist at a useful rate, and the organism would die.

Enzymes accelerate cellular activities such as:

  • Building muscle
  • Destroying toxins
  • Breaking down food particles for digestion
  • Converting dietary molecules into energy

An enzyme's active site is the portion of the enzyme where a reaction occurs. What is the substrate of an enzyme? An enzyme substrate, the substance upon which an enzyme acts, lands in the active site and is converted into an enzyme-substrate product, the substance produced as a result of the reaction. Enzymes hold substrates in positions that make molecular bond-breaking and bond-forming easier to accomplish.

Examples of enzymes in the human body include:

  • ATP synthase: Located within the inner mitochondrial membrane, it adds a phosphate molecule (substrate one) to adenosine diphosphate (ADP, substrate two) to make adenosine triphosphate (ATP). ATP is considered "cellular currency" because it is a high-energy molecule that helps drive multiple reactions.
  • Lactase: Located in the small intestine, lactase breaks lactose (milk sugar) into glucose and galactose. When a person is lactose intolerant, they do not possess the enzyme lactase.
  • Trypsin: Also found in the small intestine, trypsin breaks down protein (substrate) into amino acids for the purposes of digestion.

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Enzyme-substrate Complex

What is an enzyme-substrate complex? The enzyme-substrate complex definition is a temporary molecule formed when the substrate binds to the enzyme. When an enzyme binds to its substrate, it undergoes a conformational change or temporary change in shape. However, once the product(s) is released, the enzyme will regain its original shape.

This enzyme-substrate complex diagram demonstrates how a substrate binds to an enzyme, is converted into a product, and is released for more reactions to follow.

Diagram of how an enzyme works.

Enzymes are:

  • Regenerative: Enzymes are neither created nor destroyed by the reactions they catalyze.
  • Reuseable: Because they are not destroyed and resume their original shape, they can be reused for multiple reactions in quick succession.
  • Specific: Enzymes are specific to the substrates they bind. This is called substrate specificity. Some molecules that have a similar molecular structure can get caught in the active site, but they will not be able to undergo the reaction.

There are two models of the enzyme-substrate complex that scientists have proposed over the years:

  • Lock and key model
  • Induced fit model

The lock and key model was the first, and now the outdated, model used to describe the physical and molecular interaction between enzyme and substrate. It was proposed that the shape of the enzyme never changes and that the substrate has a complementary shape that fits the enzyme perfectly, much like a lock and key.

Scientists eventually discovered that the enzyme changes its shape slightly (through a conformational change) to maintain a tight fit on the substrate. This is called induced fit. Even with the conformational change, enzymes will only catalyze a specific reaction when the proper substrate is bound.

The most accurate model to describe how an enzyme binds to a substrate is the induced fit model.

A diagram of the induced fit enzyme model.

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Frequently Asked Questions

What is an example of an enzyme and its substrate?

Lactase is an example of an enzyme. It binds the substrate lactose, a milk sugar, to create the two monosaccharides glucose and galactose.

What is the substrate of an enzymatic reaction?

The substrate of an enzymatic reaction is the substance or molecule upon which an enzyme acts. It is the object that is transformed into a product, and a product is a substance produced as a result of the reaction.

What is meant by "substrate?"

A substrate is a substance or molecule that binds to an enzyme. When the two are combined, they create an enzyme-substrate complex.

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