Enzyme-Substrate Complex: Definition & Overview

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  • 0:00 What are Enzymes?
  • 1:13 How Do Enzymes Work?
  • 3:02 The Effect of pH and…
  • 4:20 Lesson Summary
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Lesson Transcript
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.

Enzymes are almost everywhere in your body. Discover where they are found, how they work and why they are important. Learn two models for how the enzyme-substrate complex is formed as well as the changes that could cause this bonding to fail.

What Are Enzymes?

Examples of enzymes are found pretty much everywhere in your body. Amylase is in your saliva and starts the digestion of food. If you can't drink milk without getting sick, that means you're lactose intolerant, which means you are lacking the enzyme lactase.

Lactic acid is a byproduct of hard exercise and is responsible for that burning feeling in your muscles when you work out. Your body removes lactic acid with the assistance of lactate dehydrogenase. DNA polymerase helps your cells copy DNA so that the cells can multiply and you can grow.

On a side note, did you notice that all the enzymes mentioned end with the suffix -ase? This is a good thing to keep in mind when studying biology.

Enzymes are proteins that catalyze, or speed up, chemical reactions, allowing for greater cellular activity in a shorter period of time. Enzymes do this by lowering the activation energies of the chemical reactions. In other words, they lower the amount of energy required for the reaction to start. Almost all biochemical reactions require the aid of an enzyme. Without enzymes, the chemical reactions in cells would be much too slow to maintain life.

How Do Enzymes Work?

The molecule (or molecules) with which the enzyme bonds is referred to as the substrate; the molecule (or molecules) that are created at the end of the reaction is referred to as the product. The substrate bonds to a small area of the enzyme termed the active site. When the enzyme is bonded to the substrate, we call this the enzyme-substrate complex. Once the reaction is complete, the enzyme releases the product and is ready to bond with another substrate.

Enzymes are only able to function if they bond with the correct substrate. And enzymes are highly selective; they'll activate with one and only one particular substrate. So how do they know whether or not the molecule next to them is the substrate that they want? Two models attempt to explain this.

Lock and Key Model

In the lock and key model, the substrate and the enzyme's active site are exact matches for each other, similar to puzzle pieces fitting together. Only a single substrate is the 'key' that matches the 'lock' of the active site. Once the enzyme locates the molecule that fits it exactly, the chemical reaction begins. After the products are released, the enzyme searches for another molecule that is an exact match.

Induced Fit Model

The induced fit model is a modification of the lock and key model and is generally thought to be the more accurate version. In this representation, the active site is not an exact fit for the substrate. Rather, the substrate induces a change in the enzyme, causing it to modify its shape until the binding is complete.

This model explains why some molecules are able to bond with the enzyme but are unable to produce a reaction. Only the proper substrate is able to induce the correct shape to bring about the reaction.

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