Question Video: Identifying the Enzymes, Substrates, and Products Involved in the Lock-and-Key Theory Biology

The diagram below shows a basic outline of the lock-and-key theory. Which of the following correctly identifies the structures represented by labels X, Y, and Z? [A] X: substrate; Y: enzyme; Z: products [B] X: products; Y: substrate; Z: enzyme [C] X: enzyme; Y: substrate; Z: products [D] X: enzyme; Y: products; Z: substrate

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Video Transcript

The diagram below shows a basic outline of the lock-and-key theory. Which of the following correctly identifies the structures represented by labels X, Y, and Z? (A) X: substrate; Y: enzyme; Z: products. (B) X: products; Y: substrate; Z: enzyme. (C) X: enzyme; Y: substrate; Z: products. Or (D) X: enzyme; Y: products; Z: substrate.

To answer this question, let’s start by defining what an enzyme is and then take a closer look at how they function in the lock-and-key theory.

An enzyme is a biological catalyst which speeds up the rate of reactions by reducing the energy that is required for a reaction to occur. Each enzyme has a specific shape that is complementary to a particular substrate or substrates. During a biochemical reaction, the substrate or substrates bind to a region of the enzyme called the active site, which we can see in the diagram here. This forms a structure called the enzyme–substrate complex. This reaction will produce a product or products that are then released from the active site.

Enzymes are not used up in enzymatic reactions. So this leaves the enzyme unchanged and available to catalyze more reactions. This process is sometimes called the lock-and-key theory. Similarly to how only a particular key will be able to fit into a particular lock, only a particular substrate or substrates will be able to fit into a particular enzyme’s active site to effectively unlock the catalyzing power of an enzyme.

Let’s take a look at the diagram provided by the question to determine which structures are which. In this diagram, the structure labeled X is the enzyme, or the lock. It has an active site that the substrate can bind to and remains unchanged throughout the reaction. Structure Y is the substrate, or the key. It has the particular shape that fits into the active site and is changed over the course of the reaction. Finally, the structures labeled Z are the products of the reaction. This means that we have found the correct answer to this question.

The option that correctly identifies the structures in the diagram is (C): X: enzyme; Y: substrate; Z: products.

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