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.
