Video Transcript
A diagram of an enzyme and some
substrates is shown. Which substrate will the enzyme
bind to?
This question asks us which of four
substrates will bind to the enzyme in our diagram. In order to answer this question,
let’s review how an enzyme and its substrate will bind together.
Enzymes are biological catalysts
that speed up the rate of reactions without being used up. Without enzymes metabolic processes
would happen too slowly to maintain life. So, how do enzymes work? Enzymes are globular proteins, with
a specific region called an active site, shown here in pink. To catalyze a reaction, the
reactant, or reactants, bind to the active site on the enzyme. We refer to the reactant as the
enzyme’s substrate. The substrate binds to the enzyme’s
active site, forming an enzyme–substrate complex. This is where the reaction takes
place, following which the product, or products, are released.
The enzyme remains unchanged, so it
can then bind with another substrate molecule and catalyze further reactions. You may have noticed that in our
example, the substrate molecule fits perfectly into the active site. This is true for all enzymes. They are highly specific and will
only catalyze reactions when their particular substrate molecule binds to their
active site. We can say that the enzyme has a
complementary shape to a specific substrate molecule.
Now that we have reviewed how
enzymes and substrates bind, let’s take another look at our question.
We need to establish which of the
four substrates the enzyme will bind to. We know that we need to find a
substrate with a shape that is complementary to the active site of the enzyme so
that they fit together perfectly. If we look at substrates Y and Z,
we can see they are rectangular with square-shaped indentations. Our enzyme is curved. So we can eliminate these choices
straightaway, as they will not fit into the active site.
So, which of V and X is the correct
substrate? V is the same shape as our enzyme,
so won’t fit into its active site. X has a rounded shape that we can
see would fit within the enzyme’s concave active site. So the substrate the enzyme will
bind to is X.
