Which of the following best
describes the lock-and-key theory of enzyme action? Option (A) the substrate is the
lock into which the enzyme, or the key, fits. Option (B) the enzyme and substrate
have identical shapes, like a lock and key. Option (C) once the enzyme and
substrate have joined, they are locked together and cannot be separated. Option (D) the enzyme is the lock
into which the substrate, or the key, fits.
To answer this question, we need to
define the key terms enzyme and substrate and understand the lock-and-key theory of
enzyme action. An enzyme is a biological
catalyst. It speeds up reactions. Enzymes are proteins that are
usually globular in shape. And every enzyme has a specific
region called an active site. The active site is unique to each
enzyme and specific for a certain substrate.
Now, recall that the substrate
refers to the specific molecule that an enzyme acts on. The substrate must be complementary
to its particular enzyme’s active site as this is where it binds. Upon binding of the substrate to
the enzyme’s active site, it is referred to as the enzyme–substrate complex.
The last part of our question that
we need to consider is the lock-and-key theory. The lock-and-key theory of enzyme
action proposes that the enzyme’s active site and the shape of the substrate
molecule are complementary to one another. This allows the substrate to fit
into the enzyme, like how a key would fit into a lock. If the substrate doesn’t fit, then
the enzyme will not act on it. Only the correct substrate will
bind with the active site. When they fit together correctly,
the reaction proceeds and converts the substrate into its products. Recall that enzymes are not
consumed during this process.
Now that we’ve reviewed some
terminology, let’s take a look at our answers. Because lock-and-key theory of
enzyme action refers to a substrate fitting into an enzyme’s active site, we need to
find an answer that corresponds to this description. Answer (D) states that the enzyme
is the lock into which the substrate, or key, fits. This answer is consistent with the
lock-and-key theory, so option (D) is the correct choice.