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Question Video: Describing the Lock and Key Theory of Enzyme Action Biology

Which of the following best describes the lock and key theory of enzyme action? [A] The substrate is the “lock” into which the enzyme, or the “key,” fits. [B] The enzyme and substrate have identical shapes, like a “lock and key.” [C] Once the enzyme and substrate have joined, they are locked together and cannot be separated. [D] The enzyme is the “lock” into which the substrate, or the “key,” fits.

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Video Transcript

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.

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