Enzymes
Senior 2 · Biology
Enzymes
Lesson 2 of 4
16 min
Let's begin
Amylase in your mouth breaks down the starch in cassava, but it does nothing at all to the protein in beans or the fat in groundnuts. Pepsin in your stomach digests protein but ignores starch. Why is each enzyme so choosy?
The answer lies in the shape of the enzyme molecule. It also explains why fever, strong acid and boiling can destroy enzymes.
An enzyme is a large protein folded into a precise three-dimensional shape. On its surface is a small pocket or groove called the active site. This is where the substrate binds and the reaction happens.
The shape of the active site is complementary to the shape of the substrate: they fit together the way a key fits its lock. Only a substrate with the right shape can fit, and this makes each enzyme specific.
E + S → ES complex → E + P (E = enzyme, S = substrate, P = products)
The lock-and-key hypothesis, proposed by Emil Fischer in 1894, compares the enzyme to a lock and the substrate to the key. Just as only one key opens a lock, only one substrate fits a particular active site. The model explains specificity simply.
Scientists have since found that the active site is not completely rigid. In the induced-fit model, the active site changes shape slightly as the substrate binds, gripping it more tightly, like a glove moulding round a hand. For S2 you need the lock-and-key idea; induced fit is a refinement of it.
The shape of an enzyme is held by weak bonds between parts of the protein chain. High temperature or an unsuitable pH breaks these bonds, and the protein unfolds. The active site loses its shape, the substrate no longer fits, and the enzyme stops working. This permanent change is called denaturation. A denatured enzyme is not "killed", because it was never alive. It is a damaged molecule.
Question: Using the lock-and-key hypothesis, explain why amylase breaks down starch but not protein.
Solution: The active site of amylase has a shape that is complementary to part of the starch molecule, so starch fits and forms an enzyme-substrate complex, and the reaction happens. A protein molecule has a different shape, so it cannot fit into amylase's active site. No complex forms and no reaction occurs. Each enzyme is specific because only substrates of the right shape fit its active site.
Question: Molecule M has a shape very similar to the normal substrate of an enzyme. Predict and explain its effect.
Solution: M can enter the active site and block it, so the real substrate cannot bind, and the reaction slows. M is a competitive inhibitor: it competes with the substrate for the active site. Many medicines work in this way, by blocking a particular enzyme.
Tip
Memory hook: one lock, one key. The enzyme is the lock; the substrate is the key.
Exam technique: in explanations, use the words active site, complementary shape and enzyme-substrate complex. For denaturation, write that the active site changes shape so the substrate no longer fits.
What is the active site of an enzyme?
Solution: The region of the enzyme, a groove or pocket, where the substrate binds and the reaction takes place. Its shape is complementary to the substrate.
Put these in order: products released; substrate enters the active site; enzyme-substrate complex forms; enzyme is free again.
Solution: substrate enters the active site → enzyme-substrate complex forms → products released → enzyme is free again.
Explain what happens to an enzyme when it is denatured.
Solution: Heat or extreme pH breaks the bonds that hold the protein's shape, so the enzyme unfolds and its active site changes shape. The substrate no longer fits, no enzyme-substrate complexes form, and the enzyme stops working permanently.
How is the induced-fit model different from the lock-and-key model?
Solution: In the lock-and-key model the active site is a fixed shape. In the induced-fit model it changes shape slightly as the substrate binds, to fit it more closely.
Summary
Sources and credits
The step-by-step account, the worked examples and the practice questions are written by Ganzaa.
