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Biology for Advanced Level Secondary Schools

Mechanisms of enzymatic action fit hypothesis. In the lock-and-key
Figure 1.33 Structure of the zwitterion

Task 1.8 hypothesis, an enzyme holds the substrate
as a lock holds the key, while the induced

Search from internet sources the fit hypothesis, the active site expands

simulations for enzymatic reaction and contracts to form enzyme-substrate
showing enzyme–substrate specificity. interaction. The two molecules form a
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Note down the relationship between temporary structure called an enzyme-
substrate and enzyme in comparison substrate complex, as an intermediate
to lock and key theory. product of substrate to be converted into

product. The products have different

Several steps of enzymatic action result in shapes from the substrates; therefore, once
Figure 1.34 Formation of dipeptide molecule linked by the peptide bond
the formation of products. There are two the product is formed, it escapes from the

theories which explain the mechanism active site, leaving it free to accommodate

of enzymatic action. These are the lock- other substrate molecules.

and-key hypothesis and the induced

Lock-and-Key theory

The theory states that, as the key fits in one lock, the same happens to a substrate which

fits only in one active site of the enzyme to form an enzyme-substrate complex (Figure

1.43). The specific action of an enzyme with a single substrate can be explained using

a Lock-and-Key analogy, first postulated in 1894 by Emil Fischer. In this analogy, the
lock is the enzyme and the key is the substrate. Only the correctly sized key (substrate)

fits into the key hole (active site) of the lock (enzyme). Smaller keys, larger keys, or
incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules)
do not fit into the lock (enzyme). Only the correctly shaped key opens a particular lock.

Substrate

Active site

Key (substrate) Lock (enzyme) Enzyme

Lock-key
complex Enzyme substrate
complex
Figure 1.43: A model explaining the lock and key hypothesis

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