BOTANY Higher Secondary Second Year - Textbooks Online

5.1.3. Theories explaining the mechanism of enzyme action
Two theories have been proposed to explain the mechanism and enzyme
action. They are Fischer’s Lock and key theory and Koshland’s induced
fit theory.
Fischer’s Lock and key theory
Lock and key theory was proposed by Fisher. According to this
theory, first a physical contact is made between the enzyme and the substrate.
As only a specific key fits in a particular lock to open it, a specific
substrate combines with the active site of specific enzyme. This combination
leads to the production of enzyme - substrate complex. Then the enzyme
acts on the substrate and changes it into products. After the reaction is
+ → →
+
Active site
Enzyme Substrate
Enzyme-susbstrate
complex
Products
Released enzyme
Fig.5.2 Fischer’s Lock and key theory of enzyme action
over, enzyme is released from the enzyme - substrate complex and is ready to
bind with another molecule of the substrate for further action. The cyclic reaction
is summarized by the equation
E + S [ES] P + E
(where, E - enzyme, S - substrate and P - product)
When a dissimilar substrate approaches the enzyme, it cannot combine
with the active site of the enzyme, as a wrong key cannot open the lock.
Thus, the enzyme action is inhibited.
Substrate
Koshland’s induced fit theory
Enzyme-substrate
Induced fit theory was pro-
complex
posed by Koshland. Proteins are
Enzyme
not rigid. The substrate induces the
enzyme to adjust its shape leading
to the formation of enzyme substrate
complex. Then, the enzyme Fig. 5.3 Induced fit theory
acts on substrate and forms products. Many enzymes function in this way.
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