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Cytology
Induced fit theory
The induced-fit theory assumes that the substrate plays a role in determining the final
shape of the enzyme; thus, the enzyme is partially flexible (Figure 1.44). This explains
why certain compounds can bind to the enzyme but do not react because the enzyme
has been distorted too much. Other molecules may be too small to induce the proper
alignment. Therefore, they cannot react, since only the specific substrate is capable of
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inducing the proper alignment of the active site.
Enzyme
Substrate entering the active
site of enzyme
Free enzyme + Substrate
E S
Enzyme-substrate complex Enzyme-products complex Free enzyme + Products
ES EP E P
Figure 1.44: Model of the induced fit theory
Factors governing rate of enzyme This point is called enzyme saturation
activity point. Thus, extra substrates have to wait
Several factors affect enzyme activities. for the enzyme to release the product and
These include substrate concentration, become free to accommodate another
enzyme concentration, temperature, and substrate molecule (Figure 1.45).
pH level.
Maximum reaction rate
(a) Substrate concentration
At fixed concentration of enzymes, an
increase in substrate concentration will Rate approaching
maximum rate
increase the rate of reaction. This is
because more substrate molecules will Rate of reaction Rate increases directly with
be colliding with enzyme molecules. substrate concentration
Therefore, more products will be formed
at a time. However, after a certain Substrate concentration
concentration, any increase will have Figure 1.45: The effect of substrate
no effect on the rate of reaction, since concentration on the rate of enzyme-catalysed
all enzyme’s active sites have bound to reaction
substrate, and the remaining substrate will
be unable to bind to enzymes. (b) Enzyme concentration
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