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Cytology

prevented from working normally, as the the active site of the enzyme. They fix
substrate cannot move into the active themselves elsewhere on the enzyme
site. The substrate is then prevented molecule. In so doing, they completely
from binding to the same active site and alter the shape of the enzyme molecule
therefore, decreases the enzyme affinity to in such a way that the active site cannot
other substrates. The effects of competitive accommodate the substrate. Since the
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inhibitors decrease with increased inhibitor does not compete with the
concentration of substrates. A good example substrates for the active site of the enzyme,
of the competitive inhibitors is malonic
acid, which competes with succinic acid an increase in substrate concentration does
for the active site of a respiratory enzyme, not reduce the effect of the non competitive
succinic dehydrogenase. inhibitor. An example of non-competitive
inhibitor is cyanide, which attaches itself
Non-competitive inhibitors are substances to copper prosthetic group of cytochrome
that do not resemble substrates. Therefore, oxidases, thereby inhibiting respiration
they neither compete for nor attach to process (Figure 1. 49b).

Inhibitor Substrate
Substrate Active site

Enzyme
Enzyme
Inhibitor
Substrate
Conformation
Inhibitor blocks change of active site
active site

(a) (b)

Figure 1.49: Inhibitors (a) competitive (b) non-competitive

Enzyme regulation in which metabolic pathways in the cells
Regulation of enzymatic activity is an are regulated involve allosteric enzymes,
important biological activity in controlling which are designed to change the shape
different metabolic processes. Enzymes and are regulated by the compounds,
are important for regulation of different which act as non-competitive inhibitors.
metabolic activities, such as biochemical The regulation of these enzymatic
pathways, homeostasis, gastro-intestinal activities includes; end-product inhibition,
digestion and growth. The common way zymogens and genetic control.

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