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Biology for Advanced Level Secondary Schools

End-product inhibition control: If a given they are synthesised, so that they cannot
physiological process involves several steps attack the pancreas.
and various intermediates, the end product
of the pathway may inhibit the enzyme at Genetic control: This is the regulation of
the start (Figure 1.50). In this example, the enzymatic activity by control of the synthesis
product “Y” acts as an inhibitor to enzyme of the enzyme. The synthesis of enzymes is
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“a” (ea). If the level of product Y falls, the regulated by genes. Genes carry the code for
inhibition is reduced. If the level of Y rises making enzymes. These mechanisms, which
above normal, inhibition of “ea” increases; are controlled by hormones can accelerate

therefore, the level of Y is reduced. In this or decelerate enzyme synthesis. The genetic
way, homeostatic control of Y is achieved. control strategy is particularly useful for
This mechanism is termed as negative enzymes which are needed only at certain
feedback, because the information from stages of development. Depending on the
the end of the pathway which is feedback genetics and other environmental factors,
to the start has a negative effect; that is the the body can produce many enzymes during
high concentration of Y reduces its own the life-time. As body ageing commences,
production rate. The enzymes that are the body tries to automatically conserve its
inhibited in this way are called allosteric decreasing resources and produces fewer
enzymes. These enzymes can have more enzymes of all types. When this lifetime
than one shape. One shape renders the enzyme potential is reached, the body can
enzyme active (by allosteric activator), no longer sustain life. Death occurs because
another renders it inactive (by allosteric nothing takes place in the body without
inhibitor). enzymes. Enzymes constitute the ‘`life
force” which powers the whole system in
Inhibition (negative feedback)
the organism’s life expectancy.
Intermediate compounds
Properties of enzymes
(Substrate) U V W X Y (Product)
ae eb ed ed They are proteins (globular proteins) in
nature and biocatalysts, as they lower
Figure 1.50: End product inhibition of enzyme
the activation energy of the reaction they
catalyse. They are never used up, never
Zymogens (proenzymes) control: Some
enzymes are synthesised in inactive affected by the reaction they catalyse,
forms that differ in composition from the meaning that they remain unchanged at
active forms. Activation of such enzymes, the end of reaction. Since they are proteins,
known as zymogens or proenzymes, enzymes are coded for by DNA. They
requires a chemical reaction that either have active sites where the substrates are
adds or splits off part of the molecule. accommodated; and the sites have specific
Some enzymes that digest proteins, such shapes which make them to be specific not
as trypsinogen, chemotrypsinogen, and only to the substrates they act upon, but
proelastase are produced in the pancreas. also to the reactions they catalyse. They are
These enzymes must be inactive when sensitive to temperature and pH changes,
and they can also be affected by enzyme

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