Summary
IB Biology: Metabolism Summary
Metabolism Unit 8 Summary IB HL
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8.1 METABOLISMMetabolism describes the sum total of all reactions that occur within an organism in
order to maintain life. Most chemical changes in a cell result from a series of
reactions (pathways), with each step controlled by a specific enzyme. Metabolic
pathways allow for a greater level of regulation, as the chemical change is controlled
by numerous intermediates.
Every chemical reaction requires a certain amount of energy in order to proceed this
is the activation energy. Enzymes speed up the rate of a biochemical reaction by
lowering the activation energy. When an enzyme binds to a substrate it stresses and
destabilises the bonds in the substrate. This reduces the overall energy level of the
substrate transition state, meaning less energy is needed to convert it into a product
and the reaction proceeds at a faster rate.
If the reactants contain more energy than the products, the free energy is released
into the system (exergonic). These reactions are usually catabolic (breaking down),
as energy is released from broken bonds within a molecule.
If the reactants contain less energy than the products, free energy is lost to the
system (endergonic). These reactions are usually anabolic (building up), as energy is
required to synthesise bonds between molecules.
In a normal reaction, a substrate binds to an enzyme (via the active site) to form an
enzyme-substrate complex. The shape and properties of the substrate and active
site are complementary, resulting in enzyme-substrate specificity. When binding
occurs, the active site undergoes a conformational change to optimally interact with
the substrate (induced fit). This conformational change destabilises chemical bonds
within the substrate, lowering the activation energy. As a consequence of enzyme
interaction, the substrate is converted into product at an accelerated rate.
Competitive inhibition involves a molecule, other than the substrate, binding to the
he molecule (inhibitor) is structurally and chemically similar to
enzyme’s active site. T
the substrate (hence able to bind to the active site). The competitive inhibitor blocks
the active site and thus prevents substrate binding. As the inhibitor is in competition
with the substrate, its effects can be reduced by increasing substrate concentration.
Non-competitive inhibition involves a molecule binding to a site other than the active
site (an allosteric site). The binding of the inhibitor to the allosteric site causes a
conformational change to the enzyme’s active site. As a result of this change, the
active site and substrate no longer share specificity, meaning the substrate cannot
bind. As the inhibitor is not in direct competition with the substrate, increasing
substrate levels cannot mitigate the inhibitory effect.
, End-product inhibition (or feedback inhibition) is a form of negative feedback by
which metabolic pathways can be controlled. In end-product inhibition, the final
product in a series of reactions inhibits an enzyme from an earlier step in the
sequence. The product binds to an allosteric site and temporarily inactivates the
enzyme (via non-competitive inhibition). As the enzyme can no longer function, the
reaction sequence is halted and the rate of product formation is decreased.
End-product inhibition functions to ensure levels of an essential product are always
tightly regulated. If product levels build up, the product inhibits the reaction pathway
and hence decreases the rate of further product formation. If product levels drop, the
reaction pathway will proceed unhindered and the rate of product formation will
increase.
The rate of an enzyme-catalysed reaction can be calculated and plotted according to
the time taken for the reaction to proceed. The time taken can be measured
according to either the amount of product formed or the amount of substrate
consumed. Reaction rate is the inverse of time taken, meaning that the reaction rate
is higher when less time is taken (and vice versa).
The rate of reaction can be calculated according to the following formula:
Rate of reaction (s–1) = 1 / time taken (s)
Competitive inhibitors bind directly to the active site and hence exist in direct
competition with the substrate. Increasing substrate levels will increase the likelihood
of the enzyme colliding with the substrate instead of the inhibitor. The maximum rate
of enzyme activity (Vmax) can still be achieved, although it requires a higher
substrate concentration.
Non competitive inhibitors bind to an allosteric site
and hence do not exist in direct competition with the
substrate. Increasing substrate concentrations will
not affect the level of inhibition caused by the
non-competitive inhibitor. The maximum rate of
enzyme activity (Vmax) is therefore reduced.
Malaria is a disease caused by parasitic protozoans of the genus Plasmodium. The
life cycle of the parasite requires both a human and mosquito host – hence the
disease is transmitted via mosquito bites. The maturation and development of the
parasite in both human and mosquito host is coordinated by specific enzymes. By
targeting these enzymes for inhibition, new anti-malarial drugs and medications can
be produced.
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