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Metabolism

Topic 8
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Others include both

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Metabolism

Topic 8
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Fisher, 1890’s, proposed the lock-and-key model of enzyme action
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Substrate molecules fit into a rigid section of the enzyme like a key fits into
a lock

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This model has been modified

➢ Enzymes undergo significant changes in their conformation when substrates
combine with their active site
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This new model is called induced-fit model

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Think of a glove and a hand
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Biology HL

Mechanism of enzyme action
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The enzyme changes shape to accommodate the substrate
3
...
The AE is lowered and the substrate is altered by the rearrangement of the existing
atoms
5
...
The unchanged enzyme is then free to combine with other substrate molecules
E + S ↔ ES ↔ E + P
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E: enzyme

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S: substrate

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ES: enzyme-substrate complex

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P: product

Inhibition
➢ If a molecule affects an active site in some way, the activity of the enzyme may be
altered
➢ The activity of enzymes is strictly controlled in the body
➢ Competitive inhibition
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A molecule called competitive inhibitor competes directly with the usual
substrate for the active site or an enzyme

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The substrate will have fewer encounters with the active site and rate of the
chemical reaction will be decreased

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The competitive inhibitor must have a structure similar to the substrate

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No further reactions can take place

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Example: the use of sulfanilamide (a sulfa drug) to kill bacteria during an
infection
■

Folic acid is essential to bacteria as a coenzyme

■

It is produced by enzyme action on PABA (para aminobenzoic acid)

■

The sulfanilamide competes with PABA and blocks the enzyme

■

This prevents the production of folic acid resulting in the death of
the bacteria

■
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Humans are unaffected by the drug

Reversible competitive inhibition may be overcome by increasing the
substrate concentration

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Metabolism

Topic 8
...
coli uses a metabolic pathway to produce the amino acid isoleucine
from threonine
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A 5-step process

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Metabolism
○

Topic 8
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coli, it inhibited the first
enzyme in the pathway and isoleucine will not be synthesised

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This situation will exist until the isoleucine is used up

➢ The inhibition of the first enzyme prevents the build-up of intermediates in the
cell and it is a form of negative feedback
➢ A coenzyme has an essential role in the normal actions of a enzyme
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Is not usually a protein

➢ Many potential new medicines can come from controlling enzymes
➢ By the use of massive databases of inhibitors, scientists have been able to find new
potential medicines for malaria

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