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Enzymes
Enzymes are globular proteins that speed up chemical reactions
by acting as biological catalysts
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They can be
reused repeatedly and therefore effective in small amounts
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The
active site is the part of the enzyme where the substrate molecules
bind to
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In a chemical reaction, a certain amount of energy needs to be
supplied to the chemicals before the reaction will start
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Enzymes lower the amount of
activation energy that’s needed, often making reactions happen at
a lower temperature than they could without an enzyme
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When a substrate fits into the enzyme’s active site it forms an
enzyme-substrate complex – it’s this that lowers the activation
energy
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Early
scientists studying the action of enzymes came up with the lock
and key model, which is where the substrate fits into the enzyme in
the same way that a key fits into a lock – the active site and the
substrate have a complementary shape
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This locks the substrate even
more tightly to the enzyme
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The induced fit model

The substrate doesn’t only have to be the right shape to fit the
active site but also has to make the active site change shape in the
right well as well
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Enzymes
are very specific – they usually only catalyse one reaction
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The active
site’s shape is determined by the enzymes tertiary structure
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If the tertiary structure of a protein is altered in any way, the shape
of the active site will change
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The tertiary structure of an enzyme may be altered
by changes in pH or temperature
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If mutations occur in that gene, it
could change the tertiary structure of the enzyme produced
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By
measuring the amount of substrate molecules left at different times
during the experiment the reaction can be calculated
Temperature
When temperature increases, the rate of enzyme-controlled
reaction increases
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This makes the substrate more likely to
collide with the enzymes’ active sites
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But if the temperature gets too high, the
reaction stops
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If the temperature goes above a certain level, this
vibration breaks some of the bonds that hold the enzyme in shape
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At this point the enzyme has denatured
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Most human enzymes work
best at pH 7 (neutral)
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The enzyme becomes denatured, and the active
site changes shape
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This is only true up until a ‘saturation point’ though
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Enzyme Inhibitors
Enzyme activity can be prevented by enzyme inhibitors molecules that bind to the enzyme that they inhibit
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Competitive inhibitor
Competitive inhibitor molecules have a similar shape of that of
substrate molecules
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Instead they
block the active site, so no substrate molecules can fit
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If there’s a high concentration it’ll take
up nearly all the active sites and hardly any of the substrates will
get to the enzyme
Non-competitive inhibitor
Non-competitive inhibitor molecules bind to the enzyme away from
its active site
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They don’t ‘compete’
with the substrate molecule to bind to the active site because they
are a different shape
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