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• explain the induced fit model of enzyme action
• evaluate the strengths of the induced fit model
against the lock and key theory
• apply knowledge of tertiary structure to explain
enzyme specificity and the formation of enzymesubstrate complexes.
Structure of Enzymes:
All enzymes are tertiary globular
proteins, where the protein chain
is folded back on itself into a
spherical or globular shape.
Each enzyme has its own
sequence of amino acids and is
held in its tertiary structure by
hydrogen bonds, disulfide
bridges and ionic bonds.
This complex
3D shape gives
the enzyme
many of its
properties.
How Enzymes Work:
Enzymes are biological catalysts - they speed up the rate
of metabolic reactions, without being used up themselves.
These reactions can be of two types:
1. Reactions where larger molecules are broken down into
smaller molecules.
2. Reactions where small molecules are built up into larger,
more complex, molecules.
• Enzymes react with another molecule called the SUBSTRATE.
• Each enzyme has its own special shape, with an area, the
ACTIVE SITE, onto which the substrate molecules bind.
• This is known as the lock and key theory. You will have seen this
at GCSE.
Models of enzyme action: lock-and-key
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How Enzymes Work:
Modern interpretations of the lock and
key theory suggest that in the presence
of the substrate, the active site may
change in order to fit the substrate’s
shape.
This is called the INDUCED FIT
HYPOTHESIS.
Enzyme + Substrate  Enzyme-Substrate complex  Enzyme + Product
Models of enzyme action: induced fit
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Properties of Enzymes:
Enzymes are specific i.e. each enzyme will
catalyse only one particular reaction, for
example, sucrASE acts on the sugar,
sucrOSE.
Chemical reactions need energy to
Enzymes
are- very
and have
start
them off
this isefficient
called ACTIVATION
a high TURNOVER NUMBER.
ENERGY.
• Thisenergy
meansisthat
they can
convert
• This
needed
to break
the
many molecules
of substrate
existing
chemical bonds
insideper unit
time, for example, catalase, which
molecules.
breaks down the waste product
• In
the body,
enzymes
the has
hydrogen
peroxide
in lower
the body,
activation
energy
of of
reactions
so
a turnover
number
several and
million
reduce
the input of energy needed,
per second!
allowing reactions to take place at
lower temperatures.
Why do enzymes increase the rate?
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a) 3 marks max
Enzyme/active site has a (specific) tertiary
structure 
Only glucose has the correct shape/ is
complementary/ will fit 
To active site 
(Forming) enzyme-substrate complex 
DO NOT allow ‘same’ shape! 
DO NOT allow active site is on the substrate 
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b) 2 marks max
(Only detects glucose whereas) Benedict’s
detects (all) reducing sugars/ example(s) 
Provides a reading/ is quantitative/ Benedict’s
only gives a colour/ doesn’t measure
concentration/ is qualitative/semiquantitative 
Is more sensitive/ detects low concentration 
Red colour/ colour of blood masks result 
Can monitor blood glucose conc. Continuously
DO NOT credit ‘quicker’/ ‘more accurate’