ENZYMES
A protein with catalytic properties due to its
power of specific activation
Chemical reactions
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Chemical reactions need an initial input of energy =
THE ACTIVATION ENERGY
During this part of the reaction the molecules are
said to be in a transition state.
Reaction pathway
Making reactions go faster
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Increasing the temperature makes molecules move
faster
Enzymes can increase the rate of reactions without
increasing the temperature.
They do this by lowering the activation energy.
They create a new reaction pathway “a short cut”
An enzyme controlled pathway
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Enzyme controlled reactions proceed ~100 to 1000 times
faster than corresponding non-enzymic reactions.
Enzyme structure
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Enzymes are
proteins
They have a
globular shape
A complex 3-D
structure
Human pancreatic amylase
The active site
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One part of an enzyme,
the active site, is
particularly important
The shape and the
chemical environment
inside the active site
permits a chemical
reaction to proceed
more easily
Active Site (continued)
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The active site is where the substrate fits into
in order for the reaction to take place. The
shape of the active site must complement the
substrate
Cofactors
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An additional nonprotein molecule that is
needed by some
enzymes for the reaction
to take place
Cofactors that are bound
and released easily are
called coenzymes
Many vitamins are
coenzymes
Nitrogenase enzyme with Fe, Mo and ADP cofactors
Jmol from a RCSB PDB file © 2007 Steve Cook
H.SCHINDELIN, C.KISKER, J.L.SCHLESSMAN, J.B.HOWARD, D.C.REES
STRUCTURE OF ADP X ALF4(-)-STABILIZED NITROGENASE COMPLEX AND ITS
IMPLICATIONS FOR SIGNAL TRANSDUCTION; NATURE 387:370 (1997)
The substrate
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The substrate of an enzyme are the reactants
that are activated by the enzyme
Enzymes are specific to their substrates (i.e.
enzymes can only catalyse one type of
substrate
The specificity is determined by the active
site
The Lock and Key Hypothesis
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Fit between the substrate and the active site of the enzyme is
exact
Like a key fits into a lock very precisely
The key is analogous to the enzyme and the substrate
analogous to the lock.
Temporary structure called the enzyme-substrate complex
formed
Products have a different shape from the substrate
Once formed, they are released from the active site
Leaving it free to become attached to another substrate
The Lock and Key Hypothesis
S
E
E
E
Enzymesubstrate
complex
Enzyme may
be used again
P
P
Reaction coordinate
The Lock and Key Hypothesis
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This explains enzyme specificity
This explains the loss of activity when
enzymes denature
The Induced Fit Hypothesis
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Some proteins can change their shape
(conformation)
When a substrate combines with an enzyme, it
induces a change in the enzyme’s conformation
The active site is then moulded into a precise
conformation
Making the chemical environment suitable for the
reaction
The bonds of the substrate are stretched to make the
reaction easier (lowers activation energy)
The Induced Fit Hypothesis
Hexokinase (a) without (b) with glucose substrate
Enzyme Regulation
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Enzymes do not work all the time
Our bodies must control the rate of chemical
reactions
Enzymes can be switched on and off by a
mechanism known as “Allosteric Modulation”
Allosteric Modulation of Enzymes
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An enzyme has a site that is away from the active site
When a regulator molecule enters the site it changes the shape
of the active site. It can activate the enzyme or deactivate it
Factors affecting Enzymes
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substrate concentration
pH
temperature
inhibitors
Substrate concentration: Non-enzymic reactions
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The increase in velocity is proportional to the
substrate concentration
Substrate concentration: Enzymic reactions
Vmax
Reaction
velocity
Substrate concentration
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Faster reaction but it reaches a saturation point when all the
enzyme molecules are occupied.
If you alter the concentration of the enzyme then Vmax will
change too.
The effect of pH
Optimum pH values
Enzyme
activity
Trypsin
1
3
5
7
pH
9
11
Pepsi
n
All enzymes have an optimal pH that
they work best at
The effect of pH
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Extreme pH levels will produce denaturation
The structure of the enzyme is changed
The active site is distorted and the substrate
molecules will no longer fit in it
At pH values slightly different from the enzyme’s
optimum value, small changes in the charges of the
enzyme and it’s substrate molecules will occur
This change in ionisation will affect the binding of
the substrate with the active site.
The effect of temperature
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Q10 (the temperature coefficient) = the increase in
reaction rate with a 10°C rise in temperature.
For chemical reactions the Q10 = 2 to 3
(the rate of the reaction doubles or triples with every
10°C rise in temperature)
Enzyme-controlled reactions follow this rule as they
are chemical reactions
BUT at high temperatures proteins denature (lose
their shape)
The optimum temperature for an enzyme controlled
reaction will be a balance between the Q10 and
denaturation.
The effect of temperature
Q10
Enzyme
activity
0
10
20
30
40
Temperature / °C
Denaturation
50
The effect of temperature
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For most enzymes the optimum temperature is about
30°C and depends on the organism
Many human enzymes have an optimal temperature
of 37 °C. Why?
A few bacteria have enzymes that can withstand very
high temperatures up to 100°C
Most enzymes however are fully denatured at 70°C
Inhibitors
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Inhibitors are chemicals that reduce the rate of
enzymic reactions.
The are usually specific and they work at low
concentrations.
They block the enzyme but they do not
usually destroy it.
Many drugs and poisons are inhibitors of
enzymes in the nervous system.
The effect of enzyme inhibition
Irreversible inhibitors: Combine with the
functional groups of the amino acids in the
active site, irreversibly.
Examples: nerve gases and pesticides,
containing organophosphorus, combine with
serine residues in the enzyme acetylcholine
esterase.
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The effect of enzyme inhibition
Reversible inhibitors: These can be removed
from the active site of the enzyme over time.
There are two categories.
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The effect of enzyme inhibition
1. Competitive: These
compete with the
substrate molecules for
the active site.
The inhibitor’s action is
proportional to its
concentration.
Resembles the substrate’s
structure closely.
E+I
Reversible
reaction
EI
Enzyme inhibitor
complex
Competitive Inhibition
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Inhibitor gets into the active site of the
enzyme and blocks it off
Non-competitive Inhibition
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The inhibitor enters the allosteric (regulatory
site) of the enzyme and changes its shape so
the substrate no longer fits
The effect of enzyme inhibition
2. Non-competitive: These are not influenced by the
concentration of the substrate. It inhibits by binding
irreversibly to the enzyme but not at the active site.
Example
 Cyanide combines with the Iron in the enzymes
cytochrome oxidase.
Applications of inhibitors
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Negative feedback: end point or end product
inhibition
Poisons snake bite, plant alkaloids and nerve
gases.
Medicine antibiotics, sulphonamides,
sedatives and stimulants