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ENZYMES
A protein with catalytic properties due to its
power of specific activation
© 2007 Paul Billiet ODWS
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.
© 2007 Paul Billiet ODWS
Reaction pathway
© 2007 Paul Billiet ODWS
Making reactions go faster
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Increasing the temperature make molecules move
faster
Biological systems are very sensitive to temperature
changes.
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”
© 2007 Paul Billiet ODWS
An enzyme controlled pathway
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Enzyme controlled reactions proceed 108 to 1011 times faster
than corresponding non-enzymic reactions.
© 2007 Paul Billiet ODWS
Enzyme structure
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Enzymes are
proteins
They have a
globular shape
A complex 3-D
structure
Human pancreatic amylase
© Dr. Anjuman Begum
© 2007 Paul Billiet ODWS
The active site
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© H.PELLETIER, M.R.SAWAYA
ProNuC Database
© 2007 Paul Billiet ODWS
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
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
The specificity is determined by the active
site
© 2007 Paul Billiet ODWS
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
© 2007 Paul Billiet ODWS
The Lock and Key Hypothesis
S
E
E
E
Enzymesubstrate
complex
Enzyme may
be used again
P
P
© 2007 Paul Billiet ODWS
Reaction coordinate
The Lock and Key Hypothesis
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This explains enzyme specificity
This explains the loss of activity when
enzymes denature
© 2007 Paul Billiet ODWS
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)
© 2007 Paul Billiet ODWS
The Induced Fit Hypothesis
Hexokinase (a) without (b) with glucose substrate
http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.html
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This explains the enzymes that can react with a
range of substrates of similar types
© 2007 Paul Billiet ODWS
Factors affecting Enzymes
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substrate concentration
pH
temperature
inhibitors
© 2007 Paul Billiet ODWS
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.
© 2007 Paul Billiet ODWS
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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© 2007 Paul Billiet ODWS
The effect of enzyme inhibition
Reversible inhibitors: These can be washed
out of the solution of enzyme by dialysis.
There are two categories.
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© 2007 Paul Billiet ODWS
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.
© 2007 Paul Billiet ODWS
E+I
Reversible
reaction
EI
Enzyme inhibitor
complex
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.
Examples
o  Cyanide combines with the Iron in the enzymes
cytochrome oxidase.
o  Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such
as EDTA.
© 2007 Paul Billiet ODWS
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
© 2007 Paul Billiet ODWS