Enzymes are proteins that catalyze biochemical reactions by lowering the activation energy needed. They have a globular structure and an active site that binds specifically to substrates. Some enzymes require cofactors to function. The lock and key and induced fit hypotheses explain how enzymes bind substrates specifically. Enzyme activity is affected by factors like substrate concentration, pH, temperature, and inhibitors. Inhibitors bind to enzymes and reduce reaction rates, and are used in areas like medicine and biochemistry.

1. ENZYMES
A protein with catalytic properties due to its
power of specific activation
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2. Chemical reactions
 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.
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3. Reaction pathway
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4. Making reactions go faster
 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”
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5. An enzyme controlled pathway
 Enzyme controlled reactions proceed 108 to 1011 times faster
than corresponding non-enzymic reactions.
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6. Enzyme structure
 Enzymes are
proteins
 They have a
globular shape
 A complex 3-D
structure
Human pancreatic amylase
© Dr. Anjuman Begum
© 2007 Paul Billiet ODWS

7. The active site
 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
© H.PELLETIER, M.R.SAWAYA
ProNuC Database
more easily © 2007 Paul Billiet ODWS

8. Cofactors
 An additional non-protein
molecule that is
needed by some
enzymes to help the
reaction
 Tightly bound cofactors
are called prosthetic
groups
 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) © 2007 Paul Billiet ODWS

9. The substrate
 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
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10. The Lock and Key Hypothesis
 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
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11. The Lock and Key Hypothesis
Enzyme may
be used again
Enzyme-substrate
complex
E
S
P
E
E
P
Reaction coordinate
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12. The Lock and Key Hypothesis
 This explains enzyme specificity
 This explains the loss of activity when
enzymes denature
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13. The Induced Fit Hypothesis
 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)
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14. The Induced Fit Hypothesis
Hexokinase (a) without (b) with glucose substrate
http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.html
 This explains the enzymes that can react with a
range of substrates of similar types
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15. Factors affecting Enzymes
 substrate concentration
 pH
 temperature
 inhibitors
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16. Substrate concentration: Non-enzymic reactions
Reaction
velocity
Substrate concentration
 The increase in velocity is proportional to the
substrate concentration
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17. Substrate concentration: Enzymic reactions
Reaction
velocity
 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.
Substrate concentration
Vmax
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18. The effect of pH
Optimum pH values
Enzyme
activity Trypsin
Pepsin
1 3 5 7 9 11
pH
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19. The effect of pH
 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.
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20. The effect of temperature
 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
 The optimum temperature for an enzyme controlled
reaction will be a balance between the Q10 and
denaturation.
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21. The effect of temperature
Q10 Denaturation
Temperature / °C
Enzyme
activity
0 10 20 30 40 50
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22. The effect of temperature
 For most enzymes the optimum temperature is about
30°C
 Many are a lot lower,
cold water fish will die at 30°C because their
enzymes denature
 A few bacteria have enzymes that can withstand very
high temperatures up to 100°C
 Most enzymes however are fully denatured at 70°C
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23. Inhibitors
 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.
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24. 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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25. 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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26. 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 EI
Enzyme inhibitor
complex
Reversible
reaction
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27. The effect of enzyme inhibition
Succinate Fumarate + 2H++ 2e-
Succinate dehydrogenase
CH2COOH
CHCOOH
COOH
CH2COOH CHCOOH
COOH
CH2
Malonate
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28. 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
 Cyanide combines with the Iron in the enzymes
cytochrome oxidase.
 Heavy metals, Ag or Hg, combine with –SH groups.
These can be removed by using a chelating agent such
as EDTA.
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29. Applications of inhibitors
 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