1. ENZYMES
A protein with catalytic properties due to its power of specific activation
© 2007 Paul Billiet ODWS

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.
© 2007 Paul Billiet ODWS

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”
© 2007 Paul Billiet ODWS

5. Enzymes Without Enzyme With Enzyme Free Energy
Progress of the reaction
Reactants
Products
Free energy of activation

6. Enzyme structure Enzymes are proteins They have a globular shape
A complex 3-D structure
Human pancreatic amylase

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 more easily

8. Enzyme-Substrate Complex
The substance (reactant) an enzyme acts on is the substrate
Enzyme
Joins
Substrate

9. Cofactors
An additional non-protein molecule that is needed by some enzymes to help the reaction
coenzymes are organic molecules that are required by certain enzymes to carry out catalysis.
Many vitamins are coenzymes
Cofactors are often classified as inorganic substances that are required for, or increase the rate of, catalysis
Fe+3,Fe+2, Zn+2
Nitrogenase enzyme with Fe, Mo and ADP cofactors )
© 2007 Paul Billiet ODWS

10. 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
© 2007 Paul Billiet ODWS

11. 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
© 2007 Paul Billiet ODWS

12. The Lock and Key Hypothesis
Enzyme may be used again
Enzyme-substrate complex E S P
Reaction coordinate
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13. The Lock and Key Hypothesis
This explains enzyme specificity
This explains the loss of activity when enzymes denature
© 2007 Paul Billiet ODWS

14. 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)
© 2007 Paul Billiet ODWS

15. The Induced Fit Hypothesis
Hexokinase (a) without (b) with glucose substrate
This explains the enzymes that can react with a range of substrates of similar types
© 2007 Paul Billiet ODWS

16. Factors affecting Enzymes
Concentration of substrate
Concentration of enzyme
Temperature pH
Inhibitors
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17. Substrate concentration: Non-enzymic reactions
Reaction velocity
Substrate concentration
The increase in velocity is proportional to the substrate concentration
© 2007 Paul Billiet ODWS

18. Substrate concentration: Enzymic reactions
Reaction velocity
Substrate concentration
Vmax
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.
© 2007 Paul Billiet ODWS

19. The effect of pH Optimum pH values Enzyme activity Trypsin Pepsin pH 13 5 7 9 11
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20. 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 ionization will affect the binding of the substrate with the active site.
© 2007 Paul Billiet ODWS

21. 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.
© 2007 Paul Billiet ODWS

22. The effect of temperature
Temperature / °C
Enzyme activity 10 20 30 40 50
Q10
Denaturation
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23. 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
© 2007 Paul Billiet ODWS

24. 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.
© 2007 Paul Billiet ODWS

25. 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 containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase.
© 2007 Paul Billiet ODWS

26. 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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27. The effect of enzyme inhibition
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.
Enzyme inhibitor complex
Reversible reaction
E + I EI
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28. The effect of enzyme inhibition
Succinate
Fumarate + 2H++ 2e-
Succinate dehydrogenase
CH2COOH
CHCOOH
COOH
CH2
Malonate
© 2007 Paul Billiet ODWS

29. The effect of enzyme inhibition
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.
© 2007 Paul Billiet ODWS

30. Applications of inhibitors
Negative feedback: end point or end product inhibition
Poisons snake bite, plant alkaloids and nerve gases.
Medicine antibiotics, sulphonamides.
© 2007 Paul Billiet ODWS

31. Enzyme pathways A B C D E F
Cell processes (e.g. respiration or photosynthesis) consist of series of pathways controlled by enzymes
A B C D E F eF eD eC eA eB
Each step is controlled by a different enzyme (eA, eB, eC etc)
This is possible because of enzyme specificity
© 2008 Paul Billiet ODWS

32. End point inhibition
The first step (controlled by eA) is often controlled by the end product (F)
Therefore negative feedback is possible
A B C D E F eA eB eC eD eF
Inhibition
The end products are controlling their own rate of production
There is no build up of intermediates (B, C, D and E)
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33. Example: Phosphofructokinase and ATP
Substrate: Fructose-6-phosphate
Reaction
Fructose-6-phosphate + ATP  fructose-1,6-bisphosphate + ADP
phosphofructokinase
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34. ATP is the end point
This reaction lies near the beginning of the respiration pathway in cells
The end product of respiration is ATP
If there is a lot of ATP in the cell this enzyme is inhibited
Respiration slows down and less ATP is produced
As ATP is used up the inhibition stops and the reaction speeds up again
© 2008 Paul Billiet ODWS