1. Unit 2.5 Enzymes

2. Metabolism—the sum total of all chemical reactions occurring in a biological system at a given time
Anabolic reactions join simple molecules to form complex ones.
They require energy inputs; energy is captured in the chemical bonds that form.
Catabolic reactions break down complex molecules into simpler ones.
Energy stored in the chemical bonds is released.

3. Enzymes are a type of protein that makes or breaks specific covalent bonds.

4. Covalent bonds require energy to form and release energy when they are broken.
Enzymes are required to make/break covalent bonds.
Example: sucrase enzyme breaks apart sucrose into glucose and fructose.

5. Gibbs Free Energy, G Gproducts- Greactants = ΔG
Direction of Reaction
Direction of Reaction
Negative ΔG result in “spontaneous” reactions.

6. Enzymes assist chemical reactions by providing an active site that promotes the reaction.
Promotes formation and breaking of covalent bonds.
Extremely specific to the substrate and chemical reaction.

7. Which of the following are true statements?
A: The reaction rate without an enzyme is affected by the concentration of the substrate. B: Once the reaction rate with an enzyme reaches a maximum, the rate is unaffected by increased concentration of the substrate. C: The reaction rate is more strongly affected by the presence of the substrate than by the presence of an enzyme. D: A and B E: A, B and C

8. Enzyme movie showing catabolic and anabolic reactions

9. 3.6.1 Define enzyme and active site
Definition – enzymes are globular proteins that work as catalysts.
• A catalyst speeds up chemical reactions.
• The enzyme converts a substrate into a product
• Enzymes are found in all living cells and are secreted by some to work outside the cell
• Enzymes can be used again and again without altering its shape/structure
Definition – the active site is the location on an enzyme where a substrate binds

10. 3.6.2 Explain enzyme-substrate specificity
Living organisms produce thousands of enzymes because most enzymes only catalyze one biochemical reaction.
Enzyme Specificity
• An enzyme will only work with a specific substrate for a specific reaction
• The enzyme’s 3-dimensional structure results in a specific shape that only a specific substrate will fit
Called “the lock and key” model
home.mira.net

11. Induced fit model: Interaction between substrate and enzyme changes shape of enzyme.

12. 7.6.2 Describe the induced fit model
The lock-and-key model problems:
it can't explain events that happen at the same time.
The enzyme will change shape
when substrate binds, it change shape which will allow another substrate to bind to the enzyme
The induced fit model explains it better
Definition - it is not an exact “lock and key” model, but the enzyme has the ability to slightly change its shape

13. 3.6.3 Explain the effect of temperature, pH and substrate concentration on enzyme activity
Simply, if there is more substrate, the reaction will be quicker
Another substrate cannot bind to the binding site until the product has been
Produced, so eventually the reaction rate will slow down.

14. 3.6.4 Define denaturation
Definition – the structural change of a protein resulting in its loss of its biological properties
• Caused by high temperatures and extreme pH

15. 3.6.3 Explain the effect of temperature, pH and substrate concentration on enzyme activity
Relationship between enzyme activity and temperature

16. Can you explain the effect of T and pH on enzyme reaction rates?

17. 3.6.5 Explain the use of lactase in the production of lactose-free milk
Lactose is the sugar that is naturally present in milk
It is converted into glucose and galactose by the enzyme lactase
70% of humans in the world are lactose intolerant because the gene that produces the enzyme, lactase, is turned off.
The enzyme can be obtained from yeast, purified and then added to the milk.
The lactose is broken down into glucose and galactose to allow lactose intolerant people to drink milk, eat ice cream, etc…

18. 3.6.5 Explain the use of lactase in the production of lactose-free milk
A more effective way to convert lactose into glucose + galactose is called enzyme immobilization
Procedure:
First the enzyme (lactase) is immobilized in alginate beads.
Next the beads are placed in a container over which milk can be passed.
The milk is collected and re-circulated (pumped) to convert any remaining lactose to glucose and galactose.
The circulation is maintained until all lactose has been converted.
This procedure is cheaper and more efficient

19. Movie of use of nanoparticles for enzyme immobilization

20. 3.6.5 Explain the use of lactase in the production of lactose-free milk
Percentage of the world population that is lactose intolerant. Are you part of the majority in your country?

21. 7.6.1 State that metabolic pathways consist of chains and cycles of enzyme catalyzed reactions
Most chemical changes do not occur as the result of a single reaction

22. 7.6.3 Explain that enzymes lower the activation energy of chemical reaction that they catalyze
Enzymes work by lowering the amount of energy required to activate the reacting molecule
In other words, in the absence of enzymes, a lot more energy would be needed to produce products that are necessary for metabolic processes
The boulder model

23. Regulation of enzyme function by chemical inhibitors
1. Reversible inhibition
Competitive: Inhibitor competes with substrate for active site.
Noncompetitive: Inhibitor binds to enzyme, alters enzyme shape and decreases enzyme function.
Common in biological systems. Why?

24. A competitive inhibitor competes with natural substrate for active sites.
As substrate concentration increases, the active sites become saturated with the substrate and the effect of the inhibitor in decreased.
ANIMATED TUTORIAL 3.2 Enzyme Catalysis

25. A noncompetitive inhibitor binds at a site distinct from the active site
This causes allosteric change in the shape of the enzyme that decreases the maximum enzyme function.

26. Allosteric regulation
Covalent modification of amino acid by addition of a phosphate group
Non-covalent interactions of regulatory molecule

27. 7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each
Competitive inhibitors
The structure of a molecule is very similar to the substrate and both compete for the active site of the enzyme this will prevent the substrate from binding
Example:
Prontosil (antibacterial drug) that binds to an enzyme and inhibits the synthesis of folic acid which is needed by the bacteria to survive

28. 7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each
Non-competitive inhibitors
The structure of the molecule is not similar to the substrate, but when it binds to the enzyme (not the active site) it either blocks the active site or it changes it shape
Adding more substrate will not overcome the effect of the inhibitor
Example:
Sarin is a nerve gas that binds to the enzyme acetyl cholinesterase it changes the shape of the active site and doesn't allow transmission to occur across a synapse
If the transmission cannot occur, the result is seizures, paralysis and death

29. 7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites
Allostery – a type of non-competitive inhibition
Allosteric enzymes are made of at least two polypeptide chains and have two sites (an active site for the substrate and a site for the allosteric effector)
Allosteric effectors are divided into two groups
allosteric activators – speed up reactions
allosteric inhibitors – slow down the reaction
End-product inhibition – the final product of a metabolic pathway will inhibit the beginning of the pathway

30. Good tutorial and animation at:
7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites
Example - glycolysis
Glucose is broken down into two pyruvate molecules and eventually ATP (the end product/allosteric effector)
In the first few steps of glycolysis fructose-6-phosphate is changed into fructose-1,6-diphosphate catalyzed by the enzyme phosphofructokinase (the allosteric enzyme)
When enough ATP is produced it will bind to the effector site and halt the metabolic pathway
Good tutorial and animation at:

31. Which is a true statement?
A: Competitive inhibitors slow enzyme function by occupying active sites of enzymes. B: Competitive inhibitors are not affect by substrate concentration. C: As substrate concentration increases, the effectiveness of the inhibitor increases. D: Competitive inhibitors decrease the maximum rate of a catalyzed reaction.

32. Why is the maximum reaction rate lowered in the presence of a non-competitive inhibitor?