1. Enzymes
Lesson 2

2. Starter
How does the induced fit model of enzyme action differ from the lock and key theory?

3. Success criteria
The role of enzymes in catalysing both intracellular and extracellular reactions
To include catalase as an example of an enzyme that catalyses intracellular reactions and amylase and trypsin as examples of enzymes that catalyse extracellular reactions.
The need for coenzymes, cofactors and prosthetic groups in some enzyme-controlled reactions
To include Cl – as a cofactor for amylase, Zn2+ as a prosthetic group for carbonic anhydrase and vitamins as a source of coenzymes.

4. Location, Location, location
Enzymes can only catalyse very specific reactions.
(link to last lesson: lock & key and induced fit)
Due to the substrate being very specific, the enzymes may have to catalyse the reaction in a particular location.
Enzymes are either:
Intracellular or
Extracellular
Discuss what these words mean, and write down the meaning in your notes.

5. Intracellular enzymes
These are enzymes that catalyse reactions within the confines of a cell.
This may be in the cytoplasm, or within individual organelles.
Examples include:
Enzymes involved in aerobic respiration (Kreb’s cycle).
Which organelle would this be in?
Enzymes that modify and process lipids.
Mitochondria
Smooth endoplasmic recticulum
There may be thousands of metabolic reactions occurring in a cell at any moment.

6. Metabolism
All the reactions that take place in an organism are known as metabolism
Anabolic – metabolic reactions that build large molecules
Catabolic - metabolic reactions that break large molecules into smaller ones

7. Metabolic pathways
Recap enzyme action: Discuss with a partner, how lock & key, and induced fit explain enzyme action...
Both of the theories provide an explanation about how the SUBSTRATE is converted into the PRODUCT.
However, the final product is rarely formed by the action of a single enzyme...
To get to the final product, requires a metabolic pathway involving several enzymes.
substrate
products

8. The diagram shows a metabolic pathway involving 3 enzymes.
substrate 2
substrate 1
substrate 3
The diagram shows a metabolic pathway involving 3 enzymes.
What would happen if the DNA coding for enzyme 2 mutated?
enzyme 3
product molecules

9. Intracellular enzyme example:
Oxygen is essential for cells to function. (Why?)
However, it may form reactive compounds like hydrogen peroxide (H2O2), a potentially harmful byproduct of many metabolic reactions, which may damage the cell.
The cells of most living things contain the enzyme catalase, which breaks down H2O2.
Catalase is fast-acting and is found in small vesicles called peroxisomes.
When white blood cells ingest pathogens they use catalase to help kill them
It is a quaternary protein.
What does this mean?
It contains a prosthetic group (haem).
More on this later...
You could show a simple catalase demo here:
Eg: liver & hydrogen peroxide

10. Extracellular Enzymes
Read the bottom two paragraphs of page 124 of the AS class textbook. Or
The middle paragraph of page 102 of the new, replacement textbook.
Answer the questions:
Where in the body is there an example of extracellular enzymes at work?
Give examples of amylase and trypsin
Are these enzymes catalysing anabolic or catabolic reactions?
You could show a simple catalase demo here:
Eg: liver & hydrogen peroxide

11. Extracellular enzymes
Extracellular enzymes are secreted from cells.
Amylase – produced by salivary glands (mouth) and in pancreas (digestion in lumen)
polysaccharide starch  disaccharide maltose
Trypsin – made in pancreas (digestion in lumen)
Protein  smaller polypeptides by hydrolysing peptide bonds

12. Some enzymes need the help of...
Cofactors

13. Prosthetic groups
A cofactor that is permanently bound, by covalent bonds to an enzyme molecule
Carbonic anhydrase with zinc ion permanently bound to its active site.
Found in red blood cells
Catalyses conversion of carbon dioxide and water to carbonic acid, which then breaks down into protons and hydrogencarbonate ions.
Important for the removal of CO2 from respiring tissues to the lungs

14. Cofactors
Some enzymes can only work if another small, non-protein molecule is attached to them (non permanent).
The presence of cofactors such as certain ions may help the formation of the enzyme-substrate complex.
Some cofactors are free and can even join with the substrate to make the correct, complementary shape required (co-substrates).
Some cofactors change the charge distribution on the surface of the substrate or enzyme and make the temporary bonds in the ES-complex easier to form.
e.g. Amylase will only digest starch in the presence of chloride ions
cofactor
enzyme
active site
substrate

15. Coenzymes
Coenzymes (small organic non-protein molecules) may also help the enzyme-substrate complex form by temporarily binding to the enzymes active site
Coenzymes bind temporarily to the active site of enzymes.
Many vitamins act as coenzymes.
Vitamin C is a very important coenzyme.
Unlike prosthetic groups and other cofactors, coenzymes are changed in a reaction.
What’s the implication of this?
need to be recycled or need a source of more
Implication: need to be recycled or need a source of more.

16. Question
Nicotinamide (NAD) is a very important coenzyme needed by cells. The RDA for humans is 18mg.
The amount of NAD used in metabolic reactions is a great deal more than 18mg. Suggest why the RDA is so low.
The NAD is constantly recycled, which means that there is a always a supply of it. Therefore not much is needed in the diet.
Implication: need to be recycled or need a source of more.

17. Plenary Discuss with a partner:
The difference between intracellular and extracellular enzymes, and examples of each.
The similarities and differences between:
Cofactors, Prosthetic Groups and Coenzymes.
Implication: need to be recycled or need a source of more.

18. Success criteria
The role of enzymes in catalysing both intracellular and extracellular reactions
To include catalase as an example of an enzyme that catalyses intracellular reactions and amylase and trypsin as examples of enzymes that catalyse extracellular reactions.
The need for coenzymes, cofactors and prosthetic groups in some enzyme-controlled reactions
To include Cl – as a cofactor for amylase, Zn2+ as a prosthetic group for carbonic anhydrase and vitamins as a source of coenzymes.