13. Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are proteins that catalyse specific chemical reactions

Question 2

Describe the importance of enzymes? (4pts)

Answer 2

1. Fulfil important functions in cells
2. Changes in enzymes are associated with inherited diseases
3. Used as drug targets
4. Enzymes used as catalysts

Question 3

Describe the functions of enzymes? (5pts)

Answer 3

1. Digestion- carbohydrates, fats
2. Blood clotting- fibrin clot is catalysed by thrombin
3. Defence in the immune system
4. Movement
5. Nerve conduction

Question 4

Describe enzymes (3pts)

Answer 4

1. Enzymes are intracellular or secreted
2. Enzyme examples include- Proteases, nucleases, polymerases and kinases.
3. Enzyme defect causes disease. Examples of inherited diseases include Phenylketonuria, Glycogen storage disease and Tay-sachs disease.

Question 5

Describe Phenulketonuria

Answer 5

Cannot convert Phe to tyr

Question 6

Describe glycogen storage disease

Answer 6

Cannot mobilise glucose

Question 7

Describe Tay-sachs disease

Answer 7

A defect in processing a membrane ganglioside. This can lead to neuronal damage and death

Question 8

Describe how enzymes are drug targets? (3pts)

Answer 8

1. Antibiotics target enzymes e.g penicillin inhibit cell wall synthesis in bacteria.
2. Anti-inflammatory agents target enzymes- aspirin blocks prostaglandin synthesis.
3. Anticancer drugs target enzymes and inhibit cancer cells.

Question 8

Describe how enzymes are drug targets? (3pts)

Answer 8

1. Antibiotics target enzymes e.g penicillin inhibit cell wall synthesis in bacteria.
2. Anti-inflammatory agents target enzymes- aspirin blocks prostaglandin synthesis.
3. Anticancer drugs target enzymes and inhibit cancer cells.

Question 9

Describe Key enzyme properties? (5pts)

Answer 9

1. Increase reaction rate up to 10 billion fold
2. Increase the rate of reaction by lowering the activation energy
3. Enzymes show specificity with the substrates they work on.
4. Unchanged at the end of the reaction
5. Do not alter reaction equilibrium but catalyse the reaction in both directions.

Question 10

Describe an enzymes active site? (3pts)

Answer 10

1. Active site= Is a 3D cleft that binds to the substrate with specificity through electrostatic, hydrophobic, hydrogen bonding and van der walls interactions.
2. In an enzyme catalysed reaction the enzyme binds to the substrate. Not available in the uncatalysed reaction.
3. Evidence for active sites come from:
4. X-ray crystallography
5. Kinetic studies of enzyme activity

Question 11

Describe structure determination by x-ray crystallography (6pts)

Answer 11

1. Purify a large amount of protein/ enzyme of interest.
2. Crystallise the protein/enzyme
3. Place the crystal in the path of a powerful xray beam
4. Molecules in the crystal diffract the x-ray beam
5. Record the diffraction pattern which have varying intensities.
6. You can then work back to identify the individual molecules of the crystal.

Question 12

Describe the Lock and key model theory for how enzymes recognise their substrate? (2pts)

Answer 12

1. The enzyme active site is directly complimentary in structure to the substrate.
2. They then form an enzyme substrate complex.

Question 13

Describe the Lock and key model theory for how enzymes recognise their substrate? (2pts)

Answer 13

1. The enzyme active site is directly complimentary in structure to the substrate.
2. They then form an enzyme substrate complex.

Question 14

Describe the Induced fit model? (2pts)

Answer 14

1. The enzyme active site is not directly complementary to the structure of the substrate
2. As the substrate binds to the active site the enzyme changes its shape so that the active site becomes complementary to the structure of the substrate.

Question 15

Describe the binding of glucose to hexokinase? (4pts)

Answer 15

1. Glucose binds to hexokinase via an induced fit mechanism.
2. Hexokinase is involved in the early stages of glycolysis
3. Can get the structure off hexokinase via xray crystallography.
4. Hexokinase changes its shape so that the active site becomes complimentary to the structure of the glucose substrate.

Question 16

How do enzymes use enzyme-substrate binding energy? (6pts)

Answer 16

1. To bring molecules together in the active site.
2. To constrain substrate movement
3. To strain particular bonds in the substrate making breakage easier, Substrate is distorted on binding to resemble a transition state.
4. To stabilise positive and negative charges in the transition state.
5. To exclude water from the active site- this makes the reaction go faster.
6. To provide a reaction pathway of lower energy.
7. Use cofactors

Question 17

How do Lysosomes cause strain to cleave its bacterial substrate? (3pts)

Answer 17

• lysosomes strains bonds causing a break in the polysaccharide chain.

1. Lysosomes recognises a polysaccharide on the bacterial cell wall and cleaves this polysaccharide.
2. This wakens the cell wall of the bacterium.
3. The strong osmotic pressure inside the bacterium causes its rupture.

Question 18

How do enzyme kinetics provide evidence for active sites? (6pts)

Answer 18

1. Take a series of tubes
2. Place the same concentration of enzymes in each tube
3. Use increasing amounts of the substrate
4. Measure the initial rate at which the substrate is converted to the product.
5. Plot substrate concentration v reaction velocity
6. The rate increases as the concentration increases until it reaches its maximum velocity. This is because all the enzyme active sites are filled up therefore increasing the substrate conc will have no effect on the rate.

Question 19

Michealis-Menten equation ?

Answer 19

V= VMAX S/ (S)+KM

Question 20

What is the KM?

Answer 20

KM is the measure of the affinity of the substrate for the active site. values in uM to mM range.

Question 21

What is the turnover number?

Answer 21

Max number of substrate molecules handled per active sites per second. Found by dividing the v max by the total concentration.

Question 22

Descrribe Competitive Inhibition? (2pts)

Answer 22

1. Inhibitor will compete with the substrate to bind to the active site forming an inactive complex
2. In the presence of a competitive inhibitor Km increases as it takes more substrate to reach Vmax. Vmax is unaltered.

Question 23

Describe non-competitive inhibition (2pts)

Answer 23

1. Inhibitor binds at a different site and does not compete with the substrate for binding at the active site.
2. The substrate km is unaltered but the Vmax is reduced.

Question 24

Describe how enzyme activity is regulated in cellls?

Answer 24

Used in:

1. Control of gene expression
2. Compartmentation
3. Allosteric regulation
4. Covalent modification of enzyme

Question 25

Describe the control of gene expression? (1pts)

Answer 25

The gene coding for the enzyme can be controlled to only produce a certain amount of enzyme

Question 26

Describe compartmentation ?

Answer 26

The enzyme can be targeted to certain organelles or structures through compartmentation. This is where a sequence in the polypeptide chain is recognised by the enzymes target and therefore directed to it.

Question 27

Describe Allosteric regulation?

Answer 27

Allosteric regulation is when a regulatory molecule changes the enzyme conformation to influence the active site and decrease or increase enzyme activity. Controls the flux of material through a metabolic pathway. occurs during glycolysis.

Question 28

Describe the covalent modification of enzyme?

Answer 28

Changes the enzyme shape and activity e.g phosphorylation.

Question 29

Describe CTP?

Answer 29

CPT inhibits enzyme and regulates its own production

Question 30

Describe allosteric enzymes? (5pts)

Answer 30

1. Multisubunit complexes
2. Regulatory sites and catalytic sites on different subunits
3. Regulation occurs via conformational changes.
4. Exhibit non- Michael’s mentes kinetics
5. Involved in feedback inhibition off metabolic pathways.

Question 31

Describe the function of the antibiotic Novobiocin?

Answer 31

The antibiotic novobiocin inhibits the enzyme gyrase which converts relaxed DNA using ATP to supercoiled DNA. As a result relaxed DNA will not be converted to supercoiled DNA in bacteria.

Question 31

Describe the function of the antibiotic Novobiocin?

Answer 31

The antibiotic novobiocin inhibits the enzyme gyrase which converts relaxed DNA using ATP to supercoiled DNA. As a result relaxed DNA will not be converted to supercoiled DNA in bacteria.