Enzymes as Drug Targets

Question 1

What happens when an enzyme is used?

Answer 1

Enzyme substrate is stabilised and therefore, activation energy is lowered.

Question 2

What are the 4 catalytic strategies?

Answer 2

• Covalent strategies
• General acid - base catalysis
• Metal ion catalysis
• Catalysis by proximity and orientation of two substrates.

Question 3

What is needed for the energy to reach the transition state?

Answer 3

An input of energy is required for the enzyme to reach the transition state.

Question 4

What is chymotrypsin?

Answer 4

It is a digestive enzyme found in the pancreatic juices of the duodenum.
This is where proteolysis occurs : which is where breakdown of proteins and polypeptides occurs.

Question 5

What substances form a catalytic triad in chymotrypsin?

Answer 5

Serine 195 / Histidine 57 / Aspartate 102.

Question 6

What catalytic strategies are used in chymotrypsin?

Answer 6

Covalent catalysis and general acid - base catalysis.

Question 7

For example, if serine 195 was missing from the catalytic triad what would happen?

Answer 7

No reactions would occur as the catalytic triad would not be present.

Question 8

What bonds to His - 57?

Answer 8

Asp - 102 bonds to His - 57 and then, orients it correctly. Essentially, Asp - 102 holds His - 57 in place.

Question 9

Who does His - 57 accept a proton from?

Answer 9

Accepts a proton from Ser - 195 as it acts as a base.

A base accepts a hydrogen proton

Question 10

What happens to the serine O- ion as it has now lost a proton to His - 57?

Answer 10

Serine O- ion is now a very strong nucleophile and is also, highly reactive to electron - deficient atoms.

Question 11

What catalytic strategies are used by carbonic anhydrase?

Answer 11

Carbonic anhydrase uses general - acid / base catalysis, metal ion catalysis and catalysis by proximity.

Question 12

What does Carbonic Anhydrase do?

Answer 12

They reduce the pKA of H20 from pH 15 to pH 7 - thus more likely to be in OH- form. (Has to bind in order to reduce the pH.)
It is a stronger nucleophile.

Question 13

Where is carbonic anhydrase important?

Answer 13

It is important in the blood for maintenance of blood pH.

Question 14

What is carbonic acid?

Answer 14

It is a weak acid and can dissociate in order to form bicarbonate ions.
- Bicarbonate ions are alkaline and therefore, can balance an acidic pH.

Question 15

What is the effective pKA of carbonic acid?

Answer 15

pH of 6.4

Carbonic anhydrase is important as it can buffer the blood.

Question 16

What are the binding interactions in enzyme catalysis?

Answer 16

• Strong enough to hold substrate long enough for reaction to occur.
• Weak enough to allow the product to leave.

Question 17

What happens in drug design?

Answer 17

Designs molecules with stronger binding interactions which results in enzyme inhibitors which block the active site.

Question 18

What is a drug target?

Answer 18

When a drug acts on a specific protein or cell in the body.

Question 19

What are the main protein drug targets?

Answer 19

• Receptors
• Enzymes
• Transporters
• Ion Channels

Question 20

What are the two types of enzyme inhibition?

Answer 20

• Reversible inhibition

- Irreversible inhibition

Question 21

Explain reversible inhibition.

Answer 21

The inhibitor binds non - covalently.
This prevents the substrate from binding.
Inhibitor rapidly dissociates from the enzyme.

Question 22

Explain irreversible inhibition.

Answer 22

The inhibitor binds tightly to the enzyme.
There is a slow dissociation of the the enzyme inhibitor complex.
The tight binding prevents the enzyme from working and thus, NO PRODUCT is formed.

Question 23

What are the 3 types of reversible inhibitors?

Answer 23

• Competitive
• Non - competitive
• Uncompetitive

Question 24

Explain competitive inhibitors.

Answer 24

They bind to the same site as the enzyme substrate.
Thus is a competition with the substrate. (CHEMICAL STRUCTURE IS SIMILAR).
No reaction happens when the inhibitor binds to the active site.

Question 25

Explain the various pathways which can be taken in order for an enzyme to create a product - Regarding competitive inhibitors.

Answer 25

Enzyme adds onto a substrate and therefore, an enzyme substrate is formed. This dissociates in order to form an enzyme and product.
Additionally, an enzyme can add onto an inhibitor and therefore, an enzyme inhibitor complex is formed. No product is formed as the reaction is blocked.
However, if the inhibitor is swapped out and a substrate is put in place = Enzyme Substrate is formed.
Then the enzyme substrate dissociates and therefore, an enzyme and product is formed.

Question 26

What is the competitive inhibitor of the enzyme dihydrofolate reductase?

Answer 26

The anti - cancer drug methotrexate because it has a similar structure.

Question 27

What is a non - competitive inhibitor?

Answer 27

They bind to a different site from the enzyme substrate.

Known as allosteric regulators - stabilise the inactive conformation of the enzyme.

Question 28

What is different from the substrate in competitive inhibitors?

Answer 28

The chemical substrate is different in competitive inhibitors.

Question 29

What happens when a substrate is added to the enzyme inhibitor complex?

Answer 29

An enzyme inhibitor - substrate complex is formed.

Indicates the effect is irreversible.

Question 30

What is possible after an enzyme inhibitor - substrate complex is formed?

Answer 30

The inhibitor can dissociate and form an enzyme substrate complex = Thus products can be formed.

Question 31

Why are no products formed when an inhibitor binds to the enzyme?

Answer 31

The inhibitor is not covering the active site fully as it is not in physical contact with the active site.
Furthermore, this results in no products being formed.

Question 32

What is the non - competitive inhibitor of bacterial proteolytic enzyme collagenase?
What is it used to treat?

Answer 32

Doxycycline.

Used as an antibiotic to treat gum disease.

Question 33

What is uncompetitive inhibition?

Answer 33

This is when the inhibitor will bind to the enzyme substrate complex.
And thus, no reaction occurs.

Question 34

What does irreversible inhibitors do?

Answer 34

They permanently inactivate enzymes.

Question 35

Why do irreversible inhibitors permanently inactivate enzymes?

Answer 35

They form covalent bonds with the enzyme which can’t be broken.
There is direct binding to the drug target

Question 36

What is an example of an irreversible inhibitor?

Answer 36

Aspirin is an irreversible inhibitor for the COX enzymes.

Question 37

What is Orlistat?

Answer 37

It is an obesity drug which inhibits pancreatic lipase : therefore, blocks digests fats in the intestine.

Question 38

What is the effect of Orlistat?

Answer 38

It reduces the biosynthesis of fat in the body.

Question 39

What are reversible competitive antagonists?

Answer 39

These compete with agonists for the binding site on receptors.

Question 40

What gives an ‘S - shaped’ curve?

Answer 40

An allosteric enzyme gives an ‘s - shaped’ curve.

Question 41

What happens to the graph in the presence of a reversible competitive antagonist?

Answer 41

The dose - response curve for an agonist shifts to the right.

Question 42

What is fractional agonist occupancy?

Answer 42

This is how many active sites are occupied.

Question 43

What happens to the graph in the presence of an irreversible competitive antagonist?

Answer 43

Binds to the same site as the agonist but it cannot be released.
Adding more agonist can’t overcome the effect and therefore, the curve shifts down.

Question 44

What happens with an irreversible competitive antagonist?

Answer 44

This reduces the number of active sites available.