W10.1_Enzyme Inhibitors

Question 1

Describe the role of enzymes in diseases with phenylketonuria as an example. Explain the enzymatic pathway of converting tyrosine to adrenaline in the body.

Answer 1

• Enzymes in diseases: as diagnostic (biomarker)/therapeutic (used as drug target)
• Ex. in enzyme deficiency
• Ex. in phenylketonuria: absence of enzyme causes accumulation of toxic levels of Phe (avoid aspartame, which is a natural sweetener, as it hydrolyses to form Phe)
• Tyrosine -tyrosine hydroxylase-> L-DOPA -DOPA decarboxylase-> dopamine -dopamine ß-hydroxylase-> noradrenaline -phenethanolamine N-methyltransferase-> adrenaline

Question 2

With reference to the pathway, describe the consequences and possible treatment for tyrosine hydroxylase deficiency and dopamine ß-hydroxylase deficiency.

Answer 2

• Tyrosine hydroxylase (TH) deficiency: cannot convert tyrosine into L-DOPA -> dopamine needed for motor control and movement -> treatment of sinemet (carbidopa + levodopa), where carbidopa is a DOPA decarboxylase inhibitor to increase bioavailability of L-DOPA and levodopa provides L-DOPA that penetrates blood-brain barrier easily
• Dopamine ß-hydroxylase deficiency: causes deficiency of norepinephrine and epinephrine/adrenaline -> dizzy, difficult to stand -> treatment of droxidopa (prodrug of norepinephrine) that produces norepinephrine without dopamine and enzymes (bypass)

Question 3

Explain the properties of competitive inhibitors.

Answer 3

• Binds reversibly, have some structural resemblance to substrate
• Only binds to free enzyme without reaction
• Substrate blocked from entering active site, can be reversed by increasing [S]
• Inhibitor with low k(i) (inhibitor dissociation value): high binding strength -> more potent

Question 4

Explain the properties of non-competitive and un-competitive inhibitors.

Answer 4

• Non-competitive inhibitors
• Can be reversible/irreversible
• Binds at different site than substrate
• Low k(i): more potent
• Binding interferes with normal conformational changes (stopping induced fit and catalysis)
• Un-competitive inhibitors
• Similar action with non-competitive inhibitors
• Low k(i): more potent
• Could only bind to ES as the site does not exist before ES is formed (induced fit)

Question 5

Contrast irreversible and reversible inhibitors. Describe how competitive and non-competitive inhibitors affects K(M) and V(max) values in different ways.

Answer 5

• Irreversible: covalent modifications in active site (ex. penicillin, aspirin)
• Reversible: non-covalent interactions to form E-I complex (ex. methotrexate, captopril)
• Competitive: K(M) increases, V(max) stays the same
• Non-competitive: K(M) stays the same, V(max) decreases

Question 6

In regards to enzyme inhibitors, explain the mechanisms of penicillin, aspirin, methotrexate, azidothymidine, and co-amoxiclav.

Answer 6

• Penicillin: -OH (serine) of transpeptidase binds to penicillin -> form inactivated penicillin-enzyme- derivative -> inhibit cell wall synthesis -> bacteria cannot replicate
• Aspirin: -OH (serine) of COX binds to aspirin -> block entry of arachidonic acid -> cannot produce prostaglandin -> cannot stimulate nerve endings to kill pain
• Methotrexate: binds much tighter than THF to DHF reductase to disrupt nucleic acid synthesis
• Azidothymidine: binds to HIV Reverse Transcriptase -> stops HIV replication
• Overcoming drug resistance by enzyme inhibition (ex. co-amoxiclav)
• Bacteria evolved to produce ß-lactamase that binds penicillins -> opens ß-lactam ring that are no longer recognised to inhibit transpeptidase -> use clavulanic acid as competitive inhibitor of ß-lactamase -> let penicillins to act

Question 7

Describe the properties of allosteric activator or inhibitors.

Answer 7

• Binds reversibly to site away from active site
• Very different structure to substrate
• Distorts active site so substrate cannot fit
• Cannot be reversed by increasing [S]
• Low k(i): more potent

Question 8

Explain the mechanism and properties of feedback inhibition in enzymes.

Answer 8

• Feedback inhibition (in many metabolic pathways): end-product usually inhibits first enzyme of pathway allosterically as feedback control (ex. AT Case are used as initial enzyme in pyrimidine biosynthesis, but is inhibited by end-product CTP)
• Tense state with low substrate affinity vs Released state with high substrate affinity
• Allosteric activator binds to T->R, vice versa for allosteric inhibitor
• ∴ Allosteric control of enzymes