Enzymes Flashcards
Describe the characteristics of Michealis constant Km
-Substrate concentration at 1/2 Vmax
Characteristic for an enzyme and it’s respective substrate
- Constant that describes the affinity of the enzyme for the substrate (small Km= high affinity of enzyme for substrate, large Km low affinity of enzyme for substrate)
- Stays the same whereas Vmax increases with higher enzyme concentration
What are the types of enzyme inhibition?
Reversible (competitive and non competitive )
Irreversible
What is reversible inhibition
Inhibitor drugs bind no covalently to enzyme
-Competitive & noncompetitive
What is irreversible inhibition ?
Inhibitor drugs or poisons bind covalently to the enzyme
The abnormal enzyme is degraded
The inhibition can be overcome by synthesis of a new enzyme
Explain competitive inhibition
The competitive reversible inhibitor is a structural analog of the substrate that competes with the substrates fir the binding to the active sites of the enzyme
A statin drug that competitively inhibits HMG CoA reductase which is the regulated enzyme of cholesterol synthesis
Instead of forming ES (Enzyme - HMG CoA), an EI complex (enzyme-statin) is formed which doesn’t lead to a product and the statin is released
How does competitive inhibition affect Vm and Km?
At very high substrate concentration, the substrate molecules outnumber the inhibitor molecules leading eventually to the same Vmax as the reaction without inhibitor
-The competitive inhibitor interferes with the ES formation and a larger substrate concentration is needed to reach half Vmax which is now named appearent Km
Note: appearent Km refers only to the substrate concentration to reach half Vmax. The affinity for the substrate is not changed by the competitive drug
How does noncompetitive inhibition affect Vmax and Km?
The appearent Vmax… with noncompetitive inhibitor is smaller when compared to the uninhibited reaction but the Km (Vmax/2) is the same
Contrast the Michaelis Menten graph and Lineweaver-Burke graphs
Michealis Menten
- hyperbolic curve
- many points with high substrate concentrations are needed to establish Vmax
- Vmax is estimated from the graph
Lineweaver Burke graph
- straight line
- Less experiments with high substrate concentration saves money
- Vmax is mathematically obtained from 1/Vmax at the Y-axis
What are the irreversible inhibiters ?
- Modification of the active site
- Blocking a sulfylhydryl-group of the enzyme
- Interference with a metal cofactors
Examples: specific medical drugs or orangophosphate compounds found in nerve gas and pesticides
Explain DFP (di-isopropyl fluorophosphate) as a nerve poison
DFP is an irreversible inhibitor of acetylcholinesterase
Acetylcholinesterase cleaves the neurotransmitter acetylcholine in the postsynaptic neuromuscular junction. DFP leads to accumulation of acetylcholine with devestating results
Continuous stimulation of muscle contraction and overstimulation of the autonomic nervous system lead to: blurred vision, bronchoconstriction, seizures, respiratory arrest and death
How is anticholinesterase inhibited?
The poison blocks a specific serine that forms normally a covalent bond with the Acetyl group during catalysis
Explain the functioning of aspirin
Aspirin is a pain reducing compound that was found a very long ago in the bark of the willow tree
Aspirin is a non-steroidal anti-inflammatory drug. Most NSAIDs are reversible inhibitors. This is nit the case for aspirin
Aspirin irreversibly inhibits COX-1 and COX-2 which are needed for prostaglandin and thromboxane synthesis. COX-2 is related to pain and inflammation
What is aspirin?
Aspirin is a Acetyl salicylic acid and it acetylates a serine residue in the channel of the active site of cyclooxygenase
Aspirin is an irreversible COX inhibitor in all cells.
Aspirin is used for pain treatment (300-500 mg)
A low daily dose (81 mg) leads to reduced blood clotting
How is COX involved with blood clotting?
Needed for synthesis of:
Prostacyclin in endothelial cells
Thromboxane in platelets
Platelet release —> thromboxane stimulates blood clotting while prostacyclin in endothelial cells reduce blood clotting
Why is thromboxane released by platelets ?
- Activated platelets start thromboxane synthesis using COX
- Thromboxane synthase continues and platelets release thromboxane
Why is prostacyclin released by endothelial cells?
Endothelial cells start prostacyclin synthesis using COX
Prostacyclin synthase continues and endothelial cells release prostacyclin
What are the effects of daily aspirin?
A daily dose of aspirin changes the normal blood thromboxane/prostacyclin ratio.
Less thromboxane is available which residues blood clotting
Thromboxane: small amount
Prostacyclin: normal amount
How is the thromboxane to prostac6clin ratio changed?
Aspirin irreversibly inhibits COX in both endothelial cells and in platelets. The final outcome however is different:
Endothelial cells degrade the modified enzyme and synthesize new COX. Platelets are not able to synthesize COX and less thromboxane is released until new platelets are formed
Some medical drugs act as irreversible inhibitors only after…
They are modified by the target enzyme
What are suicide inhibitor drugs?
Some drugs look like competitive inhibitors and bind to the active site of the enzyme. But instead of being released by the enzyme, the drug is used like the substrate and modified to an irreversible inhibitor
These drugs are often named “suicide inhibitor drugs” as the enzyme itself changes the structure of the drug resulting in an irreversible inhibitor:
“The enzyme commitVsuicude”
What is Allopurinol used for?
Gout treatment
How does Allopurinol work for gout treatment ?
Xanthine oxidase uses the drug Allopurinol instead of hypoxanthine as substrate and forms the irreversible inhibitor drug alloxanthine
Alloxanthine (oxypurinol) binds irreversibly to the active site of xanthine oxudase and reduces the formation of uric acid
How is normal regulation of enzymes done?
Different than inhibition by drugs
More than one regulator may act inside the same cell and hormones allow specific regulation in different cells and tissues
Regulation can be achieved by the following:
A. Concentration of substrates and/or products
B. Modulation of enzyme concentration
C. Covalent modification of enzymes
D. Allosteric regulation by reversible conformational change
