Inhibition
Inhibition
- By enzyme inhibitors: molecules that bind to enzymes and decrease enzyme's activity
→ Why?: blocking enzyme's activity can kill a pathogen / correct metabolic imbalance
- Reversible: usually react w/the enzyme and change it chemically (can be displaced by presence of substrates)
- Irreversible: bind non-covalently and different types of inhibition resulted
Competitive Inhibition
Competitive Inhibitor
- competes directly w/substrate for the enzyme's active site
- resembles the substrate structure → have functional groups found in the substrate or mimick the overall chemical structure of the substrate
- usually binds to the active site and stay unreactive
- Effect: diminishes the rate of catalysis by reducing the proportion of enzyme molecules bound to a substrate
Example: Sulfanilamide
What?
- antibiotic useful in the treating kidney infection
- structural mimick of p-aminobenzoic acid (PABA - structural part of folic acid)
How?
- some bacteria require folic acid for growth + division Folic Acid
- sulfanilamide is a competitive inhibitor for bacterial dihydrofolate synthetase
- bacteria lack folate = cannot grow/divide
- sulfanilamide is very toxic to bacteria b/c bacteria must synthesise their own folic acid
- Humans require folate from dietary source, so not harmful to humans
Example: Methotrexate
What?
- competitive inhibitor of the dihydrofolate reductase, required in biosynthesis of purines + pyrimidines
- structural analog of dihydrofolate ( substrate of dihydrofolate reductase)
How?
- Binds to the enzyme x1000 tighter than the natural substrate binds & inhibit nucleotide base synthesis
- Used to treat cancer
of Competitive Inhibition
Inhibitor (I) only binds to free enzymes E
[E][I] =
(Assumption: inhibitor binds reversibly and in rapid equilibrium)
EI is catalytically inactive. Inhibitor lowers the [E] available for S to bind
Rate for Competitive Inhibition
Enzymes Page 1
, Rate for Competitive Inhibition
- To obtain , [S] =
- Means Vmax can be achieved by increasing [S]
-
-
- Larger the α, higher the [S] to reach
- stays the same but the changes (increases)
since higher [S] is required to reach same
Increasing inhibition lowers the available enzyme for substrate
Uncompetitive Inhibition - b/c decreases (more negative) as α increases
Uncompetitive Inhibitor
- substrate dependent inhibitor, which binds only to the E-S complex
- Binding site created only with E-S interaction
→ binds to the allosteric site (proximal site) & can cause distortion of the active site
- can be overcome by addition of more substrate
- Does not need to resemble the substrate as long as it can bind to the proximal site
Exmaple: Alkaline phosphatase
What?
- catalyses the release of inorganic phosphate from phosphate esters
- found in many tissues: liver, intestine, bone, kidney, leukocytes ---
- Serum alkaline phosphatase levels are important diagnostic markers for bone & liver disease
- Phe as the allosteric inhibitor → cause conformational change so S is not released
of Uncompetitive inhibition
Rate for uncompetitive Inhibition
- At high [S], approaches
- is affected, so increasing [S] cannot reach
- Lowering [S] lowers inhibitor's affinity
- More [S] added, original cannot be reached
- reduced
- more the inhibitor, higher the enzyme's substrate affinity
- Slope constant
Mixed Inhibition
Enzymes Page 2
Inhibition
- By enzyme inhibitors: molecules that bind to enzymes and decrease enzyme's activity
→ Why?: blocking enzyme's activity can kill a pathogen / correct metabolic imbalance
- Reversible: usually react w/the enzyme and change it chemically (can be displaced by presence of substrates)
- Irreversible: bind non-covalently and different types of inhibition resulted
Competitive Inhibition
Competitive Inhibitor
- competes directly w/substrate for the enzyme's active site
- resembles the substrate structure → have functional groups found in the substrate or mimick the overall chemical structure of the substrate
- usually binds to the active site and stay unreactive
- Effect: diminishes the rate of catalysis by reducing the proportion of enzyme molecules bound to a substrate
Example: Sulfanilamide
What?
- antibiotic useful in the treating kidney infection
- structural mimick of p-aminobenzoic acid (PABA - structural part of folic acid)
How?
- some bacteria require folic acid for growth + division Folic Acid
- sulfanilamide is a competitive inhibitor for bacterial dihydrofolate synthetase
- bacteria lack folate = cannot grow/divide
- sulfanilamide is very toxic to bacteria b/c bacteria must synthesise their own folic acid
- Humans require folate from dietary source, so not harmful to humans
Example: Methotrexate
What?
- competitive inhibitor of the dihydrofolate reductase, required in biosynthesis of purines + pyrimidines
- structural analog of dihydrofolate ( substrate of dihydrofolate reductase)
How?
- Binds to the enzyme x1000 tighter than the natural substrate binds & inhibit nucleotide base synthesis
- Used to treat cancer
of Competitive Inhibition
Inhibitor (I) only binds to free enzymes E
[E][I] =
(Assumption: inhibitor binds reversibly and in rapid equilibrium)
EI is catalytically inactive. Inhibitor lowers the [E] available for S to bind
Rate for Competitive Inhibition
Enzymes Page 1
, Rate for Competitive Inhibition
- To obtain , [S] =
- Means Vmax can be achieved by increasing [S]
-
-
- Larger the α, higher the [S] to reach
- stays the same but the changes (increases)
since higher [S] is required to reach same
Increasing inhibition lowers the available enzyme for substrate
Uncompetitive Inhibition - b/c decreases (more negative) as α increases
Uncompetitive Inhibitor
- substrate dependent inhibitor, which binds only to the E-S complex
- Binding site created only with E-S interaction
→ binds to the allosteric site (proximal site) & can cause distortion of the active site
- can be overcome by addition of more substrate
- Does not need to resemble the substrate as long as it can bind to the proximal site
Exmaple: Alkaline phosphatase
What?
- catalyses the release of inorganic phosphate from phosphate esters
- found in many tissues: liver, intestine, bone, kidney, leukocytes ---
- Serum alkaline phosphatase levels are important diagnostic markers for bone & liver disease
- Phe as the allosteric inhibitor → cause conformational change so S is not released
of Uncompetitive inhibition
Rate for uncompetitive Inhibition
- At high [S], approaches
- is affected, so increasing [S] cannot reach
- Lowering [S] lowers inhibitor's affinity
- More [S] added, original cannot be reached
- reduced
- more the inhibitor, higher the enzyme's substrate affinity
- Slope constant
Mixed Inhibition
Enzymes Page 2
