Enzymes are Biological catalyst that speeds up the rate of reaction without undergoing any change by itself.
Reversible inactivation: Inhibitor forms an unstable, non-covalently bonded, enzyme inhibitor complex. Lost activity can be regained.
Irreversible inactivation: Inhibitor forms strong covalent bonds so that it cannot be dislodged. Therefore the enzyme activity lost forever.
Three major types of Reversible Inhibition
1. Competitive (Inhibitor binds to Active site)
2. Non Competitive (Inhibitor binds to Allosteric site)
3. Uncompetitive Inhibition (Inhibitor binds to ES complex)
Competitive Inhibition or Isosteric inhibition
- The inhibitor structural homologue competes with the substrate for the same active site of an enzyme.
- Depends on relative concentration of inhibitor and its affinity
- Binds to free enzyme
- Can be reversed by adding more substrate so that inhibitor is out competed.
Lowering Km; decreases affinity (increases Km value) and Vmax remains the same; as Vmax can be achieved by adding more substrate
What is Km of an enzyme?
Michaelis constant (Km): The substrate concentration at which the reaction rate is half of V max
Km value is different for different enzymes
Km refers to the affinity of an enzyme for its substrate
Smaller the value higher is the affinity
- The inhibitor binds to allosteric site
- It never blocks E-S formation
- This binding causes conformational change preventing the conversion of E-S complex to E-P complex
- It is not fully reversible inhibition some re-activators to delock non competitive inhibitor
- Km remains same (no change in affinity) & Vmax decreases as many ES complex will not form the product due to inhibitor binding to the allosteric site
- Eg : Heavy metals like Hg++, Pb++, drugs, pesticides, cyanide on cytochrome oxidase – can be removed by using a chelating agent EDTA
- The inhibitor binds to E-S complex
- Inhibitor binds to E-S complex and not with free enzyme and facilitates tight E-S binding preventing enzyme action
- Cannot be overcome by increasing substrate concentration
- Decreases Km value (increases affinity) &Vmax
- Eg: Inhibition of placental alkaline phosphatase by phenylalanine