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What is catalysis?
A process that increases the rate at which a reaction reaches equilibrium.
What are the six catalytic mechanisms enzmes employ?
- Acid-base catalysis
- Covalent catalysis
- Metal ion catalysis
- Electrostatic catalysis
- Proximity and orientation effects
- Preferential binding of the transition state complex
What is acid base catalysis?
Partial proton transfer form a Bronsted acid lowers the free energy of the transition state
What process lowers the transition state energy in acid catalysis?
Partial proton transfer from a bronsted acid
What process lowers the transition state energy in base catalysis?
Partial proton abstraction from a bronsted base
What type of catalyst is ribonuclease A?
Acid (His 119) and base (His 12)
What histidines are important in ribonuclease a?
His 12 and His 119
Rate acceleration through transient formation of a covalent catalyst-substrate bond
Schiff reactions are?
Covalent catalysis through the formation of a Schiff base
What are the three stages of covalent catalysis
- 1. Nucleophillic reaction between catalyst and substrate
- 2. Withdrawal of electrons from reaction centre by catalyst
- 3. Elimination of catalyst
The five key residue functional groups
- Lys - ε-amino group
- His - Imidazole moeity
- Cys - Thio group
- Asp - Carboxyl function
- Ser - Hydroxyl group
30% of all known enzymes require what for catalytic activity?
The two classes of metal ion requiring enzymes.
- Metal-activated enzymes
What are metalloenzymes?
- Metal ion requiring enzymes containing tightly bound metal ions
- d-block metals
- Fe2+, Fe3+, Cu2+, Zn2+, Mn2+, or Co3+
What are metal-activated enzymes?
- Metal requiring enzymes containing loosely bound metal ions from the solution
- Alkalai and alkaline earth metals
- Na+, K+, Mg2+, or Ca2+
What are the three ways metal ion catalysts aid the reaction?
- Binding and orientation of substrate
- Mediating oxidation and reduction reactions
- Electrostatic stabilization and shielding of negative charfes
How does human carbonic anhydrase work?
- Zn2+ abstracts a proton from bound water
- The remaining bound OH- attracts bound CO2 and forms HCO3-
- Catalyst regenerated by exchanging HCO3- for H2O and deprotonating His64
What effect does water have on electrostatic interactions?
It significantly weakens them
How do electrostatic catalysts increase electrostatic interactions?
By binding the substrate such that water is excluded
Why is the charge distribution of the active site the way it is?
- Stabillize transition state
- Guide polar substrates to their binding site