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What is catalysis?
A process that increases the rate at which a reaction reaches equilibrium.
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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
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What is acid base catalysis?
Partial proton transfer form a Bronsted acid lowers the free energy of the transition state
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What process lowers the transition state energy in acid catalysis?
Partial proton transfer from a bronsted acid
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What process lowers the transition state energy in base catalysis?
Partial proton abstraction from a bronsted base
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What type of catalyst is ribonuclease A?
Acid (His 119) and base (His 12)
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What histidines are important in ribonuclease a?
His 12 and His 119
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Covalent catalysis
Rate acceleration through transient formation of a covalent catalyst-substrate bond
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Schiff reactions are?
Covalent catalysis through the formation of a Schiff base
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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
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The five key residue functional groups
- Lys - ε-amino group
- His - Imidazole moeity
- Cys - Thio group
- Asp - Carboxyl function
- Ser - Hydroxyl group
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30% of all known enzymes require what for catalytic activity?
Metal ions
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The two classes of metal ion requiring enzymes.
- Metalloenzymes
- Metal-activated enzymes
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What are metalloenzymes?
- Metal ion requiring enzymes containing tightly bound metal ions
- d-block metals
- Fe2+, Fe3+, Cu2+, Zn2+, Mn2+, or Co3+
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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+
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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
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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
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What effect does water have on electrostatic interactions?
It significantly weakens them
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How do electrostatic catalysts increase electrostatic interactions?
By binding the substrate such that water is excluded
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Why is the charge distribution of the active site the way it is?
- Stabillize transition state
- Guide polar substrates to their binding site
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