Enzyme Catalysis Flashcards
What is general base catalysis?
Partial proton abstraction from a Bronsted base.
Why is acid/base catalysis useful?
It helps the enzyme to stabilise charged intermediates during the reaction.
What is specific acid-base catalysis?
Catalysis involving protons or OH ions specifically.
What is concerted acid-base catalysis?
When both processes occur simultaneously.
What does covalent catalysis involve?
- Nucleophilic reaction between the catalyst and the substrate to form a covalent bond.
- Withdrawal of electrons from the reaction centre by the now electrophilic catalyst.
- Elimination of the catalyst.
What is limiting the rate of reaction in nucleophilic catalysis?
Covalent bond formation.
What is limiting the rate of reaction during electrophilic catalysis?
Withdrawal of electrons.
How is a schiff base formed?
Condensation of an amine with a carbonyl compound.
How does a schiff base act as an electron sink?
The positive N atom stabilises any negative charge on an adjacent carbon.
What is a good covalent catalyst?
A group with high nucleophilicity and the ability to form a good leaving group.
Give examples of coenzymes usually involved in covalent catalysis.
Thiamine pyrophosphate and pyridoxal phosphate.
What are metalloenzymes?
Enzymes with tightly bound metal ions.
What are metal-activated enzymes?
Enzymes that loosely bind metal ions from solution.
How is metal ion catalysis useful?
- metal ions bind to substrates and orientate them properly.
- metal ions facilitate redox reactions through changes in the metal’s oxidation state
- metal ions stabilise charges
- metal ions shield charges
How do metal ions stabilise charges?
By acting as Lewis acids.
How can water be made a better nucleophile?
- by abstracting a proton
- by binding a metal ion
What is electrostatic catalysis?
Enzymes arrange active site charge distributions in order to stabilise the transition state.
What effect do proximity and orientation have on catalysis?
Reactants must come together with the proper spatial relationship for a reaction to occur.
When are molecules maximally reactive?
When their orbitals are aligned so that the electronic energy of the transition state is minimised.
What is general acid catalysis?
Partial proton transfer from a Bronsted acid.
What are the features of a perfectly evolved enzyme?
Diffusion controlled. Km is much higher than physiological [S] value.
