EXAM 5 - Drug Metabolism 2 Flashcards
Explain transition state theory
Enzymes bind to a transition state and favors formation of intermediate –> lowers activation energy
How can drugs be used as transition state analogs?
Drug can be designed to look like the transition state (analog) and inhibit the enzyme.
Describe enzyme conformational selection.
The conformation of the enzyme varies, one conformation recognizes the substrate.
Describe induced fit.
Binding the substrate induces a change in the enzyme
Describe the mechanism of carboxylesterase enzymes.
- Ester (substrate) is attacked by nucleophilic oxygen of the enzyme –> forms acylated enzyme intermediate
- Water comes in to displaces alcohol group (R2OH)
- Water then attacks carbonyl group –> releases acid
- Enzyme ready to bind substrate
Two-step
Explain why the mechanism of carboxylesterases is good for drug design?
If a drug binds to the intermediate and can’t be removed by water, it is an effective inhibitor of the reaction.
What reactions are included in Phase 1 reactions?
- Hydrolysis
- Reduction
- Oxidation
What enzymes are responsible for Phase 1 hydrolysis?
Esterases and epoxide hydrolases
What is the main function of esterases and epoxide hydrolases on the substrate?
Add water to the substrate to make a product.
Describe the role of carboxylesterases and pseudocholinesterases (esterases)?
Hydrolyze esters, amides, and thioesters
* Carboxylesterases - located in microsomes, cytosol, lysosomes of most tissues and serum.
* Pseudocholinesterase - located in blood
When carboxylesterases or pseudocholinesterases add water to a carboxylic acid ester, what are the products?
Acid and alcohol
When carboxylesterases or pseudocholinesterases add water to an amide, what are the products?
Acid and amine
When carboxylesterases or pseudocholinesterases add water to a thioester, what are the products?
Acid and thiol
Describe the mechanism of carboxylesterase on lovastatin.
Lovastatin is a prodrug and is activated by carboxylesterase.
* the active form of lovastatin is a mimic of the endogenous HMG-CoA reductase substrate (lacks sulfur group)
* when activated lovastatin binds to HMG-CoA reductase, prevents the formation of the intermediate, Mevalonate.
Describe the function of epoxide hydrolases (EHs).
Catalyze the addition of H2O to alkane, alkene, and arene epoxides to form vicinal diols.
* many of these epoxides are formed by P450s.
2 major epoxide hydrolases: microsomal and soluble (in cytosol) EH.
* broad range of substrates
What are the possible products of epoxide reactions?
- activation to toxic metabolite
- detoxification - produces less toxic metabolite
Compare and contrast nucleophilic attacks of carboxylesterases and epoxide hydrolases.
Carboxylesterases
* Nucleophilic oxygen on Serine attacks carbonyl group
Epoxide hydrolases
* Nucleophilic oxygen on aspartic acid attacks electrophilic carbon on epoxide
Compare and contrast intermediate formation of carboxylesterases and epoxide hydrolases.
Caboxylesterase
* Forms alcohol
Epoxide Hydrolase
* Forms vicinal diol
Compare and contrast nucleophilic attack by water of carboxylesterases and epoxide hydrolases.
Caboxylesterase
* Water comes in and attacks substrate
* Water ends up on the product
Epoxide Hydrolase
* Nucleophilic attack by water
* Water ends up on enzyme
Is reduction a major biotransformation pathway?
No.
What is Azo and Nitro-reduction catalyzed by?
Intestinal microflora and human liver enzymes
Identify this reaction:
Azo-reduction
Azide group –> gives amine
Identify this reaction:
Nitro-reduction
Nitro group –> gives amine
Identify this reaction:
Nitro-reduction
nitro group –> gives aniline
Identify this reaction:
Carbonyl reduction
* Aldehyde is reduced to a primary alcohol
* Ketone is reduced to secondary alcohol
Identify this reaction:
Quinone reduction
* quinone to hydroquinone
