P2 - Enzymes as Drug Targets Flashcards
Describe how substrates bind to an enzyme
Very high specificity
Many chemical interactions at once
Substrate binds in the active site
Always reversible - no covalent bonds
Describe how inhibitors bind to an enzyme
High specificity
Through interactions with the amino acid R groups
Can be reversible or irreversible
How do enzymes catalyse reactions?
Reduce activation energy by getting the bonds in the correct orientation
Enzyme undergoes a conformational change
Product release is faster than the substrate binding
What are the 3 types of reversible enzyme inhibitors and what are the common features of all of them?
Reversible Competitive
Reversible Non-Competitive
Reversible Uncompetitive
All bind non-covalently, do not undergo any kind of chemical reaction when bound, and work by reducing the amount of available enzyme
What is Vmax and Km on a Lineweaver Burk plot?
Km is the substrate concentration at which the speed of product formation is at half of it’s maximum value
Vmax is the maximum rate of an enzyme catalysed reaction
The y intercept is 1/Vmax
The x intercept is -1/Km
Describe how reversible competitive inhibitors work
Have similar shape to the substrate
Compete for the active site
Vmax not affected but Km is reduced
Binds to the free enzyme only
Give 3 examples of reversible competitive inhibitors
- ACE inhibitors such as Captopril. Compete with angiotensin I. Reduces vasoconstriction and salt retention, to lower BP
- Antifreeze contains ethylene glycol, which is converted by alcohol dehydrogenase into oxalic acid. Oxalic acid chelates K+ ions in the kidney, causing renal failure. Can be treated by drinking alcohol, which competes for the alcohol dehydrogenase
- Xanthine oxidase oxidises hypoxanthine into uric acid, too much of which can cause gout. Allopurinol inhibits XO
Describe how reversible non-competitive inhibitors work
Bind to enzyme at an allosteric site
Decrease Vmax but not Km
Don’t necessarily look like the substrate because not binding to the same site
Binds to the free enzyme or the enzyme-substrate complex
Give an example of a reversible non-competitive inhibitor
6-Mercaptopurine
Inhibitor of ATase, which catalyses the first step of purine synthesis
Used to treat leukaemia
Describe how reversible uncompetitive inhibitors work
Binds only to the enzyme-substrate complex
Binding of the inhibitor must be faster than the catalysis carried out by the enzyme
Decreases both Vmax and Km
Describe how irreversible inhibitors work
Covalently modifies the enzyme active site leading to permanent inactivation
First forms specific non-covalent interactions specific to get the correct enzyme, then covalent interactions with serine or cysteine residues
Give an example of an irreversible inhibitor
Aspirin irreversibly inhibits COX enzymes. Contains acetyl group that reacts with OH in the active site. Prevents synthesis of prostaglandin H2.
Also the deacetylation of the aspirin forms salicylic acid, which reduces pain via other mechanisms
What are suicide inhibitors?
Type of irreversible inhibitor
Only becomes active upon binding to the active site of an enzyme
Covalently bonds to the active site
Key advantage is that the reactive species is only generated at the site where it is meant to react
Give an example of a suicide inhibitor
DMFO
Used to treat African sleeping sickness
Binds to ornithine decarboxylase, after which the carboxyl group and fluoride atom are removed to generate an electrophilic species which covalently bonds to cysteine
Inhibition of this enzyme in the parasite leads to build up of urea which kills it
Why does DMFO affect the parasite but not the human host?
Humans can regenerate ornithine carboxylase faster than the parasite
