Medicine - Enzyme Inhibitors 2 - Suicide Inactivators And Transition State Analogies Flashcards Preview

Pharmacy Year 2 Semester 1 > Medicine - Enzyme Inhibitors 2 - Suicide Inactivators And Transition State Analogies > Flashcards

Flashcards in Medicine - Enzyme Inhibitors 2 - Suicide Inactivators And Transition State Analogies Deck (29):

Describe the use of 5-flourouracil as a suicide inhibitor

5-flourouracil is converted in the body to the fluorinated analogue of 2'-deoxyuridine monophase (FdUMP)
This suicide inactivator is used to inhibit the enzyme thymidylate synthase, which is involved in DNA biosynthesis


Define suicide inactivator

Agents which are converted to irreversible inhibitors by the enzyme-catalysed reaction
Not active until it is bound to the active site (chemically)
React with target enzyme once contact is formed


What is the mode of interactions of suicide inactivators within an enzyme?
What is the effect of increasing the substrate?

At the enzyme active site these inhibitors form a permanent complex with an enzyme via formation of covalent bonds with certain amino acids in the active site of an enzyme
Consequently, they cannot be displaced by the addition of excess substrate
Therefore, they are irreversible, non-competitive enzyme inhibitors


Diagram/series of reactions for suicide inactivators

E+I⇌(inhibitor binds)EI(normal catalytic reaction starts at t active site⇌E...I (reactive species generated) and forms E-I (inactivated enzyme)

NB: enzyme treats the inhibitor as if it is a "normal" substrate/substance


Mechanism-based inhibitors depend on...

Depend on the specific mechanism of catalysis of a particular target enzyme


Advantages of mechanism-based enzyme inhibitors

-Absolute specificity: they depend upon substrate-like binding recognition and upon specific mechanism of catalysis in the active site of a particular enzyme
-High efficiency of enzyme inhibition (covalent complex)


What is Tienilic acid?

Drug marked as a diuretic agent
Suicide inactivator
Withdrawn due to interaction with cytochrome P450 enzymes in the liver, where it acts as a suicide substrate


What helps design powerful inhibitors?

Understanding of enzyme mechanism of reaction helps to design more powerful inhibitors


Transition state analogues

It is possible to design inhibitors that bing extremely strongly to the active site.
One approach to achieve this is to design a drug that resembles the transition state of the substrate in the active site
Drug design can be based on reaction intermediates which are closer in character to transition states than substrates or products


Define transition state

High energy, transient species
Cannot be isolated or synthesised


What are transition state analogues designed to mimic?
How strongly do they bind?

The transition state of the enzyme-catalysed reaction
They are more likely to bind more strongly than drugs mimicking the substrate or product


Penicillins are what type of enzyme inhibitor?
Is there an alternative theory?

Penicillins are irreversible suicide inhibitors of transpeptidase
Penicillin becomes covalently linked to the enzyme's active site
Alternative theory: Penicillin mimics D-Ala-D-Ala


How does penicillin resistance occur?

Via beta-lactamases
These enzymes inactivate penicillins by opening beta-lactam rings
This allows bacteria to become resistant to penicillin


Beta-lactamase mechanism of action:

Mechanism of action for beta-lactamases is identical to the mechanism of inhibition of the target enzyme (i.e.transpeptidase)
But the product is removed effectively fro the lactamase active site


How can you overcome penicillin resistance?

Use clavulanic acid


What is Clavulanic acid?

-weak, unimportant antibacterial activity
-Powerful irreversible suicide inhibitor of beta-lactamase (enzyme that causes bacterial resistance to penicillins)
-used as a guard drug to ampicillin


Where does clavulanic acid come from

Isolated from
Streptomyces clavoligenus



Ampicillin+clavulanic acid

Allows less ampicillin per dose and increased activity spectrum



Ticarcillin + clavulanic acid


Why should a transition state be bound more strongly to an enzyme that a substrate or product?

-it is proposed that the binding interactions between an enzyme and a substate are optimal during the transition state of the enzyme-catalysed reaction
-this is a reasonable proposition since the speed and effectiveness of a catalysed reaction is crucially dependent on how much the catalyst stabilises the transition state


How alter the transition state to make the reaction occur more easily and make the enzyme more effective?

The more stable the transition state, the easier the reaaction will occur and the more effective the enzyme will be as a catalyst
It is more important that enzymes form their strongest interactions with 'guest molecules' at the transition state of the reaction, rather than the substrate or the product


Strong interactions with substrate or product would be...

Detrimental because it would result in slow "turnover" with substrate and product spending too much time in the active site


Name an enzyme of a transition state inhibitor and describe how it works

Renin inhibitors (Antihypertensives)

Renin is a protease enzymes which is responsible for hydrolysing a specific peptide bond in the protein angiotensinogen
This converts angiotensinogen into angiotensin I, which then (via an ACE enzyme) is converted into angiotensin II

Angiotensin II is a vasoconstrictor and constricts blood vessels, thus raises blood pressure. Renin blocks synthesis of of angiotensin I and thus, angiotensin II


Describe how renin works

Renin contains two aspartyl residues and a bridging water in the active site
The first stage involves formation of a tetrahedral intermediate via processing through a high energy transition state
The two aspartyl resides are involved in the enzyme-catalysed reaction and the tetrahedral intermediate is involved.


What is Aliskiren? What does it do?

Aliskiren is a renin inhibitor
-it contains a hydroxyethylene transition state mimic
-it therefore mimic the tetrahedral geometry of the usual reaction intermediate when renin catalyses angiotensinogen
-it mimics one of the hydroxyl groups (binding group)
-it is stable, no leaving group present


Tetrahedral geometry of transition states is in what enzymes?

ACE inhibitors
Protease inhibitors


Is transition-state inhibition usually reversible or irreversible?

-Reversible inhibition, unless very tight binding, as no covalent bond formed


Criteria for transition-state inhibition?

-Reversible (unless v.tight binding)
-Close structural resemblance to postulated transition state
-Tighter binding to the enzyme than substrate binding
-often seen as competitive inhibition as active site is targeted and increase in substrate concentration may displace the inhibitor. This may mean that the transition state analogue forces the enzyme to adopt a new conformation
-sometimes inhibition is slow (minutes to hours) as conformation is crucial


Are transition state inhibitors competitive or non-competitive

Often seen as competitive as active site is targeted
Increase in substrate concentration may displace the inhibitor
This may mean that the transition state analogue forces the enzyme to adopt a new conformation