This document discusses suicide inhibitors, which are a type of irreversible enzyme inhibitor. Suicide inhibitors initially bind to the active site of an enzyme like a normal substrate. However, they are then modified by the enzyme into a highly reactive compound that covalently binds to and permanently inactivates the enzyme. This removes the enzyme from the system. Examples of suicide inhibitors discussed include allopurinol, 5-fluorouracil, aspirin, penicillin, and various antiviral and antituberculosis drugs.

1. SUICIDE
INHIBITORS
Guided by
Dr. V. M. Motghare
Dr. Swarnank Parmar
JR-1
Dept. Of Pharmacology
GMC Nagpur

2. Overview
• Introduction
• Types of Inhibition
• Suicide inhibitors
• Clinical examples
• Conclusion
• References

3. Introduction
• “Suicide inhibition, also known as suicide
inactivation, is a form of irreversible enzyme
inhibition that occurs when an enzyme binds a
substrate analogue and forms an irreversible
complex with it through a covalent bond during
the "normal" catalysis reaction”

4. • Inhibitor binds to active site where it is modified by
the enzyme to produce a reactive group that reacts
irreversibly to form a stable inhibitor-enzyme
complex
• Inhibitor has a functional group, usually a leaving
group, that is replaced by a nucleophile in the
enzyme active site
• Reaction with suicide inhibitor removes active
enzyme from the system; this removal is measured
as inhibition

5. ReversibleReversible
IrreversibleIrreversible
Type ofType of
InhibitorsInhibitors
CompetitiveCompetitive
UncompetitiveUncompetitive
Non- CompetitiveNon- Competitive
Active SiteActive Site
DirectedDirected
SuicideSuicide
InhibitorsInhibitors

6. • A special group of irreversible inhibitors is known as suicide
inhibitors
• A suicide inhibitor is a relatively inert molecule that is
transformed by an enzyme at its active site into a reactive
compound that irreversibly inactivates the enzyme
• Relatively unreactive until they bind to the active site of the
enzyme
Suicide Inhibitors

7. • First few steps of the reaction it functions like a normal
substrate, but then it is converted into a very reactive
compound that combines with the enzyme to block its
activity
• They use the normal enzyme reaction mechanism to
inactivate the enzyme, they are also known as mechanism-
based inhibitors or transition state analogs

8. • Suicide inhibitors that exploit the transition state-stabilizing
effect of the enzyme result in higher enzyme binding
affinity than do substrate-based inhibitor designs
• Designing drugs that precisely mimic the transition state is a
real challenge because of the unstable, poorly
characterized structure of the transition state

9. • Prodrugs undergo one or more initial reactions to form an
overall electrostatic and three-dimensional intermediate
transition state complex form with close similarity to that
of the substrate
• Serves as guidelines to further develop transition state
molecules with modifications
• Such inhibitors are called suicide inhibitors because theSuch inhibitors are called suicide inhibitors because the
enzyme appears to commit suicideenzyme appears to commit suicide

10. • A common example of a suicide inhibitor is Allopurinol,
the anti-gout drug that inhibits xanthine oxidase
activity. The enzyme commits suicide by initially
activating Allopurinol into Oxypurinol (a transition
state analog) that binds very tightly to the
molybdenum-sulfide (Mo-S) complex at the active site
of xanthine oxidase

12. • Acyclovir is one of the most commonly used antiviral agents
with very low toxicity. It is selectively converted into acyclo-
guanosine monophosphate (acyclo-GMP) by viral thymidine
kinase
• Acyclo-GMP is further phosphorylated into the active
triphosphate form, acyclo-GTP, by cellular kinases

13. • Acyclo-GTP is a very potent inhibitor of viral DNA
polymerase with over 100-fold greater affinity for viral
than cellular polymerase.
• It is incorporated into viral DNA, resulting in chain
termination
• The suicide inhibitor removes enzyme and reduces the
formation of ES complex. The Vmax value is reduced and
inhibition cannot be overcome by adding extra
substrate. In this regard, suicide inhibition resembles
non-competitive inhibition

14. Some clinical examples of
suicide inhibitors
• 5-fluorouracil acts as a suicide inhibitor of thymidylate
synthase during the synthesis of thymine from uridine
• Reaction is crucial for proliferation of cells, particularly
those that are rapidly proliferating (such as fast-
growing cancer tumors)

15. During thymidylate synthesis, NDuring thymidylate synthesis, N55
,N,N1010
- methyleneTHF is converted- methyleneTHF is converted
to 7,8-dihydrofolate; methyleneTHF is regenerated in two stepsto 7,8-dihydrofolate; methyleneTHF is regenerated in two steps

16. • Inhibition causes cell death from lack of thymine to
create more DNA
• This is often used in combination with Methotrexate, a
potent inhibitor of dihydrofolate reductase enzyme

17. • Aspirin, which inhibits cyclooxygenase
- Aspirin irreversibly inhibits COX-1 and modifies the
enzymatic activity of COX-2

18. • Low doses of aspirin (75mg-81mg/day) sufficient to
irreversibly acetylate serine530 of COX-1
• Inhibits platelets generation of TXA2 – antithrombin
effect
• Intermediate doses (650mg-4g/day) inhibit COX-1 &
COX-2 blocking PG production

19. • Penicillin, which inhibits DD-transpeptidase from
building bacterial cell walls
- Penicillin blocks the formation of the cross-link
between the N-acetylmuramic acid and N-
acetylglucosamine

20. • Benzothiazinones (BTZs) - antituberculosis drug, are
suicide substrates of the FAD-dependent
decaprenylphosphoryl-β-D-ribofuranose 2'-oxidase
DprE1, an enzyme involved in cell-wall biogenesis
• Serine proteases are attractive targets for the design
of enzyme inhibitors
• Within the class of serine proteases, Human Leucocyte
Elastase(HLE) is one of the most destructive enzymes
in the body

21. • Numerous inhibitors of serine proteases reported
during the past three decades
• Coumarin-type molecules displayed high inhibitory
potency towards various serine proteases
• Halomethyl dihydrocoumarins - shown to behave as
the first general suicide inhibitors of serine protease
• Inhibit several proteases - human leucocyte elastase,
porcine pancreatic elastase, thrombin, urokinase and
human plasmin

22. • Series of mechanism-based inhibitors of
Phospholipase A2 (SIBLINKS) were synthesized
• Phospholipid analogues that contain a para-
substituted phenyl 3,3-dimethylglutaryl group in
the place of the sn-2 acyl chain

23. • Effect of the phenyl leaving group on inhibitory
activity was studied by varying electron-
withdrawing ability of the para-substituted group
• Strong correlation observed between leaving
group potential of suicide inhibitor and inhibitory
activity of the derivative toward cobra venom
phospholipase A2.

24. • TCAs act primarily as serotonin-norepinephrine
reuptake inhibitors (SNRIs)
• Blocks serotonin transporter (SERT) and nor-
epinephrine transporter (NET)-elevation of the synaptic
concentrations of neurotransmitters
• Clavulanic acid is a suicide inhibitor, covalently bonding
to a serine residue in the active site of the β-Lactamase

25. • Restructures clavulanic acid molecule, creating a more
reactive species - attacked by another amino acid in the
active site, permanently inactivating the enzyme
• This inhibition restores the antimicrobial activity of
β-lactam antibiotics against lactamase-secreting
resistant bacteria

26. • AZT (zidovudine) and other chain-terminating
nucleoside analogues used to selectively inhibit HIV-1
reverse transcriptase in the treatment of HIV/AIDS
• Exemestane is an irreversible, steroidal aromatase
inactivator, structurally related to the natural
substrate androstenedione binds irreversibly to the
active site of the enzyme causing its inactivation, an
effect also known as suicide inhibition

27. • Sulbactum is an irreversible inhibitor of ß–lactamase,
which prohibits penicillin-resistant strains of bacteria
from metabolizing penicillin
• Sarin (Organophosphorus compound) is a potent
suicide inhibitor of acetylcholinesterase degrading
neurotransmitter Acetylcholine after its release
• Benzotriazole esters(used in SARS) reported as potent
nonpeptidic suicide inhibitors of the enzyme
proteinase, acts at active-site cysteine, acylated by
benzotriazole esters

28. Selegiline (selective irreversible MAO-B inhibitor)
although in the attached reference the compound is
called a 'suicide inactivator' (not inhibitor)

29. • Serpins are a group of proteins with similar structures
that were first identified as a set of proteins able
to inhibit proteases
• Serpins are suicide inhibitors
• Serpin1 of Arabidopsis thaliana is a Suicide Inhibitor for
Metacaspase 9
• Serpins are referred to as suicide inhibitors because
they are cleaved by their target proteases

30. • Vigabatrin, an anticonvulsant is a irreversible suicide
inhibitor of GABA-Transaminase (responsible for
catabolism of GABA) increasing level of GABA in brain
• ß-haloamines represent a new series of suicide
inhibitors of lysyl oxidase which can inactivate the
enzyme faster than BAPN (ß-aminoproprionitrile) and
may have antifibrotic potential

31. • Plasminogen activator inhibitor-1(PAI-1), representative
of the serpin superfamily is the major regulator of
tissue-type (tPA) and urokinase-type (uPA)
plasminogen activators in vivo
• PAI-1 plays a role in the control of normal fibrinolysis as
well as a number of physiological processes that are
dependent on plasminogen activation

32. • PAI-1 acts as a suicide inhibitor that inactivates target
proteinases by trapping a stabilized acyl enzyme
intermediate
• Significant structural changes accompanying the
reaction result in efficient and essentially irreversible
impairment of the proteinase mechanism

33. • Eflornithine, one of the drugs used to treat sleeping
sickness, is a suicide inhibitor of Ornithine
Decarboxylase(ODC) preventing the natural substrate
Ornithine from accessing the active site
• DFMO (difluoromethylornithine) and DFMA
(difluoromethylarginine), irreversible suicide inhibitors
of ornithine and arginine decarboxylase activities (ODC
and ADC) respectively
• Inhibit the growth of six species of Microsporum and
six species of Trichophyton

34. Hit and Run drugs
• Are the drugs whose effect lasts much longer than the
drug itself
• Because, drug with a relatively short t½ still able to
produce effect long after it has been eliminated - its
intracellular action and partial binding to the receptor
long after most of the extracellular drug has been
eliminated
• Eg. Reserpine, Omeprazole, MAO inhibitors,
Methyldopa, etc.

35. Name of the
drug
Category Mechanism of
action
T1/2
(hrs.)
Uses
Reserpine Adrenergic
neuron
blocking
agent
Blocks the
ability of
aminergic
transmitter
vesicles to take
up & store
biogenic amines
24-48 Antihypertensive(rarely
used)
Omeprazole Proton
pump
inhibitor
Blocks final
common
pathway of acid
secretion in
parietal cells
0.5-1.5 Peptic, gastric,
duodenal ulcers & GERD
Methyldopa Centrally
acting
sympathetic
drug
Activates α2
adrenoceptors
2 Hypertension

36. Conclusion
• Special group of irreversible inhibitors relatively
unreactive until they bind to active site of enzyme
• Knowledge is highly useful in developing
pharmaceutical agents with minimal side effects
• Designing drugs that precisely mimic the transition
state - a real challenge because of unstable, poorly
characterized structure of transition state

37. • Main goal of this approach - to create substrates
that are unreactive and non-toxic
• Become active within that enzyme's active site and
at the same time being highly specific for that
enzyme
• Drugs based on this approach have the advantage
of very few resulting side effects
• Suicide inhibitors are used in what is called
"rational drug design" where the aim is to create a
novel substrate, based on already known
mechanisms and substrates

38. References
• Journal of chemistry & biology 2008.04.011
-A ‘Structural View of the Inactivation of the SARS
Coronavirus Main Proteinas by Benzotriazole Esters’
• ‘Development and evolution of therapies targeted to
the estrogen receptor for the treatment and
prevention of breast cancer’ 2007 January;72(1): 7–25.
doi:10.1016/j.steroids.2006.10.009.
• Bioorg Med Chem Lett. 1998 Aug 18;8(16): 2129-32. ‘The
structure-activity relationships of a series of suicide
inhibitors of phospholipase A2’

39. • ‘Benzothiazinones are suicide inhibitors of
mycobacterial decaprenylphosphoryl-β-D-ribofuranose
2'-oxidase DprE1’ 2011 American Chemical Society
• Journal of American Chemical Society -2009 Mar
4;131(8):3057-62. doi: 10.1021/ja809683v
• Journal of American Chemical Society -2012 Jan
18;134(2):912-5. doi: 10.1021/ja211042r. Epub 2011 Dec 21