This document discusses inhibitors and uncouplers of the electron transport chain and oxidative phosphorylation. It begins by describing the electron transport chain and its components. It then discusses various inhibitors that block electron transport at different sites in the chain, including complexes I-IV. Uncouplers are also described that allow electron transport without phosphorylation by increasing membrane permeability to protons. Physiological uncouplers and their role in thermogenesis are covered. The document concludes with some inherited disorders related to deficiencies in the electron transport chain.

1. Inhibitors of electron transport chain and inhibitors
and uncouplers of oxidative phosphorylation
DONE BY:
STEFFY THOMAS
I MSc BIOCHEMISTRY

2. ELECTRON TRANSPORT CHAIN
•An electron transport
chain (ETC) is a series of
complexes that transfer
electrons from electron
donors to electron
acceptors via redox (both
reduction and oxidation
occurring simultaneously)
reactions, and couples this
electron transfer with the
transfer of protons (H+
ions) across a membrane.

3. COMPONENTS AND SEQUENCE OF REACTIONS OF ELECTRON TRANSPORT
CHAIN

5. •Many site-specific inhibitors of electron transport
chain have contributed to the present knowledge of
mitochondrial respiration
•The inhibitors binds to one of the component of the
electron transport chain and block the transport of
electrons.
•This causes the accumulation of reduced
components before the inhibitor blockade step and
oxidized components after that.

6. •The synthesis of ATP (phosphorylation) is
Dependent on electron transport .
•Hence, all the site-specific inhibitors of ETC also
inhibit ATP formation.
•Three possible sites of action for the inhibitors of
ETC are identified
1. NADH and coenzyme Q :
• Fish poison rotenone,
• barbiturate drug amytal and
• antibiotic piercidin A inhibit this site.

8. Site-I (Complex-I)
• ROTENONE: A fish poison and also insecticide.
Inhibits transfer of electrons through complex-I-
NADH-Q-reductase.
• AMOBARBITAL (AMYTAL) AND SECOBARBITAL:
Inhibits electron transfer through NADH-Q reductase.
• PIERICIDIN A: An antibiotic. Blocks electron transfer
by competing with CoQ.
• DRUGS: Chlorpromazine and hypotensive drug like
guanethidine.

9. Piericidin A
amobarbitol

10. Site-II (Complex III)
• Antimycin A
• BAL (Dimer- Caprol)
• Hypoglycaemic drugs: like Phenformin
Blocks electron transfer from cyt b to c1

12. Dimercaprol, also called British
anti-Lewisite, is a medication
used to treat acute poisoning by
arsenic, mercury, gold, and lead.
It may also be used for antimony,
thallium, or bismuth poisoning,
but the evidence for these uses is
not very strong.
Antimycins are a group
of secondary metabolites
produced by Streptomyces
bacteria

13. Site-III (Complex IV)
• Cyanide
• H2S
• Azide
Inhibits terminal transfer of electrons to molecular O2
• Co (Carbon monoxide): Inhibits Cyt. oxidase by combining
with O2 binding site. It can be reversed by illumination with
light.

15. Cyanide is a rapidly acting, potentially
deadly chemical that can exist in
various forms. Cyanide can be a
colorless gas, such as
hydrogen cyanide (HCN) or cyanogen
chloride (CNCl), or a crystal form such
as sodium cyanide (NaCN) or
potassium cyanide (KCN).
Carbon monoxide is a colorless,
odorless, and tasteless
flammable gas that is slightly less
dense than air.

16. Hydrogen sulphide (H2S)
is a colorless gas that smells
like rotten eggs. At very high
levels, hydrogen
sulphide can be flammable.
Azide is the anion with the
formula N⁻ ₃. It is the conjugate
base of hydrazoic acid. N⁻ ₃ is a
linear anion that is isoelectronic
with CO₂, NCO⁻, N₂O.

17. COMPLEX II: SUCCINATE DEHYDROGENASE FAD
•Carboxin
•TTFA
Specifically inhibit transfer of reducing equivalent from
succinate dehydrogenase
•Malonate: A competitive inhibitor of succinatede hydrogenase

18. Carboxin is a systemic anilide
fungicide. It is used as a seed
treatment for control of smut, rot,
and blight on barley, oats, rice,
cotton, vegetables, corn and wheat.
It is also used to control fairy rings
on turf grass.
Thenoyltrifluoroacetone,
C₈H₅F₃O₂S, is a chemical compound
used pharmacologically as a
chelating agent. It is an inhibitor of
cellular respiration by blocking the
respiratory chain at complex II.
Perhaps the first report of TTFA as
an inhibitor of respiration was by A.
L. Tappel in 1960.

19. Malonate is a competitive inhibitor of the enzyme
succinate dehydrogenase: malonate binds to the active
site of the enzyme without reacting, and so competes
with succinate, the usual substrate of the enzyme.

21. OXIDATIVE PHOSPHORYLATION
•The process of oxidative phosphorylation is
closely associated with the functioning of the
electron transport chain.
•This was studied by fragmentation of
mitochondria.
•In the first fragmentation step, the outer
membrane is removed by treatment with various
detergents such as saponin, digitonin.

22. The two particulate fractions that result are:
1. The outer membrane, either in the form of
vesicles or completely solubilised.
2. The inner membrane and the mitochondrial
matrix enzymes. This fraction is found to
contain the enzymes of:
• The electron transport chain
• Oxidative phosphorylation
• The TCA cycle.

23. •In oxidative phosphorylation ATP is produced by combining ADP
and Pi with the energy generated by the flow of electrons from
NADH to molecular oxygen in the electron transport chain.
There are three reactions in the ETC that are exergonic to result in
the synthesis of 3 ATP Molecule. The three sites of ATP formation in
ETC are:
1. Oxidation of FMNH2 by coenzyme Q.
2. Oxidation of cytochrome b by cytochrome c1 .
3. Cytochrome oxidase reaction

24. INHIBITORS OF OXIDATIVE PHOSPHORYLATION
Oligomycin:
•It binds with the enzyme ATP synthase and blocks the proton
channels. It thus prevents the translocation of H+ into the
mitochondrial matrix, this leads to accumulation of H+ at higher
concentration in intermembrane space.
•Since protons cannot be pumped out against steep proton
gradients, electron transport stops (respiration stops).
Atractyloside:
It is a glycoside, it blocks the translocase that is responsible for
movement of ATP and ADP, across the inner mitochondrial
membrane. Adequate supply to ADP is blocked thus preventing
phosphorylation and ATP formation

25. Bongregate:
•Toxin produced by Pseudomonas.
• It acts similarly to Atractyloside.
UNCOUPLERS OF OXIDATIVE PHOSPHORYLATION
•The mitochondrial transport of electrons is tightly
coupled with oxidative phosphorylation (ATP synthesis ) .
In other words oxidation and phosphorylation proceeds
simultaneously.

26. •There are certain compounds that can uncouple (or delink) the
electron transport from oxidative phosphorylation .
•such compounds , known as uncouplers, increase the
permeability of inner mitochondrial membrane to protons (H+).
•The result is that ATP synthesis does not occur. The energy
linked with the transport of electrons is dissipated as heat.
•The uncouplers allow (often at accelerated rate) oxidation of
substrates (via NADH or FADH2) without ATP formation.

27. 2,4-dinitrophenol (DNP
•Dinitrophenol (DNP) a potent uncoupler is amphipathic
and increase the permeability of the lipoid inner
mitochondrial membrane to protons (H+), thus
reducing the electrochemical potential and short-
circuiting the ATP synthase. In this way, oxidation can
proceed without phosphorylation.
Note: DNP was used for weight loss. But it was
discontinued due to hyperthermia and other side effects

29. •The other uncouplers include dinitrocresol,
pentachlorophenol, trifluorocarbonylcyanide
phenylhydrazone( FCCP) .
•The last compound(FCCP) is said to be 100 times
more effective as an uncoupler than dinitrophenol.
•When administered in high doses the drug aspirin
acts as an uncoupler.

30. PHYSIOLOGICAL UNCOUPLERS
•Certain physiological Substances which act as
uncouplers at Higher concentration have been
identified.
•These include thermogenin, thyroxine and long
chain free fatty acids.
•The unconjugated bilirubin is also believed to act as
an uncoupler. This is, however, yet to be proved
beyond doubt

31. SIGNIFICANCE OF UNCOUPLING
•Uncoupling of respiration from oxidative phosphorylation under
natural conditions assumes biological significance.
•The maintenance of body temperature is particularly important
in hairless animals, hibernating animals and the animals adapted
to cold
•These animals possess a specialized tissue called brown adipose
tissue in the upper back and neck portions.

32. •The mitochondria of brown adipose tissue are rich in
electron carriers and are specialized to carry out an
oxidation uncoupled from phosphorylation.
•This causes liberation of heat when fat is oxidized in the
brown adipose tissue. Brown adipose tissue may be
considered as a site of non-shivering thermogenesis.
•The presence of active brown adipose tissue in certain
individuals is believed to protect them from becoming
obese.
•The excess calories consumed by these people are burnt
and
Liberated as heat, instead of being stored as fat.

33. •Thermogenin (or uncoupling protein, UCP) is a natural uncoupler
located in the inner mitochondrial membrane of brown adipose
tissue. it acts like an uncoupler, blocks the formation of ATP, and
liberates heat.
•lonophores : The term ‘ionophores’ is used to collectively represent
the lipophilic substances that promote the transport of ions across
biological membranes.
All the uncouplers (described above) are, in fact, proton ionophores
•The antibiotics valinomycin and nigercin act as ionophores for K+
ions.
Both these compounds are also capable of dissipating proton
Gradient across the inner mitochondrial membrane andinhibit
oxidative phosphorylation

34. CLINICAL ASPECTS
Inherited Disorders
Dysfunction of the respiratory chain can cause certain
diseases which may be inherited deficiency of certain enzyme
systems.
1. Infantile mitochondrial myopathy associated with renal
dysfunction:
• The condition is fatal
• There is severe diminution or absence of most of the
oxidoreductases of the electron transport chain.
2. MELAS: An inherited disorder associated with:
Mitochondrial myopathy, Lactic acidosis, Encephalopathy
and Stroke
Enzyme deficiency: NADH: Ubiquinone oxidoreductase
(complex 1) or deficiency of cytochrome
oxidase.