Download ETC Inhibitors

ELECTRON
TRANSPORT
CHAIN
ECDA
September 2009
ELECTRON TRANSPORT CHAIN
 The cells of almost all eukaryotes (animals,
plants, fungi, algae, protozoa – in other words,
the living things except bacteria) contain
intracellular organelles called mitochondria,
which produce ATP.
 Energy sources such as glucose are initially
metabolized in the cytoplasm.
 The products are imported into mitochondria.
ELECTRON TRANSPORT CHAIN
 Mitochondria continue the process of
catabolism using metabolic pathways
including the Krebs cycle, fatty acid
oxidation, and amino acid oxidation.
 The end result of these pathways is the
production of two kinds of energy-rich
electron donors, NADH and FADH2. When
metabolized:
 One NADH molecule = 3 ATP molecules
 One FADH2 molecule = 2 ATP molecules
ELECTRON TRANSPORT CHAIN
 Electrons from NADH and FADH2 are
passed through an electron transport
chain to oxygen, which is reduced to
water.
 This is a multi-step redox process that
occurs on the mitochondrial inner
membrane.
ELECTRON TRANSPORT CHAIN
ELECTRON TRANSPORT CHAIN
 Four membrane-bound complexes have been
identified in mitochondria.
 Each is an extremely complex transmembrane
structure that is embedded in the inner
membrane.
 Three of them are proton pumps (Complexes I, III,
and IV).
 The structures are electrically connected by lipidsoluble electron carriers and water-soluble
electron carriers.
ELECTRON TRANSPORT CHAIN
ETC–Complex I
 Complex I removes two electrons from NADH
and transfers them to a lipid-soluble carrier,
ubiquinone (Q)
 When NADH binds to complex I, it binds to a
prosthetic group called flavin mononucleotide (FMN),
and is immediately re-oxidized to NAD.
 FMN then receives the hydrogen from the NADH and
two electrons.
 The reduced FMN form passes the electrons to ironsulfur clusters that are part of the complex, and forces
two protons into the intermembrane space.
ETC–Complex I
ETC–Complex I
 Electrons pass from complex I to a carrier
(Coenzyme Q) embedded by itself in the
membrane.
 From Coenzyme Q electrons are passed to a
complex III which is associated with another
proton translocation event.
 Note that the path of electrons is from Complex I
to Coenzyme Q to Complex III.
 Complex II, the succinate dehydrogenase
complex, is a separate starting point, and is not a
part of the NADH pathway.
ETC–Complex II
 Complex II (succinate dehydrogenase) is not a
proton pump. It serves to funnel additional
electrons into the quinone pool (Q) by removing
electrons from succinate and transferring them
(via FAD) to Q.
 Complex II consists of four protein subunits:
SDHA, SDHB, SDHC, and SDHD. Other electron
donors (e.g., fatty acids and glycerol 3phosphate) also funnel electrons into Q (via
FAD), again without producing a proton
gradient.
ETC-Complex II
ETC-Complex III
 Complex III (cytochrome bc1 complex)
removes in a stepwise fashion two electrons
from QH2 at the QO site and sequentially
transfers them to two molecules of
cytochrome c, a water-soluble electron
carrier located within the intermembrane
space.
 From Complex III the pathway is to
cytochrome c then to a Complex IV
(cytochrome oxidase complex).
ETC-Complex III
ETC-Complex IV
 Complex IV (cytochrome c oxidase) removes
four electrons from four molecules of
cytochrome c and transfers them to molecular
oxygen (O2), producing two molecules of
water (H2O).
 Molecular oxygen serves as the final electron
sink or acceptor, clearing the way for carriers
in the sequence to be reoxidized so that
electron transport process can continue
ETC
 KEY POINTS:
 Protons are translocated across the membrane,
from the matrix to the intermembrane space
 Electrons are transported along the membrane,
through a series of protein carriers
 Oxygen is the terminal electron acceptor,
combining with electrons and H+ ions to produce
water
 As NADH delivers more H+ and electrons into the
ETS, the proton gradient increases, with H+
building up outside the inner mitochondrial
membrane, and OH- inside the membrane.
ETC INHIBITORS
 Electron transport chain may be blocked by some
compounds known as ETC Inhibitors.
 ETS inhibitors act by binding somewhere on the
electron transport chain, literally preventing
electrons from being passed from one carrier to the
next.
 They all act specifically, that is, each inhibitor binds a
particular carrier or complex in the ETS.
 No matter what substrate is used to fuel electron
transport, only two entry points into the electron
transport system are known to be used by
mitochondria: Complexes I and II.
Two entry points into the ETC system
ETC
INHIBITORS
ETC INHIBITORS
 A consequence of having separate pathways
for entry of electrons is that an ETS inhibitor
can affect one part of a pathway without
interfering with another part. In this case,
respiration can still occur depending on
choice of substrate.
 However, some poisons may completely stop
ETC and halt respiration.
ETC INHIBITORS
 Two Mechanism of Inhibition:
Irreversible inhibition results in a complete
stoppage of respiration via the blocked pathway.
2. Competitive inhibition allows some oxygen
consumption since a "trickle" of electrons can
still pass through the blocked site.
1.
Although it allows some oxygen consumption,
competitive inhibition prevents maintenance of a
chemiosmotic gradient, thus the addition of ADP
can have no effect on respiration.
ETC INHIBITORS
 An inhibitor may completely block electron
transport by irreversibly binding to a binding
site.
 For example, cyanide binds cytochrome oxidase
so as to prevent the binding of oxygen. Electron
transport is reduced to zero. Breathe all you want
- you can't use any of the oxygen you take in.
 Rotenone, on the other hand, binds competitively,
so that a trickle of electron flow is permitted.
However, the rate of electron transport is too slow
for maintenance of a gradient.
ETC Inhibitors
 Electron Transport Inhibitors
 Rotenone
 Antimycin
 Cyanide
 Oligomycin (inhibitor of oxidative
phosphorylation)
ETC Inhibitors
 ROTENONE
 ANTIMYCIN
 Used as insecticide
 Being used in researches
 Toxic to wildlife, to
 binding site for
humans as well as to
insects
 Competitive inhibitor on
complexes I and II, thus,
blocking respiration
antimycin can be
narrowed considerably
using combinations of
substrates inlcuding
succinate, NADH
ETC Inhibitors
 CYANIDE
 extremely effective
reversible inhibitor of
cytochrome oxidase
 A concentration of 1 mM
KCN is sufficient to inhibit
oxygen consumption by
mitochondria from a
vertebrate source by
>98%.
 Cyanide is one of the most
deadly compounds in a
laboratory.
ATP Synthase Inhibitor
 OLIGOMYCIN
 An antibiotic, acts by
binding ATP synthase in
such a way as to block
the proton channel
 Inhibitor of oxydative
phosphorylation
 it has no direct effect on
electron transport or the
chemiosmotic gradient
ETC Inhibitors