Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Electron Transport Chain Lecture PPT
ELECTRON TRANSPORT
CHAIN
DR. S. SHEKHAR
ASSOC. PROFESSOR
DEPT. OF
BIOCHEMISTRY
SYNTHESIS OF ATP
ATP can be synthesized in two
ways
1. Oxidative phosphorylation:
Major source of ATP in aerobic
organisms.
It is linked with mitochondrial
ETC.
2.
Substrate
level
phosphorylation:
When the energy of high energy
compound
is
directly
transferred
to
nucleoside
diphosphate
to
form
a
triphosphate without the help
from ETC.
The high-energy compounds such as
? PEP
? 1,3-bisphosphoglycerate
? Succinyl CoA
can transfer high-energy phosphate to ultimately
produce ATP.
STORAGE FORMS
? Phosphocreatine ( creatine phosphate)
? Provides high energy reservoir of ATP to regenerate
ATP rapidly, catalyzed by creatine kinase.
? Stored mainly in Muscle, Heart & Brain.
BIOLOGICAL OXIDATION
The transfer of electrons from the reduced
coenzymes through the respiratory chain to
oxygen is known as biological oxidation.
Energy released during this process is trapped
as ATP.
This
coupling
of
oxidation
with
phosphorylation
is
called
oxidative
phosphorylation.
TRANSPORT OF REDUCING EQUIVALENT
:SHUTTLE PATHWAY
? The inner mitochondrial is impermeable to
NADH.
? Therefore, the NADH produced in the cytosol
cannot directly enter the mitochondria.
? Two pathways
A. Glycerol-phosphate shuttle- In muscle and
brain
B. Malate-aspartate shuttle - In liver and heart
GLYCEROL-PHOSPHATE SHUTTLE
? Cytosolic glycerol 3-phosphate dehydrogenase
oxidizes NADH to NAD+
? The reducing equivalents are transported
through glycerol 3-phosphate into the
mitochondria.
? Glycerol 3-phosphate dehydrogenase-present
on outer surface of inner mitochondrial
membrane ? reduces FAD to FADH2.
.
? Dihydroxyacetone phosphate (DHAP) escapes
into the cytosol & the shuttling continues.
? FADH2 gets oxidized via ETC to generate
1.5ATP
GLYCEROL PHOSPHATE SHUTTLE
MALATE-ASPARTATE SHUTTLE
? In the cytosol, oxaloacetate accepts the
reducing equivalents (NADH) & becomes
malate.
? Malate enters the mitochondria where it is
oxidized by mitochondrial MDH
? In this reaction, NADH & oxaloacetate are
regenerated.
? NADH gets oxidized via ETC & 2.5 ATP are
produced.
MALATE-ASPARTATE SHUTTLE .
? In the mitochondria, oxaloacetate participates
in transamination reaction with glutamate to
produce aspartate & ketoglutarate.
? The aspartate enters the cytosol &
transaminates with -ketoglutarate to give
oxaloacetate & glutamate.
REDOX POTENTIAL
Oxidation:
? Oxidation is defined as the loss of electrons
and reduction as the gain in electrons.
? When a substance exists both in the reduced
state & in the oxidized state, the pair is called a
redox couple.
Redox potential(E0):
? The oxidation-reduction potential or redox
potential, is a quantitative measure of the
tendency of a redox pair to lose or gain
electrons.
? The redox pairs are assigned specific standard
redox potential at pH 7.0 & 250C
? The more negative redox potential represents a
greater tendency to lose electrons.
? A more positive redox potential indicates a
greater tendency to accept electrons
? The electrons flow from a redox pair with more
negative E0 to another redox pair with more
positive E0
? The redox potential (E0) is directly related to
the change in the free energy (G0)
ELECTRON TRANSFER CHAIN
? The flow of electrons occurs through
successive
dehydrogenase
enzymes
in
mitochondria , together known as the ETC.
(the electrons are transferred from higher to
lower potential.)
Significance:
? The free energy released during the transport
of electrons is utilized for the formation of ATP
MITOCHONDRIAL ORGANIZATION
? Mitochondria consists of five distinct parts
? Outer membrane, inner membrane, intermembrane
space, cristae & matrix
Inner mitochondrial membrane:
? The ETC & ATP synthesizing system are located on
inner mitochondrial membrane, which is specialized
structure, rich in proteins
? Inner membrane is highly folded to form cristae.
? Surface area of inner mitochondrial membrane is
increased due to cristae.
? The inner surface of inner mitochondrial membrane
possesses specialized particles, the phosphorylating
subunits which are centres for ATP production.
2H+
4H+
Organisation of electron transport chain and route-
map of electron flow through ETC.
ETC consists of four enzymes complexes & two
free electron carriers
Complex I: NADH-ubiquinone oxidoreductase
Complex II: Succinate dehydrogenase
Complex III: Ubiquinol cytochrome oxidoreductase
Complex IV: Cytochrome oxidase
? Two free electron carriers are coenzyme Q &
Cytochrome C.
? Complex V: It is ATP synthase.
? The complexes I-IV are carriers of electrons while
complex V is responsible for ATP synthesis.
? The enzyme complexes & mobile carriers are
collectively involved in the transport of
electrons which, ultimately, combine with
oxygen to produce water.
? Largest proportion of O2 supplied to body is
utilized by mitochondria for the operation of
ETC.
Complex I
? Of the two coenzymes NAD+& NADP+, NAD+
is more actively involved in ETC.
? Tightly bound to the inner membrane
? NAD+ is reduced to NADH + H+ by
dehydrogenases with the removal of two
hydrogen atoms from the substrates, the
substrates includes pyruvate, gly-3-P. etc.
? NADPH is more effectively utilized for
anabolic reactions - fatty acid synthesis,
cholesterol synthesis.
? The enzyme NADH dehydrogenase (NADH
coenzyme Q reductase) is a flavoprotein with FMN as
the prosthetic group.
? The coenzyme FMN accepts two electrons & a proton
to form FMNH2.
? NADH dehydrogenase is a complex enzyme closely
associated with non- heme iron proteins or iron-sulfur
proteins.
? In this, 4 protons are pumped out from mitochondria.
? NADH + H+ + FMN NAD+ + FMNH2
Complex II ? Succinate - Co Q- Reductase
? The electrons from FADH2 enter ETC at the level of
Co Q.
? Succinate DH is an enzyme found in inner
mitochondrial membrane.
? It is also a flavoprotein with FAD as coenzyme.
? The 3 major enzyme systems that transfer their
electrons directly to ubiquinone are:
a. Succinate dehydrogenase
b. Fatty acyl CoA dehydrogenase
c. Mitochondrial glycerol phosphate
dehydrogenase.
Iron-sulfur centers
? Iron-sulfur centers (Fe-S) are prosthetic groups
containing 1-4 iron atoms
? Iron-sulfur (Fe-S) proteins exist in the oxidized
(Fe3+) or reduced (Fe2+) state.
? Iron-sulfur centers transfer only one electron, even
if they contain two or more iron atoms
? Fe-S participates in the transfer of electrons from
FMN to coenzyme Q.
? Other Fe-S proteins associated with cytochrome b
& cytochrome c1 participate in the transport of
electrons.
Coenzyme Q
? It is also known as ubiquinone.
? It is a quinone derivative with isoprenoid side chain
? The ubiquinone is reduced successively to
semiquinone (QH) & finally to ubiquinol (QH2)
? It accepts a pair of electrons from NADH or
FADH2 through complex I or complex II
respectively.
? 2 molecules of cytochrome c are reduced.
? The Q cycle facilitates the switching from the 2
electron carrier ubiquinol to the single electron
carrier cytochrome c.
? This is a mobile carrier.
Complex III Cytochrome - Reductase
? This is a cluster of iron-sulphur proteins, cytochrome b
& cytochrome c1, both contain heme prosthetic group.
? Consists of a porphyrin ring with iron atom.
? The iron of heme in cytochromes is alternately oxidized
(Fe3+) & reduced (Fe2+ ) which is essential for transport
of electrons in the ETC.
? In this, 4 protons are pumped out.
? This complex transfers 2 electrons to cytochrome c from
2 molecules of CoQH2 along with the vectorial
movement of 4H+ from mitochondrial matrix to
intermembranous space.
? The property of
reversible oxidation reduction of
heme iron present in cytochromes allows them to
function as effective carriers of electrons in ETC.
? Cytochrome C:
It is a small protein containing 104 amino acids & a
heme group.
It is a loosely bound to inner mitochondrial
membrane & can be easily extracted.
Complex IV Cytochrome - Oxidase
? Contains cytochrome a and cytochrome a3 which
is the terminal component of ETC
? Tightly bound to inner mitochondrial membrane.
? Cytochrome oxidase is the only electron carrier,
heme iron of which can directly react with
molecular oxygen.
? It also contains copper that undergoes oxidation
reduction during transport of electrons.
? 2 protons are pumped out.
? In the final stage of ETC, the transported electrons,
the free protons & the molecular oxygen combine
to produce water
.
INHIBITORS OF ETC
? The inhibitors bind to one of the components of ETC
& block the transport of electrons
? This causes the accumulation of reduced
components before the inhibitor blockade step &
oxidized components after that step.
? The synthesis of ATP is dependent on ETC.
? All the site-specific inhibitors of ETC also inhibit
ATP formation.
Complex I: NADH & coenzyme Q
? Fish poison rotenone, barbiturate drug amytol &
antibiotic piercidin A inhibit this.
Complex II:
Carboxin inhibit this site.
Complex III Between cytochrome b & c1
? Antimycin A ?an antibiotic,
? British antilewisite (BAL) ?an antidote used
against war-gas
? Naphthoquinone are important inhibitors of the
site between cytochrome b & c1.
Cytochrome oxidase (Complex IV):
Carbon monoxide, cyanide, hydrogen sulphide
& azide
? Effectively inhibit cytochrome while cyanide &
azide react with oxidized form of cytochrome.
? Cyanide is most potent inhibitor of ETC
? It binds to Fe3+ of cytochrome oxidase blocking
mitochondrial respiration leading to cell death.
? Cyanide poisoning causes death due to tissue
asphyxia (mostly of CNS)
Site specific inhibitors of ETC.
Biological Oxidation:
? The transfer of electrons from the reduced co
enzymes though the respiratory chain to oxygen is
known as biological oxidation.
? Energy released during this process is trapped as
ATP.
? This coupling of oxidation with phosphorylation
is
called
as
OXIDATIVE
PHOSPHORYLATION.
? Complex V of the inner mitochondrial membrane
is the site of oxidative phosphorylation.
PHOSPHAGENS
? Phosphagens act as storage forms of high energy
phosphate and include creatine phosphate, which
occurs in vertebrate skeletal muscle, heart,
spermatozoa & brain.
? Arginine phosphate, in invertebrate muscle.
? When ATP is rapidly being utilized as a source of
energy for muscular contraction, phosphagens permit
its concentrations to be maintained, but when the
ATP/ADP ratio is high, their concentration can
increase to act as a store of high-energy phosphate.
SITES OF OXIDATIVE PHOSPHORYLATION
IN ETC
? There are 3 reactions in the ETC that are
exergonic,
Where the energy change is sufficient to
drive the synthesis of ATP from ADP and Pi.
? Site1:
Oxidation of FMNH2 by coenzyme Q.
? Site2:
Oxidation of cytochrome b by cytochrome c1
? Site3:
Cytochrome oxidase.
ENERGETICS OF OXIDATIVE
PHOSPHORYLATION
? ? O2 + NADH + H+ H2O + NAD+
The redox potential difference between these two redox
pairesis 1.14V, which is equivalent to an energy 52
Cal/mol
3 ATP are synthesized in ETC when NADH is
oxidized which equals to 21.9 Cal.
(each ATP=7.3 Cal)
The efficiency of energy conservation is calculated as
21.9 ? 100
52 = 42%
.
When NADH is oxidized, about 42% of energy
is trapped in the form of 3ATP & remaining is
lost as heat.
The heat liberation is not a wasteful process,
since it allows ETC to go on continuously to
generate ATP.
This heat is necessary to maintain body
temperature.
MECHANISM OF OXIDATIVE
PHOSPHORYLATION
? Two important hypothesis to explain the
process of oxidative phosporylation.
? Namely
Chemical coupling &
Chemiosmotic
Chemical coupling hypothesis:
? This hypothesis was put forth by Edward
Slater (1953)
? According to this, during the course of electron
transfer in respiratory chain, a series of
phosphorylated high-energy intermediates are
first produced which are utilized for the
synthesis of ATP.
? These reactions are believed to be analogous to
the substrate level phosphorylation that occurs
in glycolysis or citric acid cycle.
? This hypothesis lacks experimental evidence.
CHEMIOSMOTIC THEORY
Chemiosmotic theory,
proposed by Peter Mitchell
in 1961, postulates that the
two processes are coupled by
a proton gradient across the
inner
mitochondrial
membrane so that the proton
motive force caused by the
electrochemical
potential
difference (negative on the
matrix side) drives the
mechanism
of
ATP
synthesis.
.
? The transport of electrons through the
respiratory chain is effectively utilized to
produce ATP from ADP + Pi.
? PROTON GRADIENT:
The inner mitochondrial membrane, is
impermeable to protons (H+) & hydroxyl ions
(OH-).
The transport of electrons through ETC is
coupled with the translocation of protons
(H+)across the inner mitochondrial membrane
from the matrix to the inter membrane space.
.
? The pumping of protons results in an
electrochemical or proton gradient
? This is due to the accumulation of more H+ions
(low pH) on the outer side of the inner
mitochondrial membrane than the inner side.
? The proton gradient developed due to the
electron flow in the respiratory chain is
sufficient to result in the synthesis of ATP
from ADP +Pi.
Enzyme systems for ATP synthesis
? ATP synthase, present in the complex V, utilizes
the proton gradient for the synthesis of ATP.
? This enzyme is also known as ATPase, since it
can hydrolyze ATP to ADP + Pi.
? ATP synthase is a complex enzyme & consists of
two functional subunits, namely F1 & Fo.
? Fo unit: O stands for oligomycin,
? Fo inhibited by oligomycin.
? Fo spans inner mitochondrial membrane acting as
a proton channel through which protons enter the
mitochondria
? Fo unit has 4 polypeptide chains & is connected to
F1
.F1 UNIT
? F1 unit: It projects into the matrix.
? F1 has 9 polypeptide chains, (3 alpha, 3 beta, 1
gamma, 1 delta, 1 epsilon)
? The chains have binding sites for ATP & ADP
& beta chains have catalytic activity.
? ATP synthesis requires Mg +2 Ions.
? Its structure is comparable with lollipops.
? The
protons
that
accumulate
on
the
intermembrane space re-enter the mitochondrial
matrix leading to the synthesis of ATP
ROTOR MOTOR MODEL FOR ATP
GENERATION
? Paul Boyer in 1964 proposed that a conformational
change in the mitochondrial membrane proteins
leads to the synthesis of ATP
? This is now considered as rotary motor/engine
driving model or binding change model.
? widely accepted for the generation of ATP.
? The enzyme ATP synthase is Fo & F1 complex
? The Fo sub complex is composed of channel
protein `C' subunits to which F1-ATP synthase is
attached.
.
? F1-ATP synthase consists of a central gamma
subunit surrounded by alternating alpha & beta
subunits ( 3 & 3).
? In response to the proton flux, the gamma subunit
physically rotates.
? This induces conformational changes in the 3
subunits that finally lead to the release of ATP.
? According to the binding change mechanism, the
three subunits of F1 - ATP synthase adopt
different conformations.
? One subunit has Open (O) conformation, the
second has loose (L) conformation while the third
one has tight (T) conformation.
.
? By an known mechanism, protons induce the
rotation of gamma subunit, which in turn induces
conformation changes in subunits,.
? The substrates ADP & Pi bind to subunit in L
conformation.
? The L site changes to T conformation, & this
leads to the synthesis of ATP.
? The O site changes to L conformation which binds
to ADP + Pi.
? The T site changes to O conformation & releases
ATP.
? This cycle of conformation changes of subunits
is repeated.
? Three ATP are generated for each revolution.
.
BOYER'S BINDING CHANGE MODEL FOR ATP SYNTHESIS BY ATP SYNTHASE.
INHIBITORS OF OXIDATIVE PHOSPHORYLATION
? The mitochondrial transport of electrons is
tightly coupled with oxidative phosphorylation.
? Oxidation
&
phosphorylation
proceed
simultaneously.
? There are certain compounds that can uncouple
(or delink) the electron transport from oxidative
phosphorylation.
? Such compounds are known as uncouplers,
? Causes increase in 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
? Examples:
? 2,4-dinitrophenol (DNP):
It is small lipophilic molecule.
DNP is a proton ? carrier & easily diffuse through
the inner mitochondrial membrane.
Others ?dinitrocressol, pentachlorophenol,
trifluorocarbonylcyanide, phenylhydrazone.
PHYSIOLOGICAL UNCOUPLERS
? Certain physiological substances which act as
uncouplers at higher concentration.
? These are thermogenin, thyroxine and long chain
fatty acids & unconjugated bilirubin
Significance of uncoupling:
The maintenance of body temperature is
particularly important in hairless animals,
hibernating animals & the animals adopted to cold
? These animals possess a specialized tissue called
brown adipose tissue in the upper back & neck
portions.
.
? The mitochondria of brown adipose tissue are rich in
electron carriers & are specialized to carry out an
oxidation uncoupled from phosphorylation.
? This causes liberation of heat when fat is oxidized in
the brown adipose tissue.
? The presence of brown adipose tissue in certain
individuals is believed to protect them from
becoming obese.
? Thermogenin is a natural uncoupler located in the
inner mitochondrial membrane of brown adipose
tissue
? It acts like an uncoupler, blocks the formation of
ATP, & liberates heat.
IONOPHORES
? Ionophores: These are lipophilic substances that
are lipid soluble and increases the permeability of
inner motochondrial membrane to ions and
thereby destroy the proton gradient leading to
inhibition of ATP synthesis.
? By either forming channel or
? By binding an ion and then diffusing into
membrane.
? Valinomycin ( binds with K+) & Nigercin also act
as uncouplers
INHIBITORS OF OXIDATIVE
PHOSPHORYLATION
? Oligomycin: This antibiotic binds with enzyme
ATP synthase & blocks the proton(H+)
channels.
? Thus it prevents the translocation (re-entry) of
protons into the mitochondrial matrix and
prevent ATP synthesis
? Atractyloside: It is a plant toxin & inhibits
oxidative phosphorylation.
? It blocks the adequate supply of ADP by
inhibiting ADP/ATP transporter
INHERITED DISORDER OF OXIDATIVE
PHOSPHORYLATION
? 100 polypeptides are required for oxidative
phosphorylation.
? Of these, 13 are coded by mitochondrial DNA
& synthesized in the mitochondria, while the
rest are produced in the cytosol (coded by
nuclear DNA) & transported.
? mtDNA is maternally inherited since
mitochondria from the sperm do not enter the
fertilized ovum.
.
? Mitochondrial DNA is 10 times more susceptible
to mutations than nuclear DNA.
? mtDNA mutations are commonly seen in tissues
with high rate of oxidative phosphorylation (e.g.
CNS, skeletal & heart muscle, liver).
? Diseases:
Lethal infantile mitochondrial opthalmoplegia
Leber's hereditary optic neuropathy (LHON)
Myoclonic epilepsy
Mitochondrial encephalopathy lactic acidosis
stroke like episodes (MELAS)
STRUCTUTRE OF ATP SYNTHASE.
This post was last modified on 30 November 2021