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Oxidative Phosphorylation
•It is the process by which electrons are carried from
reduced cofactors (NADH+/ QH2) are finalled in stepwise
manner to oxygen.
•Electrons flow much like electricity in a circuit with free
energy being conserved with the formation of proton
gradient.
• In the end the investment of reduced cofactors are
utilized in the production of ATP
•Reduced cofactors like NADH+ are produced during
glycolysis, Citric acid cycle and fatty acid oxidation
•During cellular respiration oxidative phosphorylation
chemical energy of these reduced molecules are utilized to
produce ATP
•The ultimate acceptor of e- through a series of O/R
reactions is the O2 within the mitochondrion
Mitochondrial Anatomy
• The anatomy of the mitochondrion reflects its
role in oxidative phosphorylation
• The outer membrane is porous and allows the
free diffusion of molecules due to the presence
of channel protein called porins
• The inner membrane is impermeable to most
substances including ions and encloses a space
called the matrix
• The inner membrane is convoluted structure
which provides greater surface area for the
protein complexes of the oxidative
phosphorylation
• The mitochondrion
consists two membranes
separated by a
compartment called the
intermembrane space
• During oxidative
phosphorylation protons
are pumped into this
compartment
matrix
cristae
intermembrane
space
inner
membrane mitochondrion
outer
membrane
Transport Shuttles
• Since the inner mitochondrial membrane is
impermeable to most molecules, NADH+
produced by glycolysis in the cytosol must be
imported into via biochemical reaction of the
malate – aspartate shuttle
• This shuttle operates mainly in the liver, kidney
and heart
• In the skeletal muscle & brain NADH+ is imported
into - by Glycerol – phosphate shuttle
• The process is a formal currency exchange
between one region of the cell with the other.
• ATP, ADP and Pi also require transport protein for
their import and export across the inner
membrane
Redox potential of the components of
Respiratory chain
• In the respiratory chain the e- s are
transferred from reducing equivalents to a
chain of e- carriers which are arranged
sequentially
• The e- s flow from more electropositive
components to more electronegative
components i.e. to more positive re-dox
potentials
• Hydrogen and e- s move from NAD+/NADH
to O2/ H2O
Components of the Respiratory chain
• The inner membrane consists of majority of
proteins including three major electron
transporting complexes
• These complexes function in re oxidizing the
coenzymes ( NAD+/ ubiquinone, etransferring flavoproteins) that have been
reduced by dehydrogenases in the metabolic
reactions within the mitochondrion
• The terminal e- acceptor is O2 & the reaction
is coupled to ATP synthesis
Components of Respiratory chain
• The respiratory chain consists of number of redox
carriers proceeding from NAD – linked dehydrogenase
through flavoprotein and cytochromes to molecular
oxygen
• Certain substrates (fumarate/succinate) comparatively
of their more positive redox potential however are
directly linked to flavoprotein dehydrogenase further
linked to cytochromes to molecular oxygen
• Ubiquinone (Q/ Coenzyme Q) links flavoprotein to
Cytochrome b (member of cytochrome chain with
lowest redox potential)
• Ubiquinone acts as a mobile component of the
respiratory chain that collects reducing equivalents from
more fixed flavoproteins and passes them to
cytochromes
• Next component is the Iron – sulfur protein (Fe-S –
a non heme protein) associated with flavoprotein
and cytochrome b
• Electrons flow through a series of cytochromes in
order of increasing redox potentials to molecular
oxygen
• The terminal cytochrome aa3 (cytochrome oxidase)
is responsible for the final combination of reducing
equivalents with molecular oxygen.
• It has a high affinity for O2 thus allowing the
respiratory chain to function at its maximum.
• This is the only irreversible reaction in the chain
and hence provides direction to the movement of
reducing equivalents & to the production of ATP –
to which it is coupled
• The components of the respiratory chain are all present
in the inner mitochondrial membrane as four protein –
lipid respiratory chain complexes
• Cytochrome c is the only soluble cytochrome & together
with ubiquinone seems to be a mobile component
connecting the more fixed complexes
In simple outline, ETC involves the removal of hydrogen
atoms from the oxidizable substrates; these hydrogen
atoms enter the ETC, a system of membrane – bound
complexes and each soon split to yield a proton and
electron. These electrons then pass through a series of
cytochromes, finally reacting with molecular oxygen and
the protons that were released earlier, to form water
Electron carriers are Multi enzyme complexes
•
•
•
Complex I (NADH to Ubiquinone)
Also k/as NADH:ubiquinone reductase
Electron microscope reveals it as an L – shaped
structure with one arm in the membrane and the other
extending into the matrix
• An enzyme with 42 polypeptide chains and an FMN –
flavoprotein with about 6 Fe- S centres
• Complex I catalyses 2 simultaneous reactions
1. - exergonic transfer of a hydride ion (:H-)from NADH & a
proton from the matrix
NADH + H+ + Q
NAD+ + QH2
2. -endergonic transfer of 4 protons from the matrix to the
intermembrane space
Complex I – k/as proton pump driven by the energy of
electron transfer; where – protons move from one
location (matrix which then becomes negatively charged)
to the other (intermembrane space which becomes
positively charged)
Structure of iron – sulfur center
• These function as prosthetic groups which
facilitate electron transfer
• Iron - sulfur protein - The iron is not present in
heme but is found in association with inorganic
sulfur or the cysteine residues in the protein
• Rieske Iron - sulfur protein - One iron atom is
coordinated to 2 Histidine residues instead of 2
cysteine residues
• All these centers however participate in one –
electron transfer where the Fe –atom gets
oxidised/ reduced
• At least 8 Fe – S proteins are involved in
mitochondrial electron trasfer
Ubiquinone
• Lipid soluble bezoquinone with a long isoprenoid side chain
• Complete reduction of ubiquinone requires 2 electrons and 2
protons ( a 2 step rxn) through semiquinone as an
intermediate
• As it carries both e- & protons it plays a central role in coupling
electron flow to proton movement
• It always acts at the junction between a 2 e- donor and one
electron acceptor
O
O
CH3O
CH 3
CH3O
CH 3
CH3O
(CH 2 CH
O
C
CH 3
e
CH 2)nH
CH 3
CH3O
(CH 2 CH
O
coenzyme Q
C
CH 2)nH
coenzyme Q •
e + 2 H+
OH
CH3O
CH 3
CH 3
CH3O
(CH 2 CH
OH
C
CH 2)nH
coenzyme QH2
Complex II
• Also called as Succinate:ubiquinone
oxidoreductase (Succinate to ubiquinone)
• It is the only membrane bound enzyme
(succinate dehydrogenase) encountered in citric
acid cycle
• Structurally it is simpler than complex I with 2
types of prosthetic groups & 4 different proteins
• One of the protein is bound covalently to FAD &
Fe- S center with 4 Fe atoms
• Electrons pass from succinate to FAD; through
Fe – S centers finally reaching ubiquinone
Oxidation of NADH & Succinate
• NADH produced by CAC diffuses into the ETC
where the flavoprotein enzyme (NADH
dehydrogenase) oxidizes it to NAD+
• The process involves transfer of a Hydride ion
(which consists of hydrogen nucleus with 2
associated electrons) to the flavin enzyme (FMN)
which accepts the proton to be FMNH2
• To be known – Hydride ions do not have
independent existence – they just represent the
moiety transferred in biological reduction process
• Succinate dehydrogenase – flavoprotein having
FAD as the coenzyme – links CAC directly to ETC
Fate of FMNH2 & FADH2
• FMNH2 & FADH2 are oxidized by enzymes
that transfer the hydrogen atoms to a
molecule of Ubiquinone (Q) thus forming
Ubiquinol (QH2)
• Ubiquinone also accepts hydrogen atoms
transferred from other molecules that have
been oxidized by ETC ( - oxidation,
glycerol – 3 – P)
Succinate dehydrogenase (fp)
Fatty acyl CoA
dehydrogenase (fp)
Glycerol 3 phosphate
dehydrogenase (fp)
Matrix
NADH fp
Dehydrogenase Fes
Q
Q bc1
FeS
Intermembrane
space
Proton pump
Proton pump