Dr.S.Chakravarty, MD
Oxidation is defined as the removal of electrons
and reduction as the gain of electrons.
Oxidation is always accompanied by reduction of an
electron acceptor.
Oxidation and reduction:
Oxidation :
Reduction :
• Loss of electrons
• Gain of electrons
• Loss of hydrogen
• Gain of hydrogen
• Gain of oxygen
• Loss of oxygen
REDOX POTENTIAL
• When a substance exists both in the reduced
state and the oxidised state , the pair is called
a REDOX COUPLE.
• The redox potential of this couple is estimated
by measuring the EMF of a sample half cell
connected to a standard half-cell.
Salt bridge
1M H + ~ H gas
@ 1 Atmpspheric
pressure
E0’ =0 meV
• WHEN A SUBSTANCE HAS LOWER AFFINITY FOR ELECTRONS THAN HYDROGEN IT
HAS A NEGATIVE REDOX POTENTIAL
• LOWER AFFINITY FOR ELECTRONS = NEG. REDOX POTENTIAL = STRONG REDUCING
AGENT AND VICE VERSA.
• ELECTRONS MOVE ALWAYS FROM MORE ELECTRONEGATIVE TO ELECTROPOSITIVE
LOSS OF FREE ENERGY
(THIS ALWAYS ENSURES THAT FREE ENERGY DECREASES)
O
X
I
D
A
T
I
O
N
FOOD
(REDUCED )
MOLECULES
SMALLER
e-
REDUCED
COENZYMES
e.g NAD
ELECTRON
TRANSPORT CHAIN
MITOCHONDRION
ENERGY
B
I
O
L
O
G
I
O
C
A
L
02  H2O
Redox Potentials
Oxidant
Reductant
E0’ ( in V)
NAD +
NADH + H +
-0.32
Cytochrome b+++
Cytochrome b ++
+0.07
Co-eneyme Q
Co-eneyme QH2
+.010
Cytochrome c +++
Cytochrome c ++
+0.22
Cytochrome a +++
Cytochrome a +++
+0.29
½ O2 +2H
H2O
+.82
• ½ O2 + 2H+  H2O (E0’ = +0.82 )
• NAD+ + H +  NADH (E0’ = -0.32)
•  E0’ = +0.82 – (-0.32 ) = 1.14
• COMBINE BOTH OF THESE :½ O2 + NADH + H+  H2O + NAD+
Δ G0 = -nFΔE0
= - 2 x 23.06 x 1.14 = -52.6 kcal/mol
Harper’s Illustrated Biochemistry
With the help of
successive reductions
in the electron
transport chain
assembly , this
energy change is
released in small
increments so that
the body can utilize it
.
HIGH ENERGY COMPOUNDS
• THESE COMPOUNDS
WHEN HYDROLYSED
RELEASE A LARGE
AMOUNT OF ENERGY
• INDICATED BY SQUIGGLE
(~)
• FREE ENERGY VARIES
FROM -7 TO -15 kcal/mol
• Defn :- When the energy of high energy compound is directly
transferred to nucleoside diphosphate to form a triphosphate
without the help from electron transport chain.
• Examples :-
1.Bisphosphoglycerate kinase ( Glycolysis)
(1,3 bisphosphoglycerate  3-phosphoglycerate)
2. Pyruvate kinase (Glycolysis)
(Phosphoenol pyruvate  Pyruvate)
3. Succinate thiokinase (TCA cycle)
(Succinyl CoA  Succinate )
Biological Oxidation and Oxidative
Phosphorylation
• 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.
The mitochondrion contained the enzymes responsible for electron transport and
oxidative phosphorylation
Impermeable to
ions and most
other compounds
USMLE
concept!
IMPORTANT MITOCHONDRIAL
TRANSPORTERS
Pyruvate
Hydrogen
Pyruvate
Hydrogen
Malate
Citrate
Malate
Citrate
ADP
ATP
ADP
ATP
INNER MITOCHONDRIAL MEMBRANE
The mitochondrial membrane is impermeable to
NADH…..
Malate –Aspartate shuttle
Operates in Liver , Kidney and Heart
Glycerol -3 –phosphate shuttle
Operates mainly in muscle and Brain.
• The flow of electrons occurs through
successive dehydrogenase enzymes in
mitochondria , together known as the electron
transport chain (ETC).
(the electrons are transferred from lower
to higher redox potential)
The transfer of electrons is not
directly to oxygen but through coenzymes
NAD+
FMN
FeS
FAD
FeS
ubiquinone
Cyt b
ubiquinone
FeS
Cyt c1
Cyt c
Cyt a
Cyt a3
1/2 O2
Protein complexes:
• NADH-CoQ Dehydrogenase (Complex I)
• Succinate-CoQ Dehydrogenase (Complex II)
• CoQ-cytochrome c Reductase (Complex III)
• Cytochrome c Oxidase (Complex IV)
• Mobile complexes:
1. Co-enzyme Q or ubiquinone
2. Cyt C
SITE 1
4 PROTONS
PUMPED
OUT
COMPLEX I
FMN-FeS
SITE 2
4 PROTONS
PUMPED
OUT
Complex III
FeS-Cytb-Cyt c1
Co Q
SITE 3
2 PROTONS
PUMPED
OUT
Complex
IV
Cyt a-a3
Cyt C
2H +
(Fe+3,Cu+2)
COMPLEX II
FeS
Inner mitochondrial membrane
+
NAD
1.Glyceraldehyde -
FAD
8.Succinate DH(TCA CYCLE)
9. Acyl Coa A DH(fatty acid oxidn.)
10. Glycerol 3-P DH(mitochondrial)
3P
2.Isocitrate
3.Malate
4.Glutamate
5.β-OH-acyl CoA
Fp(FAD) lipoate 6.Pyruvate
7.α-ketoglurarate
H2O
Mitochondrial Matrix
Pathways for flow of electrons:
•
For NADH:
Complex 1 -> complex 3 -> complex 4
There are 2 sites of entry
for electrons into the
electron transport chain:
Using either
NAD+ or FAD
• For FADH2: (more positive redox)
Complex 2 -> complex 3 -> complex 4
Both are coenzymes for
dehydrogenase enzymes
2
1
3
4
COENZYME Q
•
The ubiquinone is reduced
successively to semiquinone (QH)
and finally to quinol (QH2)
 It accepts a pair of electrons from
NADH or FADH2 through complex
I or complex II respectively.
 Co-enzyme Q is a quinone
derivative having long isoprenoid
tail.
 2 molecules of cytochrome c are
reduced.
 The Q cycle thus facilitates the
switching from the 2 electron
carrier ubiquinol to the single
electron carrier cytochrome c.
 This is a mobile carrier.
(Peter Mitchell. N.P 1978)
• The transport of electrons from inside to outside
of inner mitochondrial membrane is
accompanied by the generation of a proton
gradient across the membrane.
• Protons accumulate outside the membrane
creating an electrochemical potential.
• This drives the synthesis of ATP by ATP synthase .
Harper’s Illustrated Biochemistry
Harper’s Illustrated Biochemistry
• The pH outside is 1.4 units lower than inside .
• The outside is positive 0.14V relative to inside.
• The proton motive force (PMF ) IS 0.224 v
corresponding to a free energy change of 5.2
kcal/mol of protons.
• ENERGETICS OF ATP SYNTHESIS
– ENERGY RELEASED = 52kcal/mol
– Synthesis of 1ATP and Pi requires 7.3 kcal/Mol
molecules
– Chemical energy trapped = 7.3 x 3 = 21.9kcal =
40%
– Rest 60% energy is dissipated as HEAT !!
F0 – F1 complex :
Fo complex: –
O STANDS FOR OLIGOMYCIN
Made of 12 subunits . H+ passes through each subunit
from membrane space to inner space rotating the Fo
complex (turbines).
F1 complex:Has 9 polypeptide chains ,(3 alpha , 3 beta , 1
gamma , 1 sigma , 1 epsilon)
the α chains have binding sites for ATP and ADP and beta
chains have catalytic activity.
ATP SYNTHESIS NEEDS Mg +2 IONS
• ADP and Pi bind the alpha subunit
• Binding change mechanism - conformation change of
beta subunits causes release of ATPs from the complex.
• ATPs formed in the mitochondrial matrix are
translocated to cytosol by ATP/ADP translocase
1 ) ADENINE NUCLEOTIDE TRANSPORTER
2)H+/Pi SYMPORT
• According to recent estimates , NADH may
generate only 2.5 ATPs while FADH2 may
generate only 1.5 ATP.
• So , instead of 38 ATP , only 32 ATPs are
generated from glucose .
ATP IS THE ENERGY CURRENCY