Biological oxidation Electron Transport chain ATP synthesis ATP synthesis inhibitors Redox potential High energy compounds

1. Biological Oxidation &
Electron Transport Chain
Dr Anurag Yadav
MBBS, MD
Associate Professor
Department of Biochemistry
Instagram page –biochem365
Email: dranurag.y.m@gmail.com

2. Stages of oxidation of food
• First stage
• Second stage
• Third stage

3. Redox potential
• It is a system of electron transfer potential E0.
• Oxidation is defined as loss of electrons
• Reduction is defined as gain of electron.
• When substance exists both in reduced state
and oxidised state, the pair is called as redox
couple.

4. Redox potential type
• Negative redox potential
– When substance has lower affinity for electron
than hydrogen.
• Positive redox potential
– When substance has higher affinity for electron
than hydrogen.
NADH is strong reducing agent with negative redox
potential and oxygen a strong oxidant with
positive redox potential.

5. Substrate level phosphorylation
• Here energy from a high energy compound is
directly transferred to nucleoside diphosphate
to form a triphosphate without the help of
electron transport chain;
• a. Bisphosphoglycerate kinase
• b. Pyruvate kinase
• c. Succinate thiokinase

6. Biological oxidation
• The transfer of electrons from the reduced co-
enzymes 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.
• In the body, this oxidation is carried out by
successive steps of dehydrogenations.

7. ELECTRON TRANSPORT CHAIN
• The electron flow occurs through successive
dehydrogenase enzymes, together known as
electron transport chain (ETC).
• The electrons flow from electronegative
potential (-0.32) to electropositive potential (+
0.82).

8. Enzymes and Co-Enzymes
• All the enzymes involved in this process of
biological oxidation belong to the major class
of oxidoreductases. They can be classified into
the following 5 headings:

9. Oxidases
Aerobic dehydrogenases
Anaerobic dehydrogenases
Hydroperoxidases
oxygenases
Enzymes and Co-Enzymes

10. 1. Oxidases
These enzymes catalyze the removal of
hydrogen from substrates, but only oxygen can
act as acceptor of hydrogen, so that water is
formed.
• AH2 + ½ O2 A + H2O

11. 2. Aerobic dehydrogenases
• These enzymes catalyze the removal of
hydrogen from a substrate, but oxygen can act
as the acceptor. These enzymes are
flavoproteins and the product is usually
hydrogen peroxide.
• AH2 + O2 A + H2O2
• Contain either FMN or FAD.

12. 3. Anaerobic Dehydrogenases
• These enzymes catalyze the removal of
hydrogen from a substrate but oxygen cannot
act as the hydrogen acceptor.
• They therefore require co-enzymes as
acceptors of the hydrogen atoms. When the
substrate is oxidized, the coenzyme is
reduced.

13. a. NAD+ linked dehydrogenases:
• When the NAD+ accepts the two hydrogen
atoms, one of the hydrogen atoms is removed
from the substrate as such. The other hydrogen
atom is split into one hydrogen ion and one
electron.
• The electron is also accepted by the NAD+ so as
to neutralize the positive charge on the co-
enzyme molecule.
• The remaining hydrogen ion is released into the
surrounding medium
3. Anaerobic Dehydrogenases

14. • H2 H + H+ + e–
• AH2 + NAD+ → A + NADH + H+
The NAD+ linked dehydrogenases are:
– i. Glyceraldehyde-3-phosphate dehydrogenase
– ii. Isocitrate dehydrogenase
– iii. Malate dehydrogenase
– iv. Glutamate dehydrogenase
– v. Beta hydroxyacyl CoA dehydrogenase
– vi. Pyruvate dehydrogenase
– vii. Alpha ketoglutarate dehydrogenase
3. Anaerobic Dehydrogenases

15. b. NADP+ linked dehydrogenases
• NADPH cannot be oxidized with concomitant
production of energy.
• NADPH is used in reductive biosynthetic
reactions like fatty acid synthesis and
cholesterol synthesis.
3. Anaerobic Dehydrogenases

16. c. FAD linked dehydrogenases:
When FAD is the coenzyme, (unlike NAD+), both
the hydrogen atoms are attached to the flavin
ring. Examples:
– i. Succinate dehydrogenase
– ii. Fatty acyl CoA dehydrogenase
– iii. Glycerolphosphate dehydrogenase
3. Anaerobic Dehydrogenases

17. • d. Cytochromes:
• All the cytochromes, except cytochrome
oxidase, are anaerobic dehydrogenases.
• All cytochromes are hemoproteins having iron
atom. Cytochrome b, cytochrome c1, and
cytochrome c are in mitochondria while
cytochrome P-450 and cytochrome b5 are in
endoplasmic reticulum
3. Anaerobic Dehydrogenases

18. 4. Hydroperoxidases
• Peroxidase: Examples of peroxidases are
glutathione peroxidase in RBCs (a selenium
containing enzyme), leukocyte peroxidase and
horse radish peroxidase.
• Peroxidases remove free radicals like hydrogen
peroxide.
• H2O2 + AH2 (peroxidase) 2 H2O + A

19. 4. Hydroperoxidases
• Catalases:
• Catalases are hemoproteins. Peroxisomes are
subcellular organelles having both aerobic
dehydrogenases and catalase.
• 2 H2O2 (catalase) 2 H2O + O2

20. 5. Oxygenases
a. Mono-oxygenases
• These enzymes are also called hydroxylases
because OH group is incorporated into the
substrate.
• A-H+ O2+ BH2--(hydroxylase)→ A-OH+ H2O+ B

21. i. Phenylalanine hydroxylase
ii. Tyrosine hydroxylase
iii. Tryptophan hydroxylase
iv. Microsomal cytochrome P-450 mono-oxygenase is
concerned with drug metabolism.
v. Mitochondrial cytochrome P-450 mono-oxygenase.
v. Nitric oxide synthase
5. Oxygenases

22. 5. Oxygenases
b. Di-Oxygenases:
• They are enzymes which incorporate both
atoms of a molecule of oxygen into the
substrate, e.g. Tryptophan pyrrolase and
homogentisic acid oxidase
• A + O2 AO2

23. High energy compounds
• These compounds when hydrolyzed will
release a large quantity of energy, that is, they
have a large ΔG0’

25. Organisation of ETC
• In the Electron transport chain, or respiratory
chain, the electrons are transferred from
NADH to a chain of electron carriers. The
electrons flow from the more electronegative
components to the more electropositive
components.
• All the components of electron transport
chain (ETC) are located in the inner
membrane of mitochondria.

26. • There are four distinct multi-protein
complexes; these are named as complex-I, II,
III and IV.
• These are connected by two mobile carriers,
co-enzyme Q and cytochrome c.
Organisation of ETC

28. Complex I
• It is also called NADH-CoQ reductase or NADH
dehydrogenase complex.
• It contains a flavoprotein (Fp), consisting of
FMN as prosthetic group and an iron-sulfur
protein (Fe-S).

30. Complex II
• The electrons from FADH2 enter the ETC at
the level of coenzyme Q.

32. Complex III
• This is a cluster of iron-sulfur proteins,
cytochrome b and cytochrome c1, both
contain heme prosthetic group.

34. Complex IV
• It contains different proteins, including
cytochrome a and cytochrome a3. The
Complex IV is tightly bound to the
mitochondrial membrane.

37. P : O ratio
• The P:O ratio is defined as the number of
inorganic phosphate molecules incorporated
into ATP for every atom of oxygen consumed.
• When a pair of electrons from NADH reduces
an atom of oxygen (½ O2), 2.5 mol of ATP are
formed per 0.5 mol of O2 consumed.
• In other words, the P:O ratio of NADH
oxidation is 2.5;
• The P:O value of FADH2 is 1.5.

38. Site of ATP synthesis
• Traditionally, the sites of ATP synthesis are
marked, as site 1, 2 and 3, as shown in Figure.
• But now it is known that ATP synthesis
actually occurs when the proton gradient is
dissipated, and not when the protons are
pumped out

39. Chemiosmotic theory
• The coupling of oxidation with phosphorylation is
termed oxidative phosphorylation.
• The transport of protons from inside to outside of
inner mitochondrial membrane is accompanied
by the generation of a proton gradient across the
membrane.
• Protons (H+ ions) accumulate outside the
membrane, creating an electrochemical
potential difference

40. Chemiosmotic theory
• This proton motive force drives the synthesis
of ATP by ATP synthase complex

41. ATP Synthase (Complex V)

42. • It is a protein assembly in the inner
mitochondrial membrane.
• It is sometimes referred to as the 5th Complex
• Proton pumping ATP synthase (otherwise
called F1-Fo ATPase) is a multisubunit
transmembrane protein.
ATP Synthase (Complex V)

43. • It has two functional units,
named as F1 and Fo.
• It looks like a lollipop since
the membrane embedded
Fo component and F1 are
connected by a protein
stalk.
ATP Synthase (Complex V)

45. Mechanism of ATP synthesis
• Translocation of protons carried out by the Fo
catalyzes the formation of phospho-anhydride
bond of ATP by F1.
• The binding change mechanism proposed by
Paul Boyer (Nobel prize, 1997) explains the
synthesis of ATP by the proton gradient.

46. • Fo is the wheel; flow of protons is the
waterfall and the structural changes in F1 lead
to ATP coin being minted for each turn of the
wheel.
• The F1 has 3 conformation states for the
alpha-beta functional unit:
– O state—Does not bind substrate or products
– L state—Loose binding of substrate and products
– T state—Tight binding of substrate and products
Mechanism of ATP synthesis

48. • According to this theory, the three beta
subunits (catalytic sites), are in three
functional states:
• O form is open and has no affinity for
substrates.
• L form binds substrate with sluggish affinity.
• T form binds substrate tightly and catalyzes
ATP synthesis.
Mechanism of ATP synthesis

49. 1. ADP and Pi bind to L binding site
2. L to T conversion is by energy driven conformational
change that catalyzes the formation of ATP
3. T state reverts to O state when ATP is released
4. L state is regenerated for further ADP binding.
For the complete rotation of F1 head through the 3 states,
10 protons are translocated.
Mechanism of ATP synthesis

50. Mechanism of ATP synthesis

51. Regulation of ATP synthesis
• The availability of ADP regulates the process.
• When ATP level is low and ADP level is high,
oxidative
• phosphorylation proceeds at a rapid rate.
• This is called respiratory control or acceptor
control.
• The major source of NADH and FADH2 is the
citric acid cycle, the rate of which is regulated by
the energy charge of the cell.

52. Inhibitors of ATP Synthesis

53. Inhibitors of ATP Synthesis

54. Uncouplers of Oxidative
phosphorylation
• Uncouplers will allow oxidation to proceed,
but the energy instead of being trapped by
phosphorylation is dissipated as heat. This is
achieved by removal of the proton gradient

55. Uncouplers of Oxidative
phosphorylation
• Sometimes, the uncoupling of oxidative phosphorylation is
useful biologically. In hibernating animals and in newborn
human infants, the liberation of heat energy is required to
maintain body temperature.
• In Brown adipose tissue, thermogenesis is achieved by this
process.
• Thermogenin, a protein present in the inner
mitochondrialmembrane of adipocytes, provides an
alternate pathway for protons.
• It is one of the uncoupling proteins (UCP).
• Thyroxine is also known to act as a physiological
uncoupler.

56. Dr Anurag Yadav
MBBS, MD
Associate Professor
Department of Biochemistry
Instagram page –biochem365
Email: dranurag.y.m@gmail.com