electron transport chain, oxidative phosphorylation, atp synthesis

1. Aerobic Respiration
Rivera Gio Treb

2. HISTORY
1948
Eugene Kennedy and Albert Lehninger
discovered that mitochondria are the site
of oxidative phosphorylation in eukaryotes
marked the beginning of the modern
phase of studies in biological energy
transductions.
Transduction is the conveyance of energy
from one electron (a donor) to another (a
receptor)

3. 2 facets of Aerobic Respiration
1. Electron Transport Chain or Respiratory Chain
2. Oxidative Phosphorylation

4. Mitochondria

6. I. NADH dehydrogenase
II. Succinate dehydrogenase
III. Q-cytochrome c oxidoreductase
IV. Cytochrome oxidase
Electron Transport Chain Enzyme Systems

7. Electron Transport Chain
Complexes I and II catalyze electron transfer to ubiquinone(Q) from
two different electron donors: NADH (Complex I) and succinate
(FADH2 From Complex II).
Complex III carries electrons from Ubiquinol (QH2)to cytochrome c, and
Complex IV completes the sequence by transferring electrons from
cytochrome c to O2.

9. COMPLEX I: NADH DEHYDROGENASE
NADH OXIDOREDUCTASE
• L shaped large enzyme composed of 42 different polypeptide chains
• Complex I catalyzes two simultaneous and obligately coupled
processes: (1) the exergonic transfer to ubiquinone of a hydride ion
from NADH and a proton from the matrix, expressed by
• (2) the endergonic transfer of four protons from the matrix to the
intermembrane space.
• NADH to Ubiquinone

10. • Complex I is therefore a proton pump driven by the energy
of electron transfer, and the reaction it catalyzes is vectorial
• it moves protons in a specific direction from one location
(the matrix, which becomes negatively charged with the
departure of protons) to another (the intermembrane space,
which becomes positively charged).

12. FMNH2
• Isoalloxazine ring as
prosthetic groups

14. Coenzymes Q Ubiquinone
• flows to the inner membrane
• Carry 2 electrons through different
complexes
• It would not associate with complex
2 but it would associate with
complex 3.

16. The detailed mechanism that couples electron and proton
transfer in Complex I is not yet known, but probably involves
a Q cycle similar to that in Complex Ill in which QH2
participates twice per electron pair,

18. COMPLEX II: Succinate to Ubiquinone
Succinate dehydrogenase
• NOT A PROTEIN PUMP
• Succinate dehydrogenase, the only
membrane-bound enzyme in the
citric acid cycle
• it contains five prosthetic groups of
two types and four different protein
subunits

19. • A

20. ALSO KNOWN AS “SUCCINATE REDUCTASE”

21. • As this reaction releases less energy than the oxidation of
NADH, complex II does not transport protons across the
membrane and does not contribute to the proton gradient.

22. COMPLEX III:Ubiquinone to Cytochrome c
Complex III also called
• Cytochrome bc1 complex
• Q-cytochrome c oxidoreductase
• Cytochrome reductase

23. • Complex III couples the transfer of electrons from
ubiquinol (QH2) to cytochrome c with the vectorial
transport of protons from the matrix to the
intermembrane space.

24. COMPLEX III CONTAINS
1. Cytochrome C1 (contains one heme group)
2. Cytochrome B (contains two heme group)
3. Rieske Center (2Fe-2S group)
Cytochrome – proteins that contain heme group and it transfers
electron

25. X-ray crystallography (1995 and 1998)

26. Q cycle Overview
• The process by which electron travel from the QH2 to cytochrome C is
known as the Q cycle
• In the Q-cycle, electrons are passed from ubiquinol (QH2) to
cytochrome c using Complex III as an intermediary docking station for
the transfer. Two pair of electrons enter from QH2 and one pair is
returned to another CoQ to re-make QH2. The other pair is donated
singly to two different cytochrome c molecules.
• Cytochrome c detaches and travels in the fluid to attach to Complex IV

27. Q cycle STEP 1
This cycle begins when 1st QH2 binds to
complex III. Upon binding, the two electrons
follow different pathways.
• 1 e- moves unto the 2F-2S group of Rieske
Center and then is transferred to the heme
group of cytochrome C1. it is then picked up
by Cytochrome C, which diffuses away and
travels to Complex IV

28. Q cycle STEP 1
• The 2nd e- moves unto the heme group of
cytochrome b before being picked up by Q.
It is partially reduced to semiquinone
radical (QH)
• QH2: 2 e-
• Cyt C: 1 e-
• Semiquinone radical: 1 e-

30. Q cycle STEP 2
The 2nd QH2 transfers a second pair of electron
through the same pathways as before, except
now a ubiquinol is generated at the end
Conclusion
In 1 Q cycle:
Two QH2 are oxidized into Q, releasing 4H+
1 Q is reduced into QH2 (recycled)
Two cytochrome c molecules are reduced

32. COMPLEX IV:Cytochrome c to O2
Cytochrome oxidase
• In the final step of the respiratory chain, Complex IV, also
called cytochrome oxidase, carries electrons from
cytochrome c to molecular oxygen, reducing it to H2O.
• Complex IV contains:
1. Two heme groups ( heme a and Heme a3)
2. Three copper atoms (Cua/Cua and CuB)

33. 1. 2 reduced cytochrome C
gives off 2electrons. One
electron stops at CuB and
Heme A3.
2. Once Heme A3 and CuB are
in their reduced form, an O2
molecule can bind an
abstract 2 electrons to form
a peroxide bridge.
3. 2 more reduced cytochrome
C are oxidized to transfer 2
electrons. Two H+ ions are
obtained from matrix to
break peroxide bridge.

34. 4. The abstraction of two
more protons from the
matrix oxides Heme a3 and
CuB into their original state
and releasing 2 H2O
In the process, Complex IV
also pumps H+ into the
intermembrane space.

36. SUMMARY
Electrons reach Q through Complexes I and II. The reduced Q (QH2)
serves as a mobile carrier of electrons and protons. It passes electrons
to Complex Ill, which passes them to another mobile connecting link,
cytochrome c. Complex IV then transfers electrons from reduced
cytochrome c to O2.

37. ATP SYNTHESIS:
Chemiosmotic model
According to the model, the
electrochemical energy inherent in the
difference in proton concentration and the
separation of charge across the inner
mitochondrial membrane
(-the proton-motive force-)
drives the synthesis of ATP as protons flow
passively back into the matrix through a
proton pore associated with ATP synthase.

39. The inner mitochondrial membrane is impermeable to protons;
protons can reenter the matrix only through proton-specific channels
(F0). The proton-motive force that drives protons back into the matrix
provides the energy for ATP synthesis, catalyzed by the F1 complex
associated with F0.

40. Generation of ATP
• ATP is generated by the phosphorylation of ADP

41. ATP Synthase Complex
• The ATP synthase has two distinct subunits:
F0 subunit, which contains a protein channel for the flow of protons.
F1 subunit, which protrudes into the matrix space and catalyzes the
synthesis of ATP from ADP and inorganic phosphate
It is embedded in the inner membrane, together with the respiratory
chain complexes .

42. ATP Synthase Complex
• The flow of protons through F0 causes it to rotate, driving the
production of ATP in the F1 complex.
• A portion of the F1 subunit termed “the stalk” links the two subunits.
• As protons flow through the channel in the F0 subunit:
they cause the embedded stalk to rotate in the stationary
F1 subunit, thereby converting the energy of the electrochemical
gradient into mechanical energy.

44. Energy released from H+
movements fuels phosphorylation
ADP + HPO4
2+ ATP + H2O

45. ATP YIELD FROM OXIDATIVE PHOSPHORILATION
• NADH 2.5 ATPs
• FADH2 1.5 ATPs

46. ATP per acetyl CoA
3 NADH x 2.5 ATP = 7.5 ATP
1 FADH x 1.5 ATP = 1.5 ATP
GTP = 1 ATP_
10 ATP

47. Reference
• Lehninger’s principle of biochemistry 5th edition
• Janice gorzynski smith general organic and biochemistry 4th edition