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Section C

Electron transport chain and oxidative phosphorylation

Electron transport chain :- In aerobic respiration, glucose is oxidized, and its electrons are passed to the electron transport chain in the inner mitochondrial membrane.The enzymes necessary for the transfer of electrons are present in the inner mitochondrial membrane in an ordered manner and also function in a specific sequence.The ten NADH that enter the electron transport originate from each of the earlier processes of respiration: two from glycolysis, two from the transformation of pyruvate into acetyl-CoA, and six from the citric acid cycle. The two FADH2 originate in the citric acid cycle. Electron flow through the mitochondrial electron transport chain which includes Complexes I, II, III, and IV.The electron transport chain also involve NADH and FADH, which act as electron transporters as they flow through the inner membrane space.

  • Complex I, (NADH:CoQ oxidoreductase,) is composed of NADH dehydrogenase with FMN as cofactor, plus non-heme-iron proteins having at least 1 iron sulfur center.In complex I, electrons are passed from NADH to the electron transport chain, where they flow through the remaining complexes.Complex I is the first coupling site in the mitochondrial membrane meaning that the redox reaction is coupled to a proton pumping activity across the membrane. It is the energy stored in the electrochemical proton gradient that is used for ATP synthesis by the H+-ATPase . NADH is oxidized to NAD in this process.
  • Complex II ( succinate dehydrogenase or succinate:CoQ oxidoreductase) Complex II oxidizes FADH, garnering still more electrons for the chain.
  • The difference in free energy, of electron flow through Complexes I and II, accounts for the fact that a pair of electrons originating from NADH and passing to oxygen supports production of 3 equivalents of ATP, while 2 electrons from succinate support the production of only 2 equivalents of ATP.
  • Complex III (Cytochrome-c reductase) which mainly composed of the heme proteins known as cytochromes b and c1 and a non-heme-iron protein, known as the Rieske iron sulfur protein.Reduced CoQ (CoQH2) diffuses in the lipid phase of the membrane and donates its electrons to Complex III.At complex III, no additional electrons enter the chain, but electrons from complexes I and II flow through it.

  • The heme iron of all cytochromes participates in the cyclic redox reactions of electron transport, alternating between the oxidized (Fe+3) and reduced (Fe+2) forms.
  • Complex IV ( cytochrome oxidase) contains cytochrome a and cytochrome a3, and is the smallest of the cytochromes.When electrons arrive at complex IV, they are transferred to a molecule of oxygen.Since oxygen is the final electron acceptor,and it gains electrons, it is reduced to water.

  • H+-ATPase (also known as complex V) The mitochondrial ATP synthase is a multi-subunit protein complex that couples a proton channel (F0 portion, integral membrane protein complex) with an ATP synthesizing unit (F1 portion, soluble mitochondrial matrix component) and is a member of the so called F-type ATPases.
  • Energy released by the "downhill" passage of electrons is captured as ATP by ADP molecules. The ADP is reduced by the gain of electrons.

Oxidative phosphorylation is the process by which NADH and FADH2 are oxidized and ATP is formed.

The energy released by the electrons as they pass through the electron transport chain is used to actively transport (pump) hydrogen ions across a membrane—across the inner mitochondrial membrane into the intermembrane space in mitochondria. As electrons are pulled from NADH and FADH2, protons (H+) also are released, and the protein complexes pump them into the intermembrane space.Inner mitochondrial membrane is impermeable to hydrogen ions(protons), they accumulate there, and thus both a proton gradient(H+ gradient)and an electrochemical gradient are established between the inner membrane space and the matrix. This diffusion is called chemiosmosis or proton motive force (PMF). This potential is the sum of the concentration difference of protons across the membrane and the difference in electrical charge across the membrane. The 2 electrons from NADH generate a 6-proton gradient.In aerobic respiration , the transport protein is an enzyme called ATP synthase with inner channels through which hydrogen ions (protons) diffuses. As the protons move down the gradient, their energy binds a phosphate group to ADP, an oxidative phosphorylation, making ATP.

NADH enters the ETS chain at the beginning, yielding 3 ATP per NADH. FADH2 enters at Co-Q, producing only 2 ATP per FADH2.

Net production of ATP per glucose molecule:- Glycolysis produces two ATPs. It does not produce any CO2 or H2O. Formation of acetyl-CoA produces two molecules of CO2. The citric acid cycle produces two ATPs and the remaining four molecules of CO2. Electron transport and chemiosmosis produce the 12 molecules of H2O and 32 to 34 ATPs. Altogether, a maximum of 36 to 38 ATPs are produced per glucose molecule.