This is the final common pathway in aerobic cells by which electrons derived from various substrates are transferred to oxygen. Different substrates may use this pathway because they are oxidized by enzymes that use NAD+ or FAD as electron acceptor cofactors. The reduced NADH and FADH2 then donate electrons to the electron transport chain. The enzymes of the electron transport chain are embedded in the inner mitochondrial membrane in association with the enzymes of oxidative phosphorylation.
NADH – derived from NAD+-linked dehydrogenases, including;
- Isocitrate, α-ketoglutarate, and malate dehydrogenase of TCA cycle
- Pyruvate dehydrogenase
- L-3-Hydroxylacy coenzyme A (CoA) dehydrogenase of fatty acid oxidation
- Miscellaneous NAD+-linked dehydrogenases
FADH2 – derived from FAD-linked dehydrogenases including;
- Succinate dehydrogenase of TCA cycle
- FAD-linked dehydrogenase of glyceraldehyde-3-phosphate shuttle
- Acyl CoA dehydrogenase of fatty acid oxidation
- Miscellaneous FAD-linked dehydrogenases
Mitochondrial membrane is impermeable to NADH but NADH equivalents generated in
glycolysis need to be transported from cytoplasm to mitochondria for oxidation. This is achieved by malate-aspartate shuttle which operates mainly in the liver, kidney and heart. The cycle is operated with the help of enzymes malate dehydrogenase (MDH) and aspartate aminotransferase.
MALATE ASPARTATE SHUTTLE
FADH2 molecules are also transported into the mitochondrial matrix through and All the components of ETC. are located in the inner mitochondrial membrane. They are four multi-protein complexes named as complex-I, II, III & IV. The complexes are connected by two mobile carriers, coenzyme Q and cytochrome c.
Complex I
It is the point of entry of electrons from NADH into the electron transport chain. This enzyme complex is called NADH-coenzyme Q reductase or NADH dehydrogenase.
Prosthetic groups
- Flavin mononucleotide (FMN)
- Iron-sulfur (Fe-S) centres
Electron acceptor
- Coenzyme Q also called ubiquinone or simple Q
Path of electron transfer
- NADH FMN Fe-S Q
Energy released
- 12 kcal/mol – utilized to drive 4 protons out of the mitochondria
Inhibitors
- Rotenone, an insecticide
- Barbiturates, such as amobarbital and secobarbital
- Piericidine A, an antibiotic
Complex II
It is the point of entry of electrons from succinate into the electron transport chain.
This enzyme complex is called succinate-coenzyme Q reductase and includes succinate dehydrogenase, which is the same enzyme that participates in the citric acid cycle.
Prosthetic groups
- FAD
- Fe-S centres
Electron acceptor
- Coenzyme Q
Path of electron transfer
- Succinate FAD FeS Q
This step does not liberate enough energy to act as a proton pump i.e. no protons are generated.
Inhibitors
- Carboxin
Complex III
Accepts electrons from both complex I (NADH) and complex II (FADH2) and donates
electrons to complex III.
The ubiquinone (Q) is reduced successively to semi-quinone (QH) and then finally to quinol (QH2).
It is the electron acceptor for coenzyme Q.
This enzyme complex is called coenzyme Q-cytochrome c reductase or just cytochrome reductase.
Prosthetic group
- Heme (cytochrome b and cytochrome c1)
Electron acceptor
- Cytochrome c
Path of electron transfer
- Q Fe-S cytochrome b cytochrome c1 cytochrome c
Energy released
- 10 kcal/mol – 4 protons are pumped out
Inhibitors
- Antimycin A, an antibiotic
Cytochrome c
It mediates the transfer of electrons from complex III to complex IV.
Prosthetic group – heme
It is the electron acceptor for cytochrome c.
Complex IV
This enzyme complex is called cytochrome c oxidase or just cytochrome oxidase.
Prosthetic group
- Copper
- Heme (cytochrome a and cytochrome a3)
Electron acceptor
- Molecular oxygen (O2) producing water
Path of electron transfer
- Cytochrome c cytochrome a+a3 O2
Energy released
- 2 protons are pumped out to the intermembrane space
Inhibitors
- Carbon monoxide (CO) – competes with O2 for its binding site on cytochrome oxidase
- Hydrogen sulfide (H2S)
- Azide (an N3-containing compound)
- Cyanide (CN-)
COMPONENT AND SEQUENCE OF REACTIONS OF ETC
Oxidative Phosphorylation
This is the process whereby free energy that is released when electrons are transferred along the electron transport chain is coupled to the formation of ATP from ADP and inorganic phosphate (Pi).
It is the main source of energy in aerobic cells. The energy released by each electron pair passing through the respiratory chain is coupled to the formation of ATP.
The energy released by the electron transfers catalysed by three coupling sites – complex I, complex III and complex IV – is sufficient for each to support the formation of approximately 1 mole of ATP.
Electrons that enter the chain from NADH support the synthesis of approximately 3 moles of ATP.
Electrons that enter the chain from FADH2 bypass complex I and support the synthesis of approximately 2 moles of ATP.
An electrochemical gradient of protons (H+) across the mitochondrial inner membrane
serves to couple the energy flow of electron transport to the formation of ATP.
The electron carriers act as pumps, which cause vectorial (directional) pumping of H+ across the membrane. Because H+ is a charged particle, the flow of free energy across the inner membrane is due to the combination of a concentration gradient and a charge gradient. In the electron transport chain, H+ is separated from the electron: As electrons move down the chain, H+ is transferred from the mitochondrial matrix to the intermembrane space. The protons in the intermembrane space pass through the inner membrane and back into the matrix via the ATP synthase. The dissipation of energy that occurs as protons pass down the concentration gradient to the matrix drives the phosphorylation of ADP to ATP by the synthase.
SUMMARY OF ATP SYNTHESIS
It is the enzyme complex that synthesized ATP and is also known as H+-ATPase or F0F1-ATPase.
Composition:
ATP synthase is composed of two units:
- F0 spans the membrane and is composed of four subunits. It forms a channel or path through which protons cross the membrane.
- F1 is tightly bound to F0 and sits on the matrix side of the mitochondrial membrane. The F1 unit is composed of five subunits. It contains the catalytic site for ATP synthesis.
Inhibitors of ATP synthase include;
- Oligomycin, an antibiotic
- Dicyclohexylcarbodiimide (DCCD)
Other enzymes that can couple ATP synthesis to the transport of ions other than hydrogen down a concentration gradient are found in other parts of the cell:
- The Ca2+-ATPase of the sarcoplasmic reticulum
- The Na+K+-ATPase of the plasma membrane
ADP must be transported into the mitochondrial matrix to be used for ATP synthesis, and ATP produced in the mitochondria must be
F1 is tightly bound to F0 and sits on the matrix side of the mitochondrial membrane.
The F1 unit is composed of five subunits. It contains the catalytic site for ATP synthesis. Inhibitors of ATP synthase include;
- Oligomycin, an antibiotic
- Dicyclohexylcarbodiimide (DCCD)
Other enzymes that can couple ATP synthesis to the transport of ions other than hydrogen down a concentration gradient are found in other parts of the cell:
- The Ca2+-ATPase of the sarcoplasmic reticulum
- The Na+K+-ATPase of the plasma membrane
ADP must be transported into the mitochondrial matrix to be used for ATP synthesis, and ATP produced in the mitochondria must be transported out for use by the cell. A membrane-bound transporter system catalyzes the exchange of ADP and ATP across the membrane. Inhibitors of the ADP/ATP transporter include:
- Atractyloside
- Bongkrekic acid
These are compounds that allow normal function of the electron transport chain without the production of ATP. Uncouplers cause leakage or transport of H+ across the membrane that collapses the proton gradient before it can be used for ATP synthesis. Energy still may be released as the electrons are transferred down the transport chain; however, this energy is not trapped as ATP but appears instead as heat. Oxidative phosphorylation uncouplers include;
- 2,4-Dinitrophenol, which was once used as a weight-loss drug but was discontinued because of its toxicity
- Dicumarol, which is an anticoagulant
- Chlorocarbonylcyanide phenylhydrazone (CCCP), which is a compound that carries protons across the membrane
- Bilirubin, which is a metabolite of heme degradation but is not normally present in mitochondria in concentrations high enough to affect normal function
The mitochondria of brown fats contain thermogenin, which also uncouples oxidative phosphorylation leading to the release of the energy as heat.
