The final stages of aerobic respiration are the electron transport chain and oxidative phosphorylation, which take place on the inner mitochondrial membrane in eurkayotes. This is when the hydrogen carriers reduced during glycolysis, the pyruvate dehydrogenase reaction, the tricarboxylic acid (TCA) cycle, and fatty acid oxidation are re-oxidised. The free energy released as their electrons are passed along a chain of carriers is used to generate a proton gradient across the inner mitochondrial membrane. This electrochemical gradient ultimately allows the formation of ATP, the universal energy currency of all cells, which can be hydrolysed to provide energy for processes within the cell.

Overview

Electrons are transferred from NADH along the electron transport chain to oxygen. This is a redox reaction as molecular oxygen is reduced to water and NADH is oxidised to NAD+. The electron transport chain consists of four membrane-bound complexes consisting of redox cofactors such as flavoproteins, cytochromes, iron-sulphur proteins, ubiquinone, and protein-bound copper:

  • NADH-Q reductase (Complex I)
  • Succinate-Q reductase (Complex II)
  • Cytochrome bc1 (Complex III)
  • Cytochrome c oxidase (Complex IV)

 

These complexes are linked by two small electron carriers; ubiquinone (also called coenzyme Q) and cytochrome c.

The electron transport chain



NADH oxidation by Complex I

NADH first binds to NADH-Q reductase (Complex I) and is re-oxidised, leaving NAD+. Two electrons and two protons (H+) are removed from NADH and transferred to the flavin mononucleotide (FMN) prosthetic group of NAD-Q reductase to produce FMNH2. Electrons and protons are then passed to iron-sulphur (FeS) clusters in FeS proteins (or non-haem iron proteins) within the complex, which change from Fe3+ (ferric) state to Fe2+ (ferrous) state when accepting an electron. 

Complex I to Complex III

Electrons and protons are transferred from NADH-Q reductase to ubiquinone to form ubiquinol. Ubiquinone / ubiquinol is small and hydrophobic and may therefore diffuse freely through the inner mitochrondrial lipid bilayer, passing electrons and protons between the less mobile protein complexes. Electrons are then transferred to the next protein complex, cytochrome bc1 (Complex III), which contains cytochrome b, cytochrome c1, and a Rieske FeS protein. Cytochrome proteins contain a haem group that can bind an electron. The complex acts as a dimer, with the globular head of the Rieske protein moving between cytochrome b of one monomer and cytochrome c1 of the other. Since cytochromes can only accept one electron, the two electrons from ubiquinol are transferred sequentially via the Q-cycle. The transfer of the second electron is very favourable as the semiquinone formed after the transfer of the first electron from ubiquinol is very unstable.

Complex III to Complex IV

Cytochrome c is a peripheral membrane protein, loosely bound to the inner mitochondrial membrane. It first binds to cytochrome bc1 (Complex III) and accepts an electron from cytochrome c1, and transfers this electron to the next complex in the chain, cytochrome c oxidase (Complex IV). The copper A (CuA) centre of cytochrome c oxidase receives electrons one at a time from cytochrome c, which are then passed on to haem a and then haem a3. Oxygen binds to haem a3 and is reduced to water by the transfer of electrons and protons.

Electrons move spontaneously

Electrons are passed spontaneously along the electron transport chain in one direction due to the increasing redox potential from NADH (most negative) to oxygen (most positive). However, many of the electron carriers in the chain utilise iron. Small distortions in the iron orbitals by the functional groups of the protein surrounding the iron atom give each subsequent carrier in the chain a higher redox potential. 

Generation of a proton electrochemical gradient

There is a large change in redox potential at each of the three complexes; NADH dehydrogenase, cytochrome bc1, and cytochrome c oxidase, which corresponds to a large change in free energy. This energy can be utilised to pump protons across the inner membrane from the mitochondrial matrix to the inter-membrane space, creating a proton electrochemical gradient. NADH-Q reductase and cytochrome bc1 each pump four protons, whereas cytochrome c oxidase pumps only two. The protons then flow down their electrochemical gradient back into the mitochrondrial matrix via an ATP synthase enzyme complex, which generates ATP.

FADH2 oxidation by Complex II

FADH2 is generated during the oxidation of succinate to fumarate by succinate dehydrogenase in the TCA cycle. Electron transport from FADH2 does not involve the NADH-Q reductase complex. Instead, electrons are transferred to the FeS clusters of the succinate dehydrogenase enzyme, which also functions as part of succinate-Q reductase (Complex II) of the electron transport chain. Electrons and protons are then passed on to ubiquinone and the rest of the complexes in the chain. The succinate-Q reductase complex does not act as a proton pump as the free energy change is insufficient to actively transport protons across the membrane. Oxidation of FADH2 therefore results in fewer protons being pumped across the inner mitochrondrial membrane than NADH, and produces less ATP.

Inhibitors of the electron transport chain

The electron transport chain can be inhibited at various stages:

  • Rotenone and amytal inhibit NADH-Q reductase
  • Antimycin A inhibits cytochrome bc1
  • Cyanide, azide, and carbon monoxide inhibit cytochrome c oxidase

Oxidative phosphorylation

The chemiosmotic hypothesis suggests that the energy released during the electron transport is used to create a proton electrochemical gradient across the inner mitochondrial membrane, which can be used to generate ATP. Protons flow down their electrochemical gradient into the mitochondrial matrix through the F1F0 ATP synthase complex located in the inner mitochondrial membrane. The F0 subunit forms a proton channel in the inner mitochondrial membrane, whilst the F1 subunit is located in the mitochondrial matrix and is responsible for ATP production. The F1 subunit is formed from five polypeptides in the arrangement (αβ)3 γδε. A ring of alternating α- and β-subunits surrounds the γ-chain. The γ- and ε-subunits form a central stalk that contacts the F0 subunit.

F1F0 ATP synthase

The F0 subunit is composed of a ring of 10-14 c-subunits along with an a-subunit and two b-subunits that contact the δ-chain of the F1 subunit. Protons flow through the F0 subunit via a channel formed between the c-ring and the a-chain. This causes the c-ring to rotate against the a-subunit stator, in turn rotating the γ-stalk. The (αβ)3 ring is held stationary by the two b-subunits and the δ-chain. The rotation of the γ-chain following proton movement is thought to induce conformational changes within the three β-subunits that allow ATP synthesis. This 'binding change mechanism' postulates that the T (tight) site binds ATP and undergoes conformational change to an O (open) site when it is released. The O site then becomes a L (loose) site when ADP and inorganic phosphate (Pi) are bound. ADP and Pi are converted into ATP when the L site becomes a T site. There is no three-fold symmetry within the γ-subunit, therefore it makes differential contacts with each of the β-subunits, which may drive these conformational changes.

Summary

  • Electrons are passed from NADH and FADH2 along the electron transport chain to oxygen, generating water.
  • Large differences in redox potential at three complexes in the chain correspond to a large change in free energy that is used to pump protons into the intermembrane space.
  • Protons flow back down their electrochemical gradient into the mitochondrial matrix through the F1F0 ATP synthase complex, which generates ATP.

References

Bioenergetics 3, Nicholls & Ferguson

Lehninger Principles of Biochemistry, Nelson & Cox

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