The electron transport chain consists of series of complexes, which take part in the flow of electrons. At each step of electron passage there is a loss of energy. Complex I-IV each plays a role in transporting electrons and establishing the proton gradient. The complexes further consist of oxidoreductases enzymes that have its cofactors and prosthetic group and coenzyme. Prosthetic groups such as FMN and iron sulphur complexes act as a carrier of electrons.

Some simple facts about the electron transport chain are

1. 34 ATP are made from the products of 1 molecule of glucose.
2. The process is a stepwise movement of the electrons from high energy to low energy that makes the proton gradient.
3. The proton gradient powers do not power the flow of electrons.
4. Electron transport chain occurs only in the presence of oxygen.

There are two routes for movement of electron
i. One pathway involves complex I, complex II, complex IV and ultimately to oxygen.
ii. Second pathway involves complex II, complex III, complex IV and ultimately to oxygen.

Complex I

The complex I is a large, multisubunit complex with approx. 40 polypeptide chains. It passes electron from NADH to coenzyme Q. It contains one molecule of flavin mononucleotide (FMN) and six to seven proteins of iron-sulfur clusters which participate in the process of electron-transport. During the transport of each pair of the electron from NADH to coenzyme Q, the complex I pumps four protons across the inner mitochondrial membrane.

Iron-sulfur proteins consist of the non-heme iron complexed to sulfur. There are two very common types of iron-sulfur proteins: designated [2Fe-2S] and [4Fe-4S]. Both these iron sulfur centers consist of equal number of iron and sulfide ions and are both coordinated to four Cys sulfhydryl groups.

Coenzyme Q (CoQ, also known as ubiquinone) is a benzoquinone linked to a number of isoprene units. The name ubiquinone is for the ubiquitous nature of the quinine. Q refers to the quinine chemical group.There are three redox states of coenzyme Q –

  • fully oxidized (ubiquinone, Q)
  • semiquinone (ubisemiquinone)
  • fully reduced (ubiquinol, QH2).

Coenzyme Q is the only electron carrier in the electron transport chain that is not a protein-bound prosthetic group. It is a carrier of hydrogen atoms that is protons plus electrons.

Complex II

Succinate dehydrogenase, an inner mitochondrial membrane-bound enzyme, is an integral component of the complex II. It converts succinate to fumarate during Krebs cycle. The two electrons released in the conversion of succinate to fumarate are transferred first to FAD, then to an iron-sulfur center and finally to CoQ. Thus, CoQ draws electrons into the respiratory chain, not only from NADH, but also from FADH2. Complex II does not pump protons during transport of electrons across the inner mitochondrial membrane.

Complex III

Complex I or complex II donates two electrons to the complex III and regenerates oxidized CoQ. Concomitantly, it releases two protons picked up on the cytosolic face into the intermembrane space generating proton gradient. Within complex III, the released electrons are transferred to an iron-sulfur center and then to two b-type cytochromes (bL, L for low affinity and bH, H for high affinity) or cytochrome c1. Finally, the two electrons are transferred to two molecules of the oxidized form of cytochrome c. Two additional protons are translocated from the mitochondrial matrix across the inner mitochondrial membrane for each pair of electrons transferred. This transfer of protons involves the proton-motive Q cycle.


Cytochromes are heme proteins having distinctive visible-light spectra. The major respiratory cytochromes are classified as b, c or a depending on the wavelength of the spectral absorption peaks. Within each class, the cytochromes are distinguished by smaller spectral differences. In the respiratory electron carriers, there are two b-type cytochromes, cytochrome c and c1 and cytochromes a and a3. The heme prosthetic groups of a and b cytochromes are tightly, but not covalently, bound to their associated proteins; whereas heme group of c-type cytochromes are covalently attached through Cys residues. Cytochrome c is present in all aerobic organisms. The degree of sequence homology in cytochrome c among species has been used as a measure of the evolutionary distances that separate species.


The mechanism of the participation of ubiquinone in the electron transport process was proposed by Peter Mitchell and termed as a proton motive Q-cycle. Ubiquinones are hydrophobic and uncharged and hence can migrate along the hydrophobic core of the membrane. Transfer of one Ubiquinol takes place to the QP binding site adjacent to the iron-sulfur protein at the P face of the mitochondrial membrane. One electron is transferred to Fe-S protein and the second electron is transferred to the heme bL and two protons are released to the P face. The Fe-S protein transfers the electron along the chain to Cyt c1 and cytochrome oxidase. The electron moves from heme bL to heme bH. Ubiquinone then binds to bH at the Qn site and electron from the reduced bH forms ubisemiquinone at this site. Now, a second ubiquinol molecule is oxidized at the QP site, the process follows as described above and the second electron formed completes the reduction of ubisemiquinone to ubiquinol. Two protons are taken from the matrix for this purpose and released to the P face. The ubiquinol then goes back to the pool and the Q-cycle is completed.

Complex IV

Complex IV or cytochrome c oxidase catalyzes the transfer of electrons from the reduced form of cytochrome c to molecular oxygen. It consists of 13 subunits and contains two heme groups and three copper ions, arranged as two copper centers. The two heme groups termed heme a and heme a3, have distinct properties because they are located in different environments within cytochrome c oxidase.

The two copper centers are designated as a and b.

  • Cua, contains two copper ions linked by two bridging cysteine residues.
  • Cub, is coordinated by three histidine residues.

Cytochrome c transports electrons, one at a time, to the complex IV. Within this complex, electrons are transferred, first to a Cua center, then to Cyt a, next to Cub center and Cyt a3 and finally to O2, the ultimate electron acceptor, yielding H2O. Together, heme a3 and Cub form the active center at which O2 is reduced to H2O.

Cyt C → Cua → Cyt a → Cub.Cyt a3 → O2

Two electrons, sequentially released from two molecules of reduced cytochrome c together with two protons from the matrix, combine with one oxygen atom to form one water molecule. Additionally, for each electron transferred from cytochrome c to oxygen, one proton is transported from the matrix to the intermembrane space, or a total of four electrons are transferred for each O2 molecules reduced to two H2O molecules.

Inhibitors of electron transport chain

  • Cyanide and carbon monoxide- Inhibitor of cytochrome oxidase.
  • Antimycin A- prevent electron transfer from Cyt b to Cyt c1.
  • Myxothiazol, Rotenone, Amytal- Prevent electron transfer from Fe-S center to ubiquinone.
  • Piericidin A and DCMU- Competes with Qs for building site in PS II.

Purpose of electron transport chain

  • Electrons move through electron transport chain carriers are arranged in increasing order of their oxidation-reduction potential. This involves a large number of redox reactions, which are exergonic and release energy.
  • This energy is used for pumping protons to inter membranous space. The backflow of protons through oxysome are coupled by synthesis of ATP.
  • Electron transport chain serves to oxidize the reduced coenzyme.
  • The electrons ultimately combine with O2 and protons and formation of metabolic H2O take place.