The Respiratory electron transport system is the chain of events that leads to ATP synthesis in the mitochondria and oxygen reduction to water. It’s found in every aerobic life on the planet. Reduced cofactors created during glycolysis, oxidative decarboxylation of pyruvate, and the Krebs cycle contribute electrons to electron receiving centers arrayed in sequence in mitochondria’s inner membrane.
The neighboring protein complex oxidizes reduced cofactors (NADH & FADH 2).
(I). The electron transfer continues until it reaches the last receiver, O2, when it is reduced to H2O. Electron transport from NADH and FADH2 to O2 occurs via a sequence of electron (e) carriers arrayed along the inner membrane of mitochondria in increasing order of their reduction potentials (E0′).
Five protein complexes are among the electron carriers. Flavin mononucleotide (FMN-complex I), flavin adenine dinucleotide (FADcomplex II), ubiquinone/CoQ, and cytochromes are among these substances (complex III & IV).
Proton (H+) transfer from the matrix to the intermembranous region of mitochondria is accompanied by electron transport. In the ensuing steps in complex III and IV containing cytochromes, the reduced coenzymes FMNH2 and FADH2 transport one electron at a time. Cytochromes are iron-containing proteins in which Fe exists in two forms: reduced (Fe2+) and oxidized (Fe3+) in a continuous cycle.
Protein complexes are classified as follows:
1. Complex NADH: CoenzymeQ Oxidoreductase
2. Succinate: CoenzymeQ Oxidoreductase
3. Coenzyme Q: Cytochrome c Oxidoreductase/ Cytochrome b cI complex
4. Cytochrome c Oxidase
5.ATP Synthase / F1 F O –ATP ase
The reduction potentials (E0′) of two complexes govern the transfer of electrons from one to the other. In an increasing direction, electrons flow from complexes with lower reduction potential (E0′) to those with higher E0′. Because NADH created in earlier processes has E0′ -0.315 V and O2 has E0′ 0.815, electron transfer from NADH to O2 occurs through several protein complexes. This is linked to energy release, which is determined by the difference in Eo’ between the two redox groups. 1.130 V is the difference in Eo’ between NADH and O2 (E 0′ acceptor 0′ donor). The difference in free energy is large enough to generate three ATP molecules, which are conserved in the system. The transfer of electrons through protein complexes in mitochondria’s inner membrane is linked to the transfer of H+ from the mitochondrial matrix to the intermembranous region (IMS). IMS is located between the mitochondria’s inner and outer membranes.
The following steps can be used to deduce the sequence of electron transport:
Complex I (NADH: CoQ Oxidoreductase) is the largest protein in the ETS system, receiving two electrons (e) from each NADH molecule and transferring them to Flavin mononucleotide (FMN). FMN has two Fe-2S and four Fe-4S clusters that help in electron transport. After receiving electrons, FMN is decreased. As a result of the electrons transiting to CoQ, FMN is oxidized while CoQ is reduced to CoQH 2. Along with the electrons, protons are also translocated from the matrix to IMS.
Complex II (CoQ Oxidoreductase: Succinate) – Succinate dehydrogenase is another name for this complex. To enhance electron transmission, this compound contains FAD with three Fe-S centers. The reduction of FAD to FADH 2 occurs during the electron transport from succinate to CoQ. The electrons are then transferred to CoQ, and two-one electrons are transferred from FADH 2 to a series of three Fe-S clusters. CoQ is reduced to CoQH2 as a result of this. Succinate is oxidized to Fumarate in this sequence.
Complex III (CoQ: Cytochrome c oxidoreductase) is made up of two Fe-2S centers, Cytochrome b, and Cytochrome c1. Cyt b comes in two varieties: Cyt bh and Cyt b l. This compound allows one molecule of CoQH2, a two-electron carrier, to decrease two molecules of Cytochrome c, each of which can only accommodate one electron at a time. The amount of cytochrome c in the body decreases.
CoQH 2+ Cyt c 1(Fe3+) —–CoQ- + Cyt c 1+ (Fe2+) +2 H + cycle 1 —Cytosolic CoQH2+ CoQ- + Cytc1 (Fe3 ) +2H + (mitochondrial)—–CoQ+CoQH2+Cytc1 (Fe2+) + H+ — Cycle2 (Cytosolic)
Cytochrome c is a peripheral membrane protein that moves between complexes III and IV. It moves one electron from Cyt c 1 (complex III) to Cu A at a time (complex IV).
The last component related to ETS is Complex IV (Cytochrome c Oxidase). Cytochromes a and a3 are present, as well as the reduction factors Cu A and Cu B. This is where electrons from Cytochrome c are received (a mobile protein ). Through Cyt a,a3, Cu A, and Cu B’, electrons flow. The electrons are transferred to O2, which is then reduced to water. Protons are translocated from reduced cofactors to the intermembranous region during this electron transfer.
ATP synthase/F1-F0 Complex V ATPase – This complex is a 180 A0 multisubunit protein with two main units, F0 (50A0) and F1 (50A0) (50 A0 ). The former F0 is an eight-subunit water-insoluble transmembrane proton channel, whereas the latter F1 is a water-soluble peripheral protein made up of five different types of protein subunits. This subunit is where ATP is produced. The F1 faces the matrix in the electron micrograph, but Fo is embedded in the inner membrane. F1 and F0 are linked together by a stalk (50 A0). The structure of the F1 protein subunits changes. They take in ADP and Pi, bind them together, and then release ATP.
Proton motive force and chemiosmotic hypothesis
Peter Mitchell (1961) proposed the chemiosmotic theory to describe the ATP generation mechanism caused by the proton motive force (pmf) that develops between the intermembranous space and the matrix of mitochondria. Because protons/H+ are transported from the matrix to the IMS along with electrons, the concentration of H+ in the IMS is higher than in the matrix, resulting in an electrochemical H+ concentration gradient between the matrix and the intermembranous space. A force known as the Proton Motive Force emerges (pmf). This gradient’s electrochemical potential is used to create ATP. (Oxidative Phosphorylation)As a result of H+ accumulation in the IMS, H+ tends to migrate to the matrix via the F!-Fo ATP ase’s H+ transport channel (complex V). As a result, the H + transport channel is called after the F1 stalk of the F! – F0 complex. This process is in charge of driving the force that causes the ATP Synthase to create ATP (complex V). Many experimental evidences and the nature of the mitochondrial inner membrane support this notion.
1. The inner membrane is impermeable to H+, OH-, K+ and Cl –, and their free movement/diffusion would cause the electrochemical gradient to be discharged. Due to H+ transport and an electrochemical potential gradient, oxidation of NADH and FADH2 is coordinated with ATP synthesis.
2. Many chemicals (chemical uncouplers) that increase membrane permeability to protons can also dissipate the electrochemical potential that has built up. As a result, ATP synthesis is slowed down.
Kennedy and Lehninger discovered that mitochondria are the sites of oxidative phosphorylation (1948). As a result of electron transport in mitochondria, ATP is created in eukaryotic aerobic respiration from ADP and pi. Oxidative phosphorylation is the name of the process. Phosphorylation of reduced cofactors NADH and FADH2 by O2 is known as oxidative phosphorylation. ATP generation necessitates energy, which is provided by redox processes occurring during electron transport. Chemical coupling is the relationship between energy conservation in the form of ATP and electron transport. The required quantity of energy to bind ADP and Pi is made available during phosphorylation by the free energy created in the following steps:
NADH+H+ ————-NAD+ (E0 -0.36 V/∆G0’ -69.5 kJ.mol-1
FADH2——– 2e- ——FAD ) (E0’ 0.190V,∆G0’ -36.7kJ/mol.)
CoQ CoQH2 —Cyt b—Cytc1——Cyt c——
ATP 3. Cyt c (red.)—2e–C u A –Cyt a–Cyt-a3 –C u B—2e—+ ½ O2+ 2H+ —àH 2 O (matrix) Cyt c (ox) (E0’ 0.580 V/∆Go’ -112 kJ.mol-1 )
FADH2 transmits electrons to CoQ in a parallel manner as well, but the free energy (G0′) of electron transfer from succinate to CoQ is inadequate to cause ATP synthesis. Because of this, only two ATP molecules are generated by FADH2.
Bacterial electron transport and oxidative phosphorylation
Bacteria lack mitochondria, the electron transport system is located in the plasma membrane, and protons are translocated from the cytosol to the plasma membrane’s exterior. In aerobic bacteria, electrons travel from CoQ through cytochrome-based oxidoreductases to O2, which is then reduced to H2O, similar to the mitochondrial system.