electron transport chain


  • In photophosphorylation, the energy of sunlight is used to create a high-energy electron donor which can subsequently reduce oxidized components and couple to ATP synthesis
    via proton translocation by the electron transport chain.

  • The energy released by reactions of oxygen and reduced compounds such as cytochrome c and (indirectly) NADH and FADH2 is used by the electron transport chain to pump protons
    into the intermembrane space, generating the electrochemical gradient over the inner mitochondrial membrane.

  • When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting
    in superoxide formation.

  • When organic matter is the electron source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar
    to Complex I in mitochondria) or succinate dehydrogenase (similar to Complex II).

  • The generalized electron transport chain in bacteria is: Electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool,
    or at the level of a mobile cytochrome electron carrier.

  • At the inner mitochondrial membrane, electrons from NADH and FADH2 pass through the electron transport chain to oxygen, which provides the energy driving the process as it
    is reduced to water.

  • An electron transport chain (ETC[1]) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions
    (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane.

  • Many of the enzymes in the electron transport chain are embedded within the membrane.

  • The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from a lower to a higher redox
    potential, is used by the complexes in the electron transport chain to create an electrochemical gradient of ions.

  • This entire process is called oxidative phosphorylation since ADP is phosphorylated to ATP by using the electrochemical gradient that the redox reactions of the electron transport
    chain have established driven by energy-releasing reactions of oxygen.

  • Complex IV[edit] In Complex IV (cytochrome c oxidase; EC, sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred
    to molecular oxygen (O2) and four protons, producing two molecules of water.

  • When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule.

  • A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane.

  • The overall electron transport chain can be summarized as follows: Complex I[edit] Further information: Respiratory complex I In Complex I (NADH ubiquinone oxidoreductase,
    Type I NADH dehydrogenase, or mitochondrial complex I; EC, two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q).

  • One example is blockage of ATP synthase, resulting in a build-up of protons and therefore a higher proton-motive force, inducing reverse electron flow.

  • Mitochondrial redox carriers[edit] Energy associated with the transfer of electrons down the electron transport chain is used to pump protons from the mitochondrial matrix
    into the intermembrane space, creating an electrochemical proton gradient across the inner mitochondrial membrane.

  • For example, electrons from inorganic electron donors (nitrite, ferrous iron, electron transport chain) enter the electron transport chain at the cytochrome level.

  • The reduced product, ubiquinol (QH2), freely diffuses within the membrane, and Complex I translocates four protons (H+) across the membrane, thus producing a proton gradient.

  • Via the transferred electrons, this energy is used to generate a proton gradient across the mitochondrial membrane by “pumping” protons into the intermembrane space, producing
    a state of higher free energy that has the potential to do work.

  • For example, E. coli (when growing aerobically using glucose and oxygen as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for
    a total of four different electron transport chains operating simultaneously.

  • Organisms that use organic molecules as an electron source are called organotrophs.

  • This type of metabolism must logically have preceded the use of organic molecules and oxygen as an energy source.

  • [9] Photosynthetic electron transport chains, like the mitochondrial chain, can be considered as a special case of the bacterial systems.

  • Two electrons are removed from QH2 at the QO site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane

  • For example, in humans, there are 8 c subunits, thus 8 protons are required.

  • Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide.

  • Electron acceptors and terminal oxidase/reductase[edit] As there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs),
    there are a number of different electron acceptors, both organic and inorganic.

  • Each electron donor will pass electrons to an acceptor of higher redox potential, which in turn donates these electrons to another acceptor, a process that continues down
    the series until electrons are passed to oxygen, the terminal electron acceptor in the chain.

  • Therefore, the pathway through Complex II contributes less energy to the overall electron transport chain process.

  • Bacterial terminal oxidases can be split into classes according to the molecules act as terminal electron acceptors.

  • [11] This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD+ and Q) with energy provided by O2.

  • Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier.

  • Here, light energy drives electron transport through a proton pump and the resulting proton gradient causes subsequent synthesis of ATP.

  • [7] As the electrons move through the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane.

  • They also function as electron carriers, but in a very different, intramolecular, solid-state environment.

  • At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient.

  • In bacteria, the electron transport chain can vary between species but it always constitutes a set of redox reactions that are coupled to the synthesis of ATP through the
    generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase.

  • Dehydrogenases: equivalants to complexes I and II[edit] Bacteria can use several different electron donors.

  • Mitochondrial Complex III is this second type of proton pump, which is mediated by a quinone (the Q cycle).

  • Individual bacteria use multiple electron transport chains, often simultaneously.

  • The two other electrons sequentially pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol.

  • The flow of electrons through the electron transport chain is an exergonic process.

  • Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain two or at least one.


Works Cited

[‘1. Lyall, Fiona (2010). “Biochemistry”. Basic Science in Obstetrics and Gynaecology. pp. 143–171. doi:10.1016/B978-0-443-10281-3.00013-0. ISBN 978-0-443-10281-3.
2. ^ Jump up to:a b Anraku Y (June 1988). “Bacterial electron transport chains”. Annual
Review of Biochemistry. 57 (1): 101–32. doi:10.1146/annurev.bi.57.070188.000533. PMID 3052268.
3. ^ Kracke F, Vassilev I, Krömer JO (2015). “Microbial electron transport and energy conservation – the foundation for optimizing bioelectrochemical
systems”. Frontiers in Microbiology. 6: 575. doi:10.3389/fmicb.2015.00575. PMC 4463002. PMID 26124754. – This source shows four ETCs (Geobacter, Shewanella, Moorella , Acetobacterium) in figures 1 and 2.
4. ^ Waldenström JG (2009-04-24). “Biochemistry.
By Lubert Stryer”. Acta Medica Scandinavica. 198 (1–6): 436. doi:10.1111/j.0954-6820.1975.tb19571.x. ISSN 0001-6101.
5. ^ Zorova LD, Popkov VA, Plotnikov EY, Silachev DN, Pevzner IB, Jankauskas SS, et al. (July 2018). “Mitochondrial membrane potential”.
Analytical Biochemistry. 552: 50–59. doi:10.1016/j.ab.2017.07.009. PMC 5792320. PMID 28711444.
6. ^ Lauren, Biochemistry, Johnson/Cole, 2010, pp 598-611
7. ^ Garrett & Grisham, Biochemistry, Brooks/Cole, 2010, pp 598-611
8. ^ Garrett R, Grisham
CM (2016). biochemistry. Boston: Cengage. p. 687. ISBN 978-1-305-57720-6.
9. ^ Jump up to:a b Stryer. Biochemistry. toppan. OCLC 785100491.
10. ^ Jonckheere AI, Smeitink JA, Rodenburg RJ (March 2012). “Mitochondrial ATP synthase: architecture,
function and pathology”. Journal of Inherited Metabolic Disease. 35 (2): 211–25. doi:10.1007/s10545-011-9382-9. PMC 3278611. PMID 21874297.
11. ^ Jump up to:a b Garrett RH, Grisham CM (2012). Biochemistry (5th ed.). Cengage learning. p. 664. ISBN
12. ^ Fillingame RH, Angevine CM, Dmitriev OY (November 2003). “Mechanics of coupling proton movements to c-ring rotation in ATP synthase”. FEBS Letters. 555 (1): 29–34. doi:10.1016/S0014-5793(03)01101-3. PMID 14630314. S2CID
13. ^ Berg JM, Tymoczko JL, Stryer L (2002-01-01). “A Proton Gradient Powers the Synthesis of ATP”. {{cite journal}}: Cite journal requires |journal= (help)
14. ^ Cannon B, Nedergaard J (January 2004). “Brown adipose tissue: function
and physiological significance”. Physiological Reviews. 84 (1): 277–359. doi:10.1152/physrev.00015.2003. PMID 14715917.
15. ^ Kim BH, Gadd GM (2008). “Introduction to bacterial physiology and metabolism”. Bacterial Physiology and Metabolism. Cambridge
University Press. pp. 1–6. doi:10.1017/cbo9780511790461.002. ISBN 978-0-511-79046-1.
16. ^ Mills EL, Kelly B, Logan A, Costa AS, Varma M, Bryant CE, et al. (October 2016). “Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to
Drive Inflammatory Macrophages”. Cell. 167 (2): 457–470.e13. doi:10.1016/j.cell.2016.08.064. PMC 5863951. PMID 27667687.
17. ^ EC
18. ^ Ingledew WJ, Poole RK (September 1984). “The respiratory chains of Escherichia coli”. Microbiological
Reviews. 48 (3): 222–71. doi:10.1128/mmbr.48.3.222-271.1984. PMC 373010. PMID 6387427.
Photo credit: https://www.flickr.com/photos/dok1/2431737309/’]