electron transport chain

Examples of electron transport chain in the following topics:

• Electron Donors and Acceptors

  • Electrons can enter the electron transport chain at three levels: dehydrogenase, the quinone pool, or a mobile cytochromeelectron carrier.
  • Individual bacteria use multiple electron transport chains, often simultaneously.
  • A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient.
  • Since electron transport chains are redox processes, they can be described as the sum of two redox pairs.
  • The situation is often summarized by saying that electron transport chains in bacteria are branched, modular, and inducible.

• Electron Donors and Acceptors in Anaerobic Respiration

  • In anaerobic respiration, a molecule other than oxygen is used as the terminal electron acceptor in the electron transport chain.
  • This method still incorporates the respiratory electron transport chain, but without using oxygen as the terminal electron acceptor .
  • Many different types of electron acceptors may be used for anaerobic respiration.
  • Acetogenesis is a type of microbial metabolism that uses hydrogen (H2) as an electron donor and carbon dioxide (CO2) as an electron acceptor to produce acetate, the same electron donors and acceptors used in methanogenesis.
  • Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase.

• Sulfate and Sulfur Reduction

  • Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain.
  • Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain.
  • Many sulfate reducers are organotrophic, using carbon compounds, such as lactate and pyruvate (among many others) as electron donors, while others are lithotrophic, and use hydrogen gas (H2) as an electron donor.
  • Before sulfate can be used as an electron acceptor, it must be activated.
  • In organisms that use carbon compounds as electron donors, the ATP consumed is accounted for by fermentation of the carbon substrate.

• Respiration and Proton Motive Force

  • Fructose 1, 6-diphosphate then splits into two phosphorylated molecules with three carbon chains that later degrades into pyruvate.
  • This is used by fermenting bacteria, which lack an electron transport chain, and which hydrolyze ATP to make a proton gradient.
  • Bacteria use these gradients for flagella and for the transportation of nutrients into the cell.
  • This creates ATP while using the proton motive force created by the electron transport chain as a source of energy.
  • A diagram of cellular respiration including glycolysis, Krebs cycle (AKA citric acid cycle), and the electron transport chain.

• Fermentation Without Substrate-Level Phosphorylation

  • Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound.
  • In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain.

• Anoxygenic Photosynthetic Bacteria

  • The electron transport chain (ETC) of green sulfur bacteria uses the reaction centre bacteriochlorophyll pair, P840.
  • If the electron leaves the chain to reduce NAD+, P840 must be reduced for the ETC to function again.
  • The electron transport chain of purple non-sulfur bacteria begins when the reaction center bacteriochlorophyll pair, P870, becomes excited by the absorption of light.
  • Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain.
  • The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.

• Anoxygenic Photosynthesis

  • Water is therefore not used as an electron donor.
  • The electron transport chain of purple non-sulfur bacteria begins when the reaction centre bacteriochlorophyll pair, P870, becomes excited from the absorption of light.
  • Excited P870 will then donate an electron to Bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain.
  • The electron returns to P870 at the end of the chain so it can be used again once light excites the reaction-center.
  • Therefore electrons are not left over to oxidize H2O into O2.

• Nitrate Reduction and Denitrification

  • Denitrification is a type of anaerobic respiration that uses nitrate as an electron acceptor.
  • In anaerobic respiration, denitrification utilizes nitrate (NO3-) as a terminal electron acceptor in the respiratory electron transport chain.
  • Protons are transported across the membrane by the initial NADH reductase, quinones and nitrous oxide reductase to produce the electrochemical gradient critical for respiration.
  • In general, it occurs where oxygen is depleted and bacteria respire nitrate as a substitute terminal electron acceptor.

• Proton Reduction

  • Biological energy is frequently stored and released by means of redox reactions, or the transfer of electrons.
  • Reduction occurs when an oxidant gains an electron.
  • The reverse reaction, respiration, oxidizes sugars (loses an electron) to produce carbon dioxide and water.
  • In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient.
  • In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy.

• Organic Acid Metabolism

  • It permits the donation of electrons to a second substrate (such as NAD+) in the process.
  • The ability to metabolize formate is also critical in bacterial anaerobic metabolism, in which case formate is also oxidized by an FDH enzyme but the electrons are donated to cytochromes (proteins involved in electron transport).
  • This process requires the β-oxidation pathway, a cyclic process that catalyzes the sequential shortening of fatty acid acyl chains to the final product, acetyl-CoA.
  • Fatty acid chains are converted to enoyl-CoA (catalyzed by acyl-CoA dehydrogenase).
  • The fatty acid chain that is left over after the thiolation step can then reenter the β-oxidation pathway, which can cycle until the fatty acid has been completely reduced to acetyl-CoA.