16. Microbial Energetics II

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  1. Why and how are electrons transferred in the electron transport chain?
    • if electrons and protons from donors were given directly to a terminal acceptor - huge amount of energy would be released in an uncontrolled way, largely as unusable heat
    • instead, bacteria perform several smaller electron transfers in sequence
    • this provides several places where the energy of a reaction can be converted into a proton gradient across the IM, which is another form of stored, usable energy
  2. characteristics of electron transport chains
    • 1) contain protein complexes that span the cytoplasmic membrane
    • 2) electron carriers are arranged from lower to higher reduction potentials
    • 3) alternate electron and proton and electron-only carriers
    • 4) generate a proton motive force at sites of large ΔE0´
  3. FMN
    • an intermediate electron carrier in electron transport chains
    • non-covalently bound to protein
    • carries both protons and electrons
  4. heme
    • cytochrome protein cofactors
    • porphyrin rings called tetrapyrroles
    • the Fe coordinated at the center cycles between the 2+ and 3+ states as an electron is accepted or donated
    • electron-only carriers
    • have different reduction potentials depending on the protein structure
  5. Iron-sulfur clusters
    • covalently bound to electron transport proteins via cysteine residues
    • electron-only carriers
    • reduction potential varies with the number of Fe and S and how the cluster is attached to the proteins
  6. Quinone molecules
    • mobile electron carriers that diffuse in the plane of the cytoplasmic membrane
    • can interact with different donor and acceptor complexes
    • in their reduced state, each molecule carriers 2e- and 2H+
    • take up H+ from the cytoplasm and release them into the periplasm, helping to generate the proton gradient
  7. generation of proton motive force during aerobic respiration
    • 1) 2H+ and 2e- pass from NADH to FMN in complex I
    • 2) 4H+ are pumped to the outside of the membrane when FMNH2 donates 2e- to an Fe/S cluster within complex I
    • 3) 2H+ are taken up from the dissociation of water in the cytoplasm when Fe/S donates 2e- to the quinone Coenzyme Q
    • 4) Complex II (succinate dehydrogenase) feeds 2H+ and 2e- from FADH2 directly into the quinone pool, bypassing complex I (important because FADH2 is made during the TCA cycle)
    • 5) the reduced quinone passes electrons to cyt bL in complex III
    • 6) when electrons pass from cyt bL to an Fe/S cluster, 2H+ are pumped outside the membrane
    • 7) Electrons can also be passed from bL to bH, all of which have similar E0´ as the quinone pool. Electrons are recycled to Q from bH along with extra protons from the cytoplasm. When Q re-donates electrons to bL and Fe/S, 2 more H+ are pumped out of the cell.
    • 8) Electrons are passed from Fe/S to cyt c1 in complex III, then to cyt c in the periplasm
    • 9) Electrons pass to cyt a and a3 in complex IV
    • 10) cyt a3 reduces O2 to H2O and pumps 2 H+ out of the cell

    • total = 10 H+ pumped out for every 2e- that enter at complex I from NADH
    • only 6H+ are pumped out for every 2e- that enter at complex II from FADH2
  8. regulation of ETC complex expression to respond to environment
    too large a voltage can disrupt the cytoplasmic membrane. In high O2, cells express NDH-2 so that NADH gets recycled, but the membrane potential created is controlled.

    • Cyt bo only works at high [O2]
    • in low [O2] cells express cyt bd, which can transfer electrons to O2, but can't pump H+ (preferable to a backup in the ETC)
  9. Why are H2 and O2 functionally the outside points on the electron tower?
    because there's so much water in cells

    if any molecule is a better electron donor than H2, it can reduce the hydrogen in water to create H2 gas

    if any molecule is a better electron acceptor than oxygen, it can oxidize the oxygen in water to create O2

    electron transport chains in cells have to use redox compounds with reduction potentials between H2 and O2
  10. oxidative phosphorylation
    • the proton gradient generated by electron transport is used to power ATP synthesis
    • subunits rotate causing a conformational change where ADP and Pi bind and react
  11. energetics balance sheet for aerobic respiration
    • Assumes
    • 10 H+ pumped per 2 e- from NADH
    • 6 H+ pumped per 2 e- from FADH2
    • 3 H+ cross IM per ATP synthesized
  12. How do bacteria maintain ion gradients across the cytoplasmic membrane during fermentation when no terminal acceptor is present and electron transport chains aren't functioning?
    ATP generated by substrate-level phosphorylation is used to run the F1F0 ATPase backward, so that ATP is hydrolyzed to create a proton gradient
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16. Microbial Energetics II
general microbiology midterm 2
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