Typed lecture notes covering chapter 14 of Microbiology: An Evolving Science, the textbook used in the "General Microbiology" course (BioM122) at UCI. Aligns with lecture 8.
Microbial Metabolism II (Ch. 14, Lec. 8)
Wednesday, October 21, 2020 4:52 PM
• Microbes transfer energy by moving electrons.
○ Electrons move from reduced food molecules -> energy carriers -> membrane protein
carriers -> oxygen/oxidized minerals.
• Membrane-embedded Electron Transport System (ETS): system that drives protons across
the membrane to generate a proton motive force (PMF); PMF powers ATP synthesis. // ETS
are membrane proteins that transfer e- from an e- donor to terminal e- acceptor that leaves
the cell.
○ Gram-neg bacteria have their ETS in the inner membrane.
• Classes of metabolism that use ETS: these YIELD ENERGY through oxidation by…
○ Organotrophy: use organic electron donors/terminal electron acceptors.
○ Lithotrophy: use inorganic electron donors/terminal electron acceptors.
○ Phototrophy: light absorption excites electrons.
○ Some prokaryotes use more than one of these metabolisms. Protein complexes used
have indicated they have evolved from a common ancestral ETS.
• Cable bacteria: can form multicellular chains that can transfer electrons across several
centimeters (usually to where oxygen is present).
○ Bacteria in marine sediment collect electrons from sulfides, then build a "snorkel" to
where O2 (STRONG e- acceptor) is present.
14.1: ETS and the PMF
• Most energy-yielding rxns involve electron (e-) transfer from a reduced e- donor to an
oxidized e- acceptor.
• Electron transport system/chain: e- transfer thru a series of membrane-soluble carriers
during respiration.
• In redox reactions, the ΔG values are proportional to the reduction potential (E) between the
oxidized form (e– acceptor) and its reduced form (e– donor).
○ Reduction potential: tendency of a compound to accept electrons. ->
○ Positive value of E has a negative deltaG -> gain of e- yields energy.
○ Negative value of E -> loss of electrons yields energy.
○ Standard reduction potential (E*): assumes all reactants/products = 1M @ pH 7.
• Table 14.1: "Electron tower":
• Electron acceptor -> electron donor. If you flip the arrow, you also flip the signs of the E and
deltaG values.
• Redox couple: the oxidized and reduced states of a compound.
• A complete redox rxn combines 2 redox couples:
○ One accepts electrons (becoming reduced).
○ One donates electrons (becoming oxidized).
○ Positive E values = rxn proceeds forward to provide energy.
• Oxidoreductases: electron transport proteins that oxidize one substrate and reduce
another.
○ Consist of multiple-protein complexes that include cytochromes-- colored proteins
whose absorbance spectrum shifts when there is a change in redox state.
• Chemiosmotic theory: process in which the ETS pumps protons out of the cell, resulting in
an electrochemical gradient of protons that drives ADP into ATP via ATP synthase.
• When protons are pumped across the membrane, energy is stored in 2 different forms:
○ Electron potential (Dy): separation of charge b/w the cytoplasm and soln outside the
cell membrane.
○ pH difference (DpH): log ratio of external : internal chemical conc of H+.
○ Proton potential (Dp)= Dy - 60DpH
• DP drives ATP synthesis, rotation of flagella, uptake of nutrients, and efflux of
toxic drugs.
• Weak acids transfer protons through the membrane until the DpH is dissipated.
○ Uncouplers: weak acids that can move acrcoss the membrane in both protonated and
unprotonated forms, and cyclically bring protons into the cell, collapsing both DpH and
Dy.
14.4: The Respiratory ETS and the ATP Synthase
• In aerobic bacteria and mitochondria, e- carriers NADH and FADH2 transfer e- to 02,
producing H2O.
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