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Coupled reactions

Coupled reactions. Coupled reaction : D G (free energy released) in the exergonic reaction (- D G) > D G required for endergonic reaction (+ D G) Oxidation : reaction in which electrons are lost; substance losing e - is e - donor or reducing agent

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Coupled reactions

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  1. Coupled reactions • Coupled reaction: DG (free energy released) in the exergonic reaction (-DG) > DG required for endergonic reaction (+DG) • Oxidation : reaction in which electrons are lost; substance losing e- is e- donor or reducing agent • Reduction : reaction in which e- are gained; substance gaining e- is e- acceptor or oxidizing agent

  2. Redox pairs • Redox reaction : couple oxidation and reduction (DG must be negative) • Redox couple : The oxidant and reductant pair • Examples: SO42-/H2S • Fe3+/Fe2+ • NO3-/NO2- • By convention: oxidant/reductant

  3. Concepts in Metabolism • Catabolism - breakdown of chemical compounds • Anabolism - biosynthesis of chemical compounds • Amphibolic pathways - metabolites in these pathways serve as substrates for catabolism and anabolism; ex. TCA cycle

  4. Metabolic Diversity “It is in the fueling reactions that bacteria display their extraordinary metabolic diversity and versitility. Bacteria have evolved to thrive in almost all natural environments, regardless of the nature of available sources of carbon, energy, and reducing power …. The collective metabolic capacities of bacteria allow them to metabolize virtually every organic compound on this planet ………” Neidhardt, Ingraham and Schaecter

  5. Energy-generating metabolism • Substrate-level phosphorylation : ~P is transferred to ADP to form ATP using energy derived from coupled exergonic reaction • Electron transport and chemiosmosis : ATP is synthesized via transfer of e-, coupled with H+ efflux • Photophosphorylation : same as ETC except e- are generated by light-induced reactions

  6. Big picture concepts • Unifying themes in fermentation • NADH is oxidized to NAD+ • Electron acceptor is pyruvate or pyruvate derivative • Limited amount of E obtained from fermentation alone

  7. TCA or Krebs cycle • Pyruvate degraded completely to CO2 in stage 3 • Result of 1 turn of cycle: 2CO2, 1ATP(GTP), 3 NADH+H+, 1FADH2 • Citrate synthetase: allosteric enzyme with lower activity as ATP binds • Provides C skeletons for biosynthesis

  8. Electron transport chain • Location: cytoplasmic membrane in bacteria; mitochondria in eukaryotes • Components: series of e- carriers that transport e- from NADH and FADH2 to O2 • Function: harness more E from ox. of glu • P/O ratios: 3 from NADH to O2; 2 from FADH2 to O2 (mitos) • Mito. and bact. ETC vary - different cytochromes; bacterial often branched

  9. Large difference in E0 of NADH and E0 of O2 Correlates to large amount of energy released

  10. Terms and equations • pH = pHi - pHo : Proton gradient •  = i - o : Electochemical gradient (separation of charge) • PMF (proton motive force) = p =  - Z pH , where Z=2.3RT/F @25 C, p =  - 59pH (in mV) • More negative p, more work can be done

  11. Aerobic respiration vs. fermentation • Get up to 3 ATPs per NADH + H+ • Get up to 2 ATPs per FADH2 • Theoretical max from glycolysis, TCA and ETC : 38 ATP (varies between bugs) • Total amt of free energy in oxidation of glc : -688 kcal/mol • Aerobic respiration: 40% efficiency; fermentation : 26% efficiency

  12. Energetics of chemolithotrophy DG0= -nFDE0’ • 2H+/H2: -0.41V; ½O2/H2O: +0.82 • Potential difference is 1.23 • 2 electrons involved: DG0= -237.34 kJ. • H+/H2 to NO3-/NO2-: DG0= -163kJ/mol • Still enough to make ATP: -31.kJ/mol • Predict kinds of chemolithotrophy found in nature

  13. Bioenergetics of Hydrogen bacteria

  14. Bioenergetics of Sulfur bacteria

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