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Bacterial Physiology (Micr430)

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  1. Bacterial Physiology (Micr430) Lecture 7 C1 Metabolism (Text Chapter: 13)

  2. Definition • C1 compounds are organic compounds lacking C-C bonds • Methylotrophs – organisms which use C1 compounds other than CO2 as sole sources of energy and carbon • Methanotrophs - organisms which use methane as sole sources of energy and carbon • Methanogens – organisms that can produce methane

  3. Carbon Dioxide Fixation • There are three major autotrophic CO2 fixation pathways in prokaryotes: • Calvin cycle (or Calvin-Benson-Bassham) • Reductive TCA cycle • The Acetyl-CoA Pathway

  4. Calvin Cycle – 2 stages • Stage 1, reductive carboxylation of ribulose-1,5-bisphosphate (RuBP) to form phosphoglyceraldehyde (PGALD), catalyzed by RubisCO • Stage 2, sugar rearrangements to regenerate three RuBPs from five of six PGALDs

  5. Calvin Cycle • Only two reactions are unique to this cycle (the others are the same as the oxidative pentose phosphate pathway): • Phosphoribulokinase • Ribulose bisphosphate carboxylase (RubisCO) • This cycle constitutes the dark reaction of photosynthesis • Six turns of the cycle result in the synthesis of 1 mol of hexose (F-6-P)

  6. Calvin Cycle: key reactions

  7. Calvin Cycle

  8. RuBP carboxylation Fig 13.1

  9. Calvin cycle, glycolysis and PPP Fig 13.3

  10. The Acetyl-CoA Pathway • Bacteria that use this pathway include methanogens, acetogenic bacteria and most autotrophic sulfate-reducing bacteria • Acetyl-CoA is made from CO2 via Acetyl-CoA pathway • Acetyl-CoA then is incorporate into cell material as carbon source

  11. Acetyl-CoA Pathway in Clostridium Fig 13.4

  12. Acetyl-CoA Pathway in Methanogens Fig 13.7

  13. Methanogensis • Methanogensis from CO2 and H2 • Methanogensis from acetate

  14. Reductive TCA cycle • Reductive TCA pathway is used by • Desulfobacter (strict anaerobic) • Chlorobium (strict anaerobic) • Hydrogenobacter (aerobic) • Archaea • Overall reaction is the synthesis of one mole of oxaloacetate from four moles of CO2.

  15. Reductive TCA cycle • Three new enzymes are needed to reverse TCA cycle: • Fumarate reductase (step 4) • a-Ketoglutarate synthase (step 6) • ATP-dependent citrate lyase (step 11) • In addition, pyruvate synthase replaces pyruvate dehydrogenase, and PEP synthetase replaces pyruvate kinase

  16. Reductive TCA cycle Fig 13.9

  17. Methylotrophs • Compounds used for methylotrophic growth include: • Methane • Methanol • Formaldehyde • Formate • Methylamine • Trimethylamine (multicarbon but without C-C)

  18. C1 carbon assimilation • Methylotrophs assimilate C1 carbon source via either ribulose-monophosphate pathway or the serine pathway

  19. Serine Pathway Fig 13.11