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

Definition C 1 compounds are organic compounds lacking C-C bonds Methylotrophs – organisms which use C 1 compounds other than CO 2 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

Carbon Dioxide Fixation There are three major autotrophic CO 2 fixation pathways in prokaryotes: Calvin cycle (or Calvin-Benson-Bassham) Reductive TCA cycle The Acetyl-CoA Pathway

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

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)

Calvin Cycle: key reactions

Calvin Cycle

RuBP carboxylation Fig 13.1

Calvin cycle, glycolysis and PPP Fig 13.3

The Acetyl-CoA Pathway Bacteria that use this pathway include methanogens, acetogenic bacteria and most autotrophic sulfate-reducing bacteria Acetyl-CoA is made from CO 2 via Acetyl- CoA pathway Acetyl-CoA then is incorporate into cell material as carbon source

Acetyl-CoA Pathway in Clostridium Fig 13.4

Acetyl-CoA Pathway in Methanogens Fig 13.7

Methanogensis Methanogensis from CO 2 and H 2 Methanogensis from acetate

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 CO 2.

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

Reductive TCA cycle Fig 13.9

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

C1 carbon assimilation Methylotrophs assimilate C 1 carbon source via either ribulose-monophosphate pathway or the serine pathway

Serine Pathway Fig 13.11