Bacterial Physiology (Micr430)

Lecture 7C1 Metabolism

(Text Chapter: 13)

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

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

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 CO2 via Acetyl-CoA pathway

Acetyl-CoA then is incorporate into cell material as carbon source

Acetyl-CoAPathway in Clostridium

Fig 13.4

Acetyl-CoAPathway in Methanogens

Fig 13.7

Methanogensis

Methanogensis from CO2 and H2

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 CO2.

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 C1 carbon source via either ribulose-monophosphate pathway or the serine pathway

Serine Pathway

Fig 13.11