Metabolic Diversity (III):
Acetogenesis
10-18-16
Overview
2. Anaerobic respiration
2.4 Acetogenesis
 CO2 is typically abundant in
anoxic habitats
- Major product of
chemoorganotrophs
 Two major groups of obligate
anaerobes use CO2 as e–
acceptor
- Acetogens
- Methanogens
Acetogens and Methanogens
 H2 is the major e– donor for both groups.
Why are there two groups of organisms that use CO2?
2.4 Acetogenesis
 Acetogens:
4H2 + 2CO2 → CH3COOH + 2H2O
ΔGº’= -105 kJ/rxn
- Obligate anaerobes
- Produce acetate as the primary product of
catabolism
- Use the reductive acetyl-CoA pathway (WoodLjungdahl pathway) to conserve energy and fix C
• Different from acetic acid bacteria: produce acetic
acid from sugars or alcohols during fermentation
(1)
Reductive Acetyl-CoA Pathway
(1) CO2 + 3H2 → [CH3]
HCOOH
(2) CO2 + H2 → [CO]
HCO–
(3) [CH3] + [CO] + CoA→
CH≡
CH2=
 Proposed to be oldest
pathway for CO2 fixation
 Fulfills two requirements
of life:
- Conserve energy
- Assimilate C
(2)
CH3–
(3) CH3–
Acetyl-CoA Pathway: Methyl Branch
H2 or
NAD(P)H
HCOOH
HCO–
CH≡
CH2=
CH3–
CH3–
 Formate dehydrogenase (FDH):
a common enzyme in obligate or
facultative anaerobes
- Catalyzes the reversible
HCOOH ↔ CO2
- Contains [NiFe], [FeS], and
Mo/W-pterin (O2-sensitive)
Acetyl-CoA Pathway: Methyl Branch
• One-carbon carrier: tetrahydrofolate (THF)
or tetrahydromethanopterin (H4MPT)
NADH
HCOOH
NADH
HCO–
CH≡
NADH
CH2=
NADH
CH3–
CH3–
CH3
Acetyl-CoA Pathway: Methyl Branch
• Corrinoid Fe-S protein (CoFeSP): contains Co
and [Fe-S] cluster
HCOOH
HCO–
Cobalamin
CH≡
CH2=
CH3–
CH3–
Acetyl-CoA Pathway: Carbonyl Branch
Fd2-red
CO2/CO: E0’= – 0.52 V
NAD+/NADH: E0’= – 0.32 V
Fdox/Fd2-red: E0’= – 0.45 V
Fdox/Fd2-red: E’ ≈ – 0.5 V
• Electron bifurcation: one e–
donor → two e– acceptors with
different E’
- Couple an endergonic redox
reaction to an exergonic one
- Often observed in organisms with
strong energy limitations
Fdox/Fdred [-0.45]
 Practice: Compare and
contrast reverse e– flow
and e– bifurcation
Acetyl-CoA Pathway: Acetyl-CoA Synthase
• CODH/ACS
- Catalyzes two
reactions:
- Contains [Ni-FeS]
cluster → active site
- Very O2-labile
Energy Conservation in
Acetyl-CoA Pathway
 NO net ATP formed by SLP
 Produce ATP by chemiosmosis
- Sodium MF or PMF
 NO enzyme in acetyl-CoA
pathway is membranebound.
- Cannot directly
translocate H+/Na+
 Chemiosmosis is coupled
to reduced ferredoxin.
Biosynthesis
Energy Conservation in Acetyl-CoA Pathway
 Acetogens also metabolize many
organic compounds (including sugars,
methyl compounds, and aromatic
compounds)
- 4 ATP is produced by SLP
Acetyl-CoA Pathway for Acetate Oxidation
 Widely used is sulfate
reducers and other
microbes
 Acetate is oxidized linearly
in two branches
 One-carbon carrier: H4F
(bacteria) or H4MPT
(archaea)
10
2[H]
2[H]
5
Tetrahydromethanopterin (H4MPT)
2[H]
10
5
Tetrahydrofolate (H4F)
2[H]
Archaeoglobus
Functions of Acetyl-CoA Pathway
Acetogens are
common in both
Gram-positive
and Gramnegative bacteria
 Reductive direction:
CO2 → acetate
 Fix CO2 for
autotrophic growth
 Oxidative direction:
acetate → CO2
 Discussion: What properties of the acetyl-CoA
pathway would support that it is the most
ancient CO2-fixation pathway?
- Function
- Distribution
- Enzyme
Acetogens in Wood-Feeding Termites
 Termite gut provides a
protective niche for
microbial symbionts
 Microbes near the
hindgut wall consume
O2 fast → anoxic
 Microbes degrade
sugars → acetate,
benefiting the host
Why acetogens outcompete
methanogens in wood-feeding
termite gut?
 Acetogens (mostly
spirochetes) are the
major H2-consuming
organisms → acetate
>> CH4