Lecture 3D Flashcards
What are acetogenesis and methanogenesis and where do they occur?
They are microbial processes that conserve energy by reducing CO₂ or oxidizing CH₄, occurring in anoxic habitats like sediments and intestines.
What gases are abundant in many anoxic habitats that are involved in one-carbon metabolism?
CO₂ and CH₄
What is the major electron donor in acetogenesis and methanogenesis?
H₂ (hydrogen gas)
What is the ecological significance of CO₂ reduction in acetogenesis?
It helps balance atmospheric CO₂ by converting it into acetate influencing the greenhouse effect.
What is the net reaction of acetogenesis using H₂ and CO₂?
4H₂ + H⁺ + 2HCO₃⁻ → CH₃COO⁻ + 4H₂O (ΔG = -105 kJ)
What are some alternate electron donors in acetogenesis besides H₂?
Methanol, methoxylated aromatics, sugars, organic acids, amino acids, alcohols, nitrogen bases
What is the key pathway used in acetogenesis?
The Reductive Acetyl-CoA Pathway (Wood–Ljungdahl pathway)
What is the major pathway for acetate production in obligate anaerobes?
Reductive Acetyl-CoA pathway
Is the reductive acetyl-CoA pathway reversible?
Yes, some microbes can reverse it to oxidize acetate
What enzyme catalyzes the conversion of CO₂ to CO in acetyl-CoA synthesis?
Carbon monoxide dehydrogenase
What coenzymes or cofactors are involved in carbon monoxide dehydrogenase?
Nickel, zinc and iron
What is the final product of the acetyl-CoA pathway?
Acetate
How is ATP generated in acetogenesis?
By substrate-level phosphorylation during conversion of acetyl-CoA to acetate
How do acetogens generate energy beyond substrate-level phosphorylation?
Through an ion motive force, often generated by Rnf or Ech complexes
What is the role of the Rnf complex in acetogenesis?
It pumps Na⁺ across the membrane using reduced ferredoxin as an electron donor and NAD⁺ as an acceptor
What is flavin-based electron bifurcation?
A process where an exergonic reaction (NAD⁺ reduction) drives an endergonic one (ferredoxin reduction) via a flavin coenzyme
What is required to produce one reduced ferredoxin and one NADH?
Oxidation of 2 H₂ molecules
Why is bifurcation important in anaerobes?
Many lack external donors strong enough to reduce ferredoxin
What happens to the methyl group in the acetyl-CoA pathway?
It is transferred from THF to CoFeSP and then combined with CO to form acetyl-CoA
What generates the Na⁺ motive force in acetogens?
Rnf or Ech complex during electron transport
What is methanogenesis and who performs it?
Anaerobic respiration where Archaea reduce CO₂ to CH₄ performed by methanogens
Where are methanogens commonly found?
Freshwater sediments, bioreactors, sewage sludge, and animal intestines
What is the main electron donor in methanogenesis?
H₂
What are the main steps of methanogenesis from CO₂ + H₂?
Activation by methanofuran, reduction to methyl group via methanopterin, transfer to CoM, reduction to CH₄
