One-Carbon (C1) Metabolism AND Fermentation

Question 1

What two one-carbon molecules are abundant in anoxic habitats and can be used by microbes to conserve energy?

Answer 1

Methane and carbon dioxide.

Methane is reduced, while carbon dioxide is oxidized.

Question 2

What two groups of prokaryotes reduce carbon dioxide to conserve energy?

Answer 2

Acetogens and methanogens.

These prokaryotes are anaerobes.

Question 3

What is the major electron donor for acetogens and methanogens?

Answer 3

Hydrogen.

Question 4

What ions can be used by acetogens and methanogens to power their ATP synthases?

Answer 4

Sodium and proton ions.

Question 5

Which group of prokaryotes can also produce ATP via substrate level phosphorylation?

Answer 5

Acetogens.

Question 6

What are some examples of organic compounds that can be oxidized by acetogens?

Answer 6

Alcohols and fatty acids.

Question 7

What two other molecules can be reduced by some acetogens?

Answer 7

Formate and acetate.

Question 8

What metabolic pathway is used by acetogens to reduce carbon dioxide?

Answer 8

The reductive acetyl-CoA pathway.

Question 9

How many NADH molecules are produced when acetogens catabolize glucose to acetate?

Answer 9

Two NADH molecules.

These NADH molecules are used for energy conservation.

Question 10

In what three ways can the reductive acetyl-CoA pathway be used by prokaryotes?

Answer 10

For carbon fixation, energy conservation, and biosynthesis.

Question 11

What four groups of prokaryotes use the reductive acetyl-CoA pathway autotrophically?

Answer 11

Acetogens, methanogens, sulfate-reducing bacteria, and some syntrophs.

Question 12

What two groups of prokaryotes run the reductive acetyl-CoA pathway in reverse?

Answer 12

Methanogens and sulfate-reducing bacteria.

Question 13

How does the reductive acetyl-CoA pathway differ from other carbon-fixation pathways?

Answer 13

It uses a unique set of enzymes and cofactors.

Question 14

What is the key enzyme in the reductive acetyl-CoA pathway?

Answer 14

Acetyl-CoA synthase.

It requires iron, nickel, and sulfur as cofactors.

Question 15

How is the ion motive force used for ATP synthesis generated in acetogens?

Answer 15

By using hydrogen as the electron donor and carbon dioxide as the electron acceptor.

Question 16

What is flavin-based electron bifurcation and why is it necessary in acetogens?

Answer 16

It is a process that allows the coupling of exergonic and endergonic reactions to conserve energy.

Question 17

What are methanogens?

Answer 17

Microorganisms that produce methane.

They are found in the Archaea domain.

Question 18

What ecological niches contain methanogens?

Answer 18

Wetlands, ruminant guts, and anaerobic sediments.

Question 19

How many electrons are required to reduce carbon dioxide to methane?

Answer 19

Eight electrons.

Question 20

What two categories of coenzymes are required for the reduction of carbon dioxide to methane?

Answer 20

Methyl-coenzymes and coenzyme M.

Examples include methanofuran and coenzyme B.

Question 21

What is the typical source of electrons for the reduction of carbon dioxide to methane?

Answer 21

Hydrogen.

Question 22

At what point during the reduction of carbon dioxide to methane are sodium ions pumped across the cell membrane?

Answer 22

During the conversion of formate to methane.

Question 23

What role does electron bifurcation play in the reduction of carbon dioxide to methane?

Answer 23

It helps to efficiently transfer electrons to different pathways.

Question 24

What are two other mechanisms for methanogenesis?

Answer 24

Acetoclastic methanogenesis and methylotrophic methanogenesis.

Question 25

How can methanogens use the reductive acetyl-CoA pathway without tetrahydrofolate?

Answer 25

By utilizing alternative cofactors.

Question 26

How is ATP produced in methanogens that reduce carbon dioxide to methane?

Answer 26

Through substrate-level phosphorylation.

Question 27

How is ATP produced in methanogens that produce methane from acetate or methanol?

Answer 27

By using the proton motive force.

Question 28

What is the difference between methylotrophs and methanotrophs?

Answer 28

Methylotrophs utilize single-carbon compounds, while methanotrophs specifically oxidize methane.

Question 29

What enzyme catalyzes the first step of the aerobic oxidation of methane?

Answer 29

Methane monooxygenase.

Question 30

What two pathways exist for the incorporation of carbon atoms into the cell material in aerobic methanotrophs?

Answer 30

The serine pathway and the ribulose monophosphate pathway.

Question 31

What are the substrates, products, key enzymes, reducing power requirements, and ATP requirements for each of these pathways?

Answer 31

Varies by pathway. ## Footnote Specific details depend on the pathway being utilized.

Question 32

What two types of prokaryotes make up consortia that can anaerobically oxidize methane?

Answer 32

ANME (Anaerobic Methanotrophic Archaea) and SRB (Sulfate-Reducing Bacteria). ## Footnote ANME oxidizes methane, while SRB reduces sulfate.

Question 33

Where are these AOM consortia found in nature?

Answer 33

In marine sediments and freshwater environments.

Question 34

How are electrons transferred from the ANME cells to the SRB cells?

Answer 34

Through direct intercellular connections or via conductive minerals.

Question 35

What other type of consortium can perform AOM?

Answer 35

Methanogenic consortia. ## Footnote They typically involve methanogens and syntrophs.

Question 36

How is Methylomirabilis oxyfera able to aerobically oxidize methane?

Answer 36

By using oxygen as an electron acceptor.

Question 37

What is syntrophy?

Answer 37

A metabolic interaction where two or more organisms cooperate to degrade a substrate.

Question 38

What are secondary fermentations?

Answer 38

Fermentations that occur after primary fermentation, often involving syntrophic relationships.

Question 39

What process forms the heart of most syntrophic reactions?

Answer 39

Hydrogen transfer.

Question 40

What groups of organisms can consume H2 produced by syntrophs?

Answer 40

Methanogens and sulfate-reducing bacteria.

Question 41

How do members of the genus Pelotomaculum grow as syntrophs?

Answer 41

By degrading fatty acids in the presence of hydrogen-consuming partners.

Question 42

How do members of the genus Syntrophomonas grow as syntrophs?

Answer 42

By fermenting butyrate and transferring hydrogen to methanogens.

Question 43

Why is the removal of H2 important in syntrophic relationships?

Answer 43

It prevents the inhibition of the fermentation process.

Question 44

In addition to H2, what else can be directly transferred from the syntroph to the partner organism?

Answer 44

Fatty acids. ## Footnote This transfer occurs through direct contact or diffusion.

Question 45

What two general mechanisms are used by syntrophs to conserve energy?

Answer 45

Substrate-level phosphorylation and electron transfer.

Question 46

What is fatty acid disproportionation?

Answer 46

The process where fatty acids are split into shorter-chain fatty acids and hydrogen.

Question 47

During syntrophic metabolism, how is H2 produced?

Answer 47

By fermentation of organic substrates. ## Footnote This process adds an energy burden on syntrophs.

Question 48

What is the ecological importance of syntrophs?

Answer 48

They facilitate the degradation of organic matter and recycling of nutrients.