Chapter 7C - Microbial Metabolism: Fermentation & Respiration

Question 1

Chemoautotrophs oxidize inorganic electron donors for energy and need a reductant for __________________ fixation.

Answer 1

Carbon dioxide

Question 2

Why is chemoautotrophic relatively inefficient?

Answer 2

Because the compounds used as electron donors are not very good reducing agents

Question 3

Although inefficient, _____________________________ allows organisms to occupya unique ecophysiological niche where there is no competition from the hoards of heterotrophic organisms.

Answer 3

Chemoautotrophic respiration

Question 4

How do electrons from inorganic substrates normally enter electron transport system?

Answer 4

At the level of quinone or cytochrome c

Question 5

Why do chemoautotrophic electron donors enter ETS at the level of quinone or cytochrome c?

Answer 5

Because the reduction potentials of most chemoautotrophic electron donors are. Ore positive than NADH

Question 6

Why do electron donors with reduction potentials lower than NADH still enter ETS at the level of quinone or cytochrome c?

Answer 6

This is because the actual reduction potentials vary with concentration. At 1 atm, these gases are potent reductants; however, at the 10-3 atm or less that are commonly encountered in natural habitats, they are considerably less effective and cannot directly reduce NADH

Question 7

Where does oxidation of inorganic electron donors usually occur?

Answer 7

Periplasm

Question 8

What are two reasons why inorganic electron donors are oxidized in the periplasm?

Answer 8

A majority are oxidized by enzymes tht pass electrons to quinone or cytochrome c

1. There is no need to transport the substrate across the membrane
2. It contributes to the protonic potential because protons are often produced during oxidation resctions

Question 9

What happens to electrons that enter the ETS as quinone or cytochrome c?

Answer 9

Some of it flows downhill to oxygen, generating a protonic potential in the process. This protonic potential is used to generate ATP and also to pump the other stream of electrons uphill to generate reductant

Question 10

Why do chemoautotrophs use reverse electron transport?

Answer 10

Because they use carbon dioxide as their carbon source, they need an electron donor for carbon dioxide reducation as well as for electron transport. It takes two pairs of electrons to reduce carbon dioxide to cell material and requires two NADP+. However, most chemoautotrophs cannot reduce NADP+ directly and reduce quinone or cytochrome c. Instead of flowing downhill to oxygen though they are pumped uphill to NAD+ via ETS enzymes operating in reverse (ie NADH dehydrogenase)

Question 11

______________________ couples NADPH production to proton entry.

Answer 11

Transhydrogenase

Question 12

What is required for carbon dioxide fixation? NADH or NADPH?

Answer 12

NADPH

Question 13

Reverse electron transport forms NADH, but carbon dioxide fixation requires NADPH. How does the cell convert NADH to NADPH?

Answer 13

Transhydrogenase, which couples the reduction of NADP+ by NADH to the entry of protons

Question 14

Electrons from sulfide go through ____________________ driven uphill by proton entry.

Answer 14

NADH dehydrogenase via quinone

Question 15

Electrons from nitrite go through ______________________ and ______________________ driven uphill by proton entry.

Answer 15

Cytochrome b/c complex via cytochrome c

NADH dehydrogenase via quinone

Question 16

What are two reasons that chemoautotrophic respiration is very inefficient?

Answer 16

1. Electrons enter the ETC at the level of quinone or cytochrome c and thus few protons are pumped per pair of electrons
2. Much of the protonic potential is expended on generating reductant than ATP

Question 17

What do anaerobic chemoautotrophic respirations use as an electron donor?

Answer 17

Molecular hydrogen (H2)

Question 18

Chemoautotrophic respiration is very inefficient for two related reasons: (1) electrons enter the ETC at the level of quinone or cytochrome c and thus ____________ protons are pumped per pair of electrons, and (2) much of the protonic potential is expending on generating _______________ rather than ____________.

Answer 18

Few

Reductant

ATP

Question 19

Why is hydrogen gas most commonly used as an electron donor in anaerobic chemoautotrophic respiration?

Answer 19

Hydrogen is a common product of fermentative metabolism, and thus, in any anaerobic environment in which there is significant amounts of fermentable organic material, large amounts of hydrogen are likely to be generated

Question 20

What are the two most notable groups of anaerobic chemoautotrophs?

Answer 20

Methanogenic archaea

Sulfur- and sulfate-reducing bacteria

Question 21

Sulfur- and sulfate-reducing bacteria are __________________ anaerobes that are found in nearly all anaerobic habitats, but they are most prominent in the sediments of coastal shelves because of the abundance of sulfate in _____________.

Answer 21

Obligate anerobes

Seawater

Question 22

In chemoautotrophic respiration by sulfate reducers, oxidation of hydrogen occurs on the ___________________ face of a membrane _____________________, which produces protons. Production of protons in the ________________ and consumption in the ________________ is equivalent to pumping protons across the membrane.

Answer 22

Periplasmic

Hydrogenase

Periplasm

Cytoplasm

Question 23

What two species did Dr. Bates highlight as examples of chemoautrotrophic respiration via sulfate reducation?

Answer 23

Chlamydomonas (algae)

Clostridium

Question 24

______________________________ pumps ions without a cytochrome-based ETS.

Answer 24

Methanogenesis

Question 25

Methanogenesis is unique to the _______________ and is the principal consumer of hydrogen in anaerobic soils, freshwater sediments, and animal intestinal tracts.

Answer 25

Archaea

Question 26

In methanogenesis, __________________________ is reduced to _______________________ usually with hydrogen as the electron donor.

Answer 26

Carbon dioxide

Methane

Question 27

Only the last few steps in the biochemical pathway of methanogenesis appear to be linked to the generation of a protonic potential. By this point, carbon dioxide has been reduced to a methyl group and carried on the unique coenzyme ________________ _____. The membrane enzyme __________________________ then catalyzes the transfer of the CoM group to the sulfhydryl group of another unique cofactor ________________________________ (HTP-SH). HTTPS-COM is reductively split to regenerate the two coenzymes.

Answer 27

Coenzyme M

Methyl-coenzyme M reductase

7-mercaptoheptanoylthreonine phosphate

Question 28

What is HTP-SH?

Answer 28

7-mercaptoheptanoylthreonine phosphate

Question 29

The initial steps of methanogenesis yield no useful energy for the cell. __________________ pumps sodium as it produces methane and heterodisulfide composed of ____________________ and methyl coenzyme M. _________________________________ acts as a proton pump by producing protons in the periplasm and consuming them in the cytoplasm as it reduces the disulfide bond.

Answer 29

Methyl-CoM reductase

HTP-SH or 7-mercaptoheptanoylthreonine phosphate

Heterodisulfide reductase

Question 30

Energy is conserved at two points in methanogenesis. What are they?

Answer 30

1. Methyl-CoM reductase pumps sodium ions out of the cell with the transfer of CoM to HTTP to contribute to the electrical potential across the membrane
2. Heterodisulfide reductase pumps protons to contribue to the protonic potential

Question 31

In methanogenesis, each carbon dioxide reduced to methane results in the pumping of __ protons and __ sodium ions.

Answer 31

2 protons

1 sodium ion

Question 32

Despite its ineffectiveness, why has methanogenesis been successful in anaerobic environments?

Answer 32

It capitalizes on two compounds found abudantly in anaerobic environments as fermentation products: carbon dioxide and hydrogen gas

Question 33

There are several ETC poisons. Seven were discussed in class. What are they?

Answer 33

1. Cyanide
2. Malonate
3. Azides
4. 2,4-dinitrophenol
5. Oligomycin
6. Antimicin

Question 34

Which ETC poison is an extremely effective reversible inhibitor of cytochrome oxidase?

Answer 34

Cyanide

Question 35

Which ETC poison is a competitive inhibitor of succinate dehydrogenase?

Answer 35

Malonate

Question 36

What ETC poison binds to heme groups in cytochromes?

Answer 36

Azides

Question 37

What ETC poisson acts as an uncoupling agent?

Answer 37

2,4-dintrophenol

Question 38

Which ETC poison inhibits proton flow through ATP synthase?

Answer 38

Oligomycin

Question 39

Which ETC poison binds to cytochrome c?

Answer 39

Antimicin