Chapter 7C - Microbial Metabolism: Fermentation & Respiration Flashcards
Chemoautotrophs oxidize inorganic electron donors for energy and need a reductant for __________________ fixation.
Carbon dioxide
Why is chemoautotrophic relatively inefficient?
Because the compounds used as electron donors are not very good reducing agents
Although inefficient, _____________________________ allows organisms to occupya unique ecophysiological niche where there is no competition from the hoards of heterotrophic organisms.
Chemoautotrophic respiration
How do electrons from inorganic substrates normally enter electron transport system?
At the level of quinone or cytochrome c
Why do chemoautotrophic electron donors enter ETS at the level of quinone or cytochrome c?
Because the reduction potentials of most chemoautotrophic electron donors are. Ore positive than NADH
Why do electron donors with reduction potentials lower than NADH still enter ETS at the level of quinone or cytochrome c?
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
Where does oxidation of inorganic electron donors usually occur?
Periplasm
What are two reasons why inorganic electron donors are oxidized in the periplasm?
A majority are oxidized by enzymes tht pass electrons to quinone or cytochrome c
- There is no need to transport the substrate across the membrane
- It contributes to the protonic potential because protons are often produced during oxidation resctions
What happens to electrons that enter the ETS as quinone or cytochrome c?
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
Why do chemoautotrophs use reverse electron transport?
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)
______________________ couples NADPH production to proton entry.
Transhydrogenase
What is required for carbon dioxide fixation? NADH or NADPH?
NADPH
Reverse electron transport forms NADH, but carbon dioxide fixation requires NADPH. How does the cell convert NADH to NADPH?
Transhydrogenase, which couples the reduction of NADP+ by NADH to the entry of protons
Electrons from sulfide go through ____________________ driven uphill by proton entry.
NADH dehydrogenase via quinone
Electrons from nitrite go through ______________________ and ______________________ driven uphill by proton entry.
Cytochrome b/c complex via cytochrome c
NADH dehydrogenase via quinone
What are two reasons that chemoautotrophic respiration is very inefficient?
- Electrons enter the ETC at the level of quinone or cytochrome c and thus few protons are pumped per pair of electrons
- Much of the protonic potential is expended on generating reductant than ATP
What do anaerobic chemoautotrophic respirations use as an electron donor?
Molecular hydrogen (H2)
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 ____________.
Few
Reductant
ATP
Why is hydrogen gas most commonly used as an electron donor in anaerobic chemoautotrophic respiration?
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
What are the two most notable groups of anaerobic chemoautotrophs?
Methanogenic archaea
Sulfur- and sulfate-reducing bacteria
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 _____________.
Obligate anerobes
Seawater
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.
Periplasmic
Hydrogenase
Periplasm
Cytoplasm
What two species did Dr. Bates highlight as examples of chemoautrotrophic respiration via sulfate reducation?
Chlamydomonas (algae)
Clostridium
______________________________ pumps ions without a cytochrome-based ETS.
Methanogenesis
Methanogenesis is unique to the _______________ and is the principal consumer of hydrogen in anaerobic soils, freshwater sediments, and animal intestinal tracts.
Archaea
In methanogenesis, __________________________ is reduced to _______________________ usually with hydrogen as the electron donor.
Carbon dioxide
Methane
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.
Coenzyme M
Methyl-coenzyme M reductase
7-mercaptoheptanoylthreonine phosphate
What is HTP-SH?
7-mercaptoheptanoylthreonine phosphate
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.
Methyl-CoM reductase
HTP-SH or 7-mercaptoheptanoylthreonine phosphate
Heterodisulfide reductase
Energy is conserved at two points in methanogenesis. What are they?
- 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
- Heterodisulfide reductase pumps protons to contribue to the protonic potential
In methanogenesis, each carbon dioxide reduced to methane results in the pumping of __ protons and __ sodium ions.
2 protons
1 sodium ion
Despite its ineffectiveness, why has methanogenesis been successful in anaerobic environments?
It capitalizes on two compounds found abudantly in anaerobic environments as fermentation products: carbon dioxide and hydrogen gas
There are several ETC poisons. Seven were discussed in class. What are they?
- Cyanide
- Malonate
- Azides
- 2,4-dinitrophenol
- Oligomycin
- Antimicin
Which ETC poison is an extremely effective reversible inhibitor of cytochrome oxidase?
Cyanide
Which ETC poison is a competitive inhibitor of succinate dehydrogenase?
Malonate
What ETC poison binds to heme groups in cytochromes?
Azides
What ETC poisson acts as an uncoupling agent?
2,4-dintrophenol
Which ETC poison inhibits proton flow through ATP synthase?
Oligomycin
Which ETC poison binds to cytochrome c?
Antimicin
