17. Anaerobic Respiration
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- means passing electrons down an ETC and reducing an exogenous electron acceptor
- electron acceptor is oxygen for almost all eukaryotes
- many bacteria can use alternative electron acceptors at the end of their ETCs - can grow using respiration in the absence of oxygen
compounds are reduced for the purpose of binding cellular macromolecules and the cell only reduces the amount needed for growth
compounds are reduced for the purpose of energy conservation, large amounts are reduced and the cell excretes the reduced product into the environment
anaerobic respiration systems
- ferric iron reduction
the conversion of nitrite into gaseous compounds that are lost from the soil and water
E.coli can use O2 or NO3- as an electron acceptor
nitrite is only reduced to nitrite by E.coli - nitrate reduction
fewer protons are transferred out of the cell during nitrate reduction than during oxygen reduction or full denitrification
nitrate reduction plus denitrification
Paracoccus denitrificans and Pseudomonas stutzeri reduce nitrate to nitrogen gas
this pathway pumps more protons
than nitrate reduction to nitrite, since protons are also extruded by the nitric oxide reductase
- often performed by facultative aerobes:
- for nitrate reduction to occur, oxygen must be absent and nitrate must be present - ensures that E.coli use the most energetically favorable electron acceptor first
Ferric iron reduction
is abundant in nature - often in the solid phase as part of minerals
- Bacteria use 3 distinct mechanisms to access mineralized Fe3+:
- 1) produce and excrete chelators for Fe3+ to make it soluble
- 2) use electron shuffling compounds that can reduce Fe3+ away from the cell - quinone-like molecules
- 3) touch the mineral directly - type IV pili are required, thought to have cytochromes attached to them to make them electrically conductive
- performed by Archaea that are strict anaerobes
- often the final stage in biodegradation of organic matter in anoxic habitats
- important part of carbon cycle
- occurs where fermenters generate large amounts of H2 and CO2
- requires cofactors that are unique to methanogens
one-carbon carriers in methanogenesis
Methanofuran: (MF)-amino nitrogen binds CO2 in the first step of methanogenesis
Methanopterin: (MP)-C1 carrier in intermediate steps of reduction to CH4
Coenzyme M (CoM): methyl carrier in next to last step of reduction to methane
last C1 carrier in methanogenesis
- Coenzyme F430
- part of the methyl reductase enzyme complex
- participates in the final step of methanogenesis by removing the methyl group from CoM to form a Ni2+-CH3 intermediate
- this is reduced by electrons from CoB to generate CH4
electron donors in methanogenesis
coenzyme F420: electron donor in intermediate steps of methanogenesis
conenzyme B (CoB): electron donor in final steps of methanogenesis
- 1) H2 reduces ferredoxin, which donates electrons to produce a formyl group attached to the amino nitrogen of MF
- 2) formyl group transferred to MP. It is dehydrated and reduced in 2 steps to a methyl group using electrons from F420.
- 3) methyl transferase enzyme transfers methyl group to CoM. this reaction is highly exergonic and yields energy to pump Na+ out of the cell.
- Na+ gradient can be converted to H+ gradient or used directly to power other cellular reactions.
- 4) In methyl reductase complex, the methyl group is moved to Ni2+ of F430. This complex is reduced by electrons from CoB, forming CH4 and a disulfide link between CoM and CoB.
- 5) Free CoM and CoB are regenerated by reduction of the dissulfide with H2.
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