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Anaerobic Respiration

Anaerobic respiration is a process that generates cell energy by coupling membrane-associated electron transfer reactions using an electron acceptor other than O2. The process creates a membrane potential across the cytoplasmic membrane called the proton motive force (pmf). The cell then uses this energy to drive ATP synthesis using the membrane-bound ATP synthase (electron transport phosphorylation).

 

Key Concepts

  • Anaerobic respiratory chains are located in the cytoplasmic membranes and generate a proton motive force (pmf).
  • The electron transport chains always consist of an electron donating dehydrogenase and an electron accepting terminal reductase.
  • Menaquinone (MQ) is the electron mediator between the enzyme complexes.
  • The proton motive force drives ATP synthesis via the membrane-bound ATP synthase.
  • Anaerobic respiration generates the majority of the cell energy under anaerobic growth conditions.
  • There are many compounds that can support anaerobic respiration in E. coli.

 

Principles of Anaerobic Respiration

Anaerobic respiration supports growth of E. coli cells under conditions when suitable electron donors (DH) and acceptors (A) are present. There are a variety of different inorganic and organic donors and acceptors that can be used, and each respiratory substrate requires a specific membrane bound enzyme for its utilization.

Together, these donor and acceptor enzymes form modular electron transport chains that consist of a membrane-associated dehydrogenase enzyme that transfers electrons to an anaerobic terminal reductase enzyme. The overall reaction is represented by:

DH + A D + AH

Example 1: Anaerobic electron transfer from formate to nitrate

In the following example of anaerobic respiration, formate (HCOO-) serves as the electron donor and nitrate (NO3-) is the electron acceptor. Formate is first oxidized by formate dehydrogenase N and electrons are then transferred to nitrate reductase which in turn reduces nitrate to nitrite (NO2-). The two enzymes form a modular electron transport chain.

This overall reaction is:
formate + NO3-  CO2 + NO2- + H2O

The individual reactions catalyzed by each enzyme in the above electron transport chain are:

Formate dehydrogenase N:
formate + MQ    CO2 + MQH2

Nitrate reductase:
MQH2  + NO3-  NO2- + H2O + MQ

Under anaerobic conditions, the lipid soluble cofactor menaquinone (MQ) mediates (i.e., transfers) electrons between the dehydrogenase and the reductase enzymes.

 

 

How E. coli Respires under anaerobic conditions

The E. coli genome encodes a variety of distinct dehydrogenase and terminal reductase enzymes that accomplish anaerobic respiration. Their synthesis usually requires the absence of O2 (anaerobiosis) and the presence of the respective enzyme substrate. Regardless of which enzymes are used, the resulting electron transport chain forms a proton motive force (pmf) that is then used for ATP synthesis and for other energy-requiring processes. 

 

Anaerobic Electron Donors

There are at least six compounds that E. coli can use as anaerobic electron donors: they include formate, hydrogen, NADH, lactate, glycerol-3-phosphate, and ethanol.

Electron donors and their dehydrogenases:

E. coli produces one or more specific dehydrogenase enzymes to oxidize each electron donor. Any of these dehydrogenases can donate electrons to any of the electron acceptor enzymes (i.e., terminal reductases) described below to form an electron transport chain. The synthesis of the individual enzyme is usually controlled by oxygen and the availability of the enzyme’s substrate.

  • Formate and formate dehydrogenase

    E. coli contains three formate dehydrogenase enzymes.

          
  • NADH and NADH dehydrogenase

    E. coli contains two NADH dehydrogenases.

          

Other electron donor enzymes:
Several other dehydrogenases may also function anaerobically to provide electrons to the terminal reductases. These include:

  • Hydrogen and hydrogenase       
  • Glycerol 3-phoshate and glycerol 3-P dehydrogenase       
  • Lactate and lactate dehydrogenase       

 

Anaerobic Electron Acceptors

There are at least five compounds that can function as anaerobic electron acceptors in E. coli. They include nitrate, nitrite, trimethylamine-N-oxide, dimemethyl-sufloxide, and fumarate. To catalyze their reduction, the cell must synthesize one or more substrate specific terminal reductase enzymes.

Electron Acceptors and their terminal reductase enzymes:

  • Nitrate and nitrate reductase

    E. coli contains genes for three distinct nitrate reductase enzymes that reduce nitrate to nitrite.

          
  • Nitrite and nitrite reductase       
  • TMAO and TMAO reductase       
  • DMSO and DMSO reductase       
  • Fumarate and fumarate reductase       

Example 2: Anaerobic electron transfer from NADH to nitrate

In the following example of anaerobic respiration, NADH serves as the electron donor and nitrate is the electron acceptor. NADH is first oxidized by NADH dehydrogenase and electrons are then transferred to nitrate reductase which in turn reduces nitrate to nitrite. The two enzymes form a modular electron transport chain.

This overall reaction is:
NADH + H+ + NO3- NAD+ + NO2- + H2O

The individual reactions catalyzed by each enzyme in the above electron transport chain are:

NADH dehydrogenase:
NADH + H+ + Q NAD+ + QH2

Nitrate reductase:
QH2  + NO3-   NO2- + H2O + Q

Under anaerobic conditions, the lipid soluble cofactor menaquinone (MQ) usually mediates (i.e., transfers) electrons between the dehydrogenase and the reductase enzyme. This is one of the few exceptions where ubiquinone (Q) is used in place of MQ.

 

 

 

Credits:

Authored by Robert Gunsalus and Imke Schröder
©The Escherichia coli Student Portal

This project acknowledges support from:
NIH Grant Award GM077678 to SRI, International
Peter Karp and coworkers at EcoCyc.org
The UCLA Department of MIMG