Anaerobic respiration

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This is a mechanism in which ATP synthesis is linked to some chemiosmotic process. Exergonic redox reactions constitute an electron transfer sequence which maintains a transmembrane electrochemical ion gradient and dissipation of the gradient through an ATP synthase complex allows the free energy so released to be harnessed as ATP.
Anaerobic respiration is named from its conceptual relation to mitochondrial aerobic respiration but it occurs under anoxic conditions. Oxygen is not available as the ultimate electron acceptor and so an alternative oxidising agent has to be used as the dump for reducing equivalents that pass along the electron transport chain. This means that water is not the reduced end product of electron transport in these bacteria.

Although the redox reactions that power ATP synthesis initiate an electron transport sequence quite analogous to mitochondrial events the "electron transport chain" typically found in these bacteria is brief.

Another difference is that the electrogenic ion gradient does not have to be a proton gradient. Sodium ion gradients are formed in some bacteria and the resulting "sodium motive force" has the necessary electrochemical energy to bring about ATP synthesis.

Anaerobic respiration in the Autotrophs

One group of autotrophic micro-organisms that employ anaerobic respiration as their sole source of ATP are the methanogens.

Another group of bacteria that can also derive all their ATP by anaerobic respiration are the homoacetogens.

In both of these groups of organisms a commonly used source of reducing equivalents for electron transport is hydrogen, which is taken up from the growth environment where it is present as a waste product of the metabolism of other ("donor") bacteria. Methanogens and homoacetogens live in syntrophic relations with the hydrogen producers and "interspecies hydrogen transfer" is a key concept underlying such syntrophy because the "donor" organisms also grow better as a result of hydrogen utilisation. The reason for this is analysed later in this CAL where we deal with methanogenisis.

When hydrogen is used in this way the oxidising agent that is often used (in place of oxygen) is carbon dioxide. This is also taken up form the growth environment, where it also is present as an excretion product of the metabolism of other bacteria.

The methanogens and homoacetogens are chemoautotrophs; they use the carbon dioxide as the sole source of carbon for growth. What makes them special is that they also use a proportion of this growth substrate as the ultimate oxidising agent in a chemiosmotic process that generates ATP.

Methane and acetate are just the end products of carbon dioxide reduction that show how different bacterial species have found ways of achieving an anaerobic chemiosmotic mechanism for ATP synthesis.

Some heterotrophic microorganisms also derive some ATP by anaerobic respiration.

Some propionate producers, which are heterotrophs, can derive some of their ATP by anaerobic respiration, to supplement the ATP obtained by substrate level phosphorylation. They take up organic nutrient chemicals from their environment which they use as carbon substrates for fermentation and to supply reducing equivalents which support anaerobic respiration [1].


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