Wood–Ljungdahl pathway

The reductive acetyl -CoA pathway ( Wood - Ljungdahl pathway also ) is a pathway of autotrophic, strictly anaerobic microorganisms and is used for carbon dioxide assimilation, but also for energy. This path is the biological equivalent of the Monsanto process is illustrated in the technical ways to acetate.

Occurrence

Of the acetyl-CoA reductive pathway is known to all, as well as many homoacetogenen sulfate reducing gram-positive bacteria. Based mud screen thermoacetica most studies of the biochemistry of the acetyl-CoA reductive pathway have been carried out.

The pathway has also been demonstrated in many methanogenic archaea of the Euryarchaeota department. The sulfate-reducing archaea of the genera Archaeoglobus (eg Archaeoglobus fulgidus ) and Ferroglobus this way also operate.

In all cases, the reductive acetyl -CoA pathway proceeds only under anaerobic conditions.

Biochemistry

In the reductive acetyl -CoA pathway is a non-cyclic path of the fixation of carbon dioxide ( CO2 ) and various C1 compounds such as formate, methanol, carbon monoxide, methyl amine or methyl. As reducing agent in this case is elemental hydrogen (H2). The above-mentioned microorganisms can grow with these C1 compounds and H2, because the acetate formation under standard conditions is energetically favorable ( ΔG0 '= -95 kJ / mol and -104 kJ / mol):

The key enzyme in this case, the high oxidation-sensitive CO dehydrogenase / acetyl-CoA synthase ( CODH / ACS, or acetyl -CoA synthase complex), which has both a carbon monoxide dehydrogenase activity (CO dehydrogenase ), as well as acetyl-CoA to synthesize ( acetyl-CoA synthase).

The fix includes two branches, which were named after their discoverers. In the " methyl branch " CO2 to a methyl group ( CH3) is reduced, which was discovered by the scientific work of Lars Ljungdahl. The metabolic pathways are present in all known living creatures in a similar form. Harland Woods work cleared the way in the so-called "carbonyl - branch ", in which the second molecule of CO2 is reduced to carbon monoxide (CO).

There are many known variants of the reductive acetyl-CoA pathway. They differ in the participating coenzymes and the electron carriers. The thereby occurring biochemical processes have so far been identified only in acetogenic, methanogenic and sulfate-reducers.

In the literature, the terms East (eastern ) and western (western ) are also known for the branch methyl or carbonyl branch. The following is the way for bacteria is described, below you will find a chapter on the way in archaea.

Methyl branch

One molecule of CO2 is reduced by formate dehydrogenase ( EC 1.2.1.43 ) under consumption of NADPH to formic acid, which is present under physiological conditions as formate ( HCOO - ). The dehydrogenase is a tungsten - and selenocysteine ​​- contained enzyme. Formate then condensed with tetrahydrofolate ( TH4 ) to N10 -formyl- FH4, one molecule of ATP is required. This reaction catalyzes a 10 -formyl -THF synthetase ( EC 6.3.4.3 ), a homotetramer in M. thermoacetica. N10 -formyl- FH4 is finally reacted by multiple reduction steps to N5 -methyl- FH4, while the formyl group is reduced to a methylene group. This methyl group is finally transmitted at Na acetogenic by a methyltransferase to the cobalt (I) atom, an organometallic Methylcobamid ( vitamin B12 - derivative), a prosthetic group of corrinoid - iron-sulfur protein ( CFeSP or CoFeSP ). Thus, the methyl group from tetrahydrofolate to the cobalt atom can be transferred, they must first be activated. It is suggested that for this purpose the nitrogen atom is protonated at position 5 of N5 -methyl- FH4.

CFeSP then binds to the acetyl-CoA synthesis complex and can thus feed the methyl group for the next condensation.

Carbonyl branch

The second molecule of CO2 is reduced by a CO - dehydrogenase to carbon monoxide, which remains bound to the enzyme acetyl-CoA synthesis complex. If the microorganisms grow on CO, this can also be bound directly. The CODH is a nickel -containing iron-sulfur protein. Of the acetyl -CoA synthesis complex finally catalyzes the condensation of the methyl residue and the Carbonylrestes coenzyme A into acetyl- CoA:

Pathway in archaea

The reductive acetyl -CoA pathway in archaea largely corresponds in bacteria. However, there are a few differences ( see also right panel).

Thus, carbon dioxide reductively bound in archaea in the methyl branch at Methanofuran, while ferredoxin is oxidized and there is Formylmethanofuran. Formylmethanofuran outputs the formyl group to Tetrahydromethanopterin, which is used in place of tetrahydrofolate. Formyltetrahydromethanopterin thereby formed ( formyl H4MPT ), which the formyl group is bonded at the N- 5 instead of N-10 as in the tetrahydrofolate. In contrast to the way in bacteria so no ATP is needed. The reduction of the formyl group is analogous as in bacteria, the electrons come from the cofactor F420.

For the reduction of carbon dioxide using the CO dehydrogenase the reducing power of the co F420.

Importance

In this type of CO2 fixation is probably the oldest and operated a billion years before the first oxygen formation. The resulting acetyl-CoA is converted to the structure of cell components to triosephosphate on. Here, another molecule of CO2 is fixed, what a ferredoxin -dependent Pyruvatsynthase catalyzed. This will eventually be implemented with the consumption of three molecules of ATP to triosephosphate.

Four molecules of ATP are consumed for the development of a molecule triosephosphate. Thus, the reductive acetyl -CoA pathway is energetically the most affordable way for carbon dioxide fixation. However, it takes a lot of coenzymes and many rare metals (Fe, Co, Ni, Mo or W).

But the path can also, as in homoacetogenen bacteria are used to produce energy ( Homoacetatgärung ). Thus, a molecule of glucose is broken down to three molecules of acetate. Here, glucose is converted into two molecules of glycolysis to pyruvate. These are oxidatively decarboxylated by pyruvate - ferredoxin oxidoreductase to two molecules of acetyl -CoA. The energy-rich thioester bond to acetyl -CoA is used to generate ATP by substrate chain ( about acetyl ). As a result, formed in addition acetate also reducing equivalents and CO2. The latter can then be implemented in the reductive acetyl -CoA pathway to acetate.

It is established, a proton gradient (eg M. thermoacetica ) across the membrane. This is used by an ATPase for further proton -driven ATP synthesis. Some acetogenic, for example Acetobacterium woodii or Propionigenium modestum, translocate sodium ions instead of protons. Accordingly, they have a sodium ion dependent ATP synthase.

Reversibility of the way

The pathway can proceed in both directions. Either - as described above - derived from C1 compounds acetate. However, some microorganisms can also grow through it on acetate, since it is degraded in the Wood - Ljungdahl pathway to two molecules of CO2.