Acetyl CoA reductive pathway

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The reductive acetyl-CoA pathway (also Wood-Ljungdahl pathway ) is a metabolic pathway of autotrophic , strictly anaerobic microorganisms and is used for carbon dioxide assimilation , but also for energy generation. This route is the biological equivalent of the Monsanto process , in which acetate is produced in a technical way .

Occurrence

The reductive acetyl-CoA pathway is known in all homoacetogenic and many sulfate - reducing gram-positive bacteria . Based on Moor Ella thermoacetica most studies have been conducted about the biochemistry of the reductive acetyl-CoA pathway.

The metabolic pathway has also been demonstrated in many methanogenic archaea in the Euryarchaeota department . The sulphate-reducing archaea of ​​the genera Archaeoglobus (e.g. Archaeoglobus fulgidus ) and Ferroglobus also operate this path.

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

biochemistry

The reductive acetyl-CoA route is a non-cyclical route of fixing carbon dioxide (CO 2 ) or various C 1 compounds such as formate , methanol , carbon monoxide , methylamine or methyl ether . Elemental hydrogen (H 2 ) serves as the reducing agent . The above-mentioned microorganisms can grow with these C 1 compounds and H 2 , since acetate formation is energetically favorable under standard conditions (ΔG 0 '= −95 kJ / mol or −104 kJ / mol):

Four molecules of hydrogen and two molecules of carbon dioxide are converted into one molecule of acetic acid (here dissociated) and two molecules of water.

The key enzyme here is the highly oxidation-sensitive CO dehydrogenase / acetyl-CoA synthase ( CODH / ACS or acetyl-CoA synthase complex ), which has both carbon monoxide dehydrogenase activity ( CO dehydrogenase ) and can synthesize acetyl-CoA ( Acetyl CoA synthase ). The fixation includes two branches named after their discoverers. In the “ methyl branch”, CO 2 is reduced to a methyl residue (CH 3 ), which was discovered through the scientific work of Lars Ljungdahl . Harland Wood's work clarified the way in the so-called “ carbonyl branch”, in which the second molecule of CO 2 is reduced to carbon monoxide (CO). In the literature, the designations eastern ( eastern ) and western ( western ) branch for the methyl and carbonyl branch are also known.

Many variants of the reductive acetyl-CoA pathway are known. The biochemical processes that take place have so far only been identified in acetogens, methanogens and sulfate reducers. They differ in the coenzymes involved and the electron carriers. In general, the enzymes of the carbonyl branch involved in the process show a high degree of homology among bacteria and archaea.

In the following the path for bacteria is described, further down there is a chapter on the path in archaea.

The reductive acetyl-CoA pathway in bacteria, please see text for details. The metabolic pathway is not a cycle, but the coenzymes involved are regenerated in smaller cycles.

Methyl branch

A molecule of CO 2 is reduced to formic acid by a formate dehydrogenase ( EC  1.2.1.43 ) with consumption of NADPH , which is present as formate (HCOO - ) under physiological conditions . Dehydrogenase is an enzyme that contains tungsten and selenocysteine . Formate then condenses with tetrahydrofolate (TH 4 ) to form N 10 -formyl-FH 4 , one molecule of ATP is required. This reaction is catalyzed by a 10-formyl-THF synthetase ( EC  6.3.4.3 ), a homotetramer in M. thermoacetica . N 10 -formyl-FH 4 is finally converted to N 5 -methyl-FH 4 in several reduction steps , the formyl group being reduced to a methyl group. In Na + acetogens, this methyl group is finally transferred to the cobalt (I) atom of an organometallic methylcobamide (a vitamin B 12 derivative), a prosthetic group of the corrinoid iron sulfur protein (CFeSP or CoFeSP ) by a methyl transferase ). In order for the methyl group to be transferred from tetrahydrofolate to the cobalt atom, it must first be activated. It is suggested that for this purpose the nitrogen atom at position 5 of N 5 -methyl-FH 4 is protonated.

CFeSP then attaches to the acetyl-CoA synthesis complex and can thus feed in the methyl group for the subsequent condensation.

Carbonyl branch

The second molecule of CO 2 is reduced to carbon monoxide by a CO dehydrogenase, which remains enzyme-bound on the 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. The acetyl-CoA synthesis complex finally catalyzes the condensation of the methyl radical, the carbonyl radical and of coenzyme A to form acetyl-CoA :

Metabolic pathway in archaea

The reductive acetyl-CoA pathway in archaea, see text for details. The metabolic pathway is not a cycle there either, but the coenzymes involved are regenerated in smaller cycles.

The reductive acetyl-CoA pathway in archaea largely corresponds to that in bacteria. There are, however, a few differences (see also the picture on the right).

In archaea, for example, carbon dioxide is reductively bound to methanofuran in the methyl branch , ferredoxin is oxidized and formylmethanofuran is produced. Formylmethanofuran gives off the formyl group to tetrahydromethanopterin , which is used in place of tetrahydrofolate. This creates formyltetrahydromethanopterin (Formyl-H 4 MPT), which the formyl group has attached to N-5 instead of N-10 as with tetrahydrofolate. In contrast to path in bacteria, no ATP is required. The reduction of this formyl group proceeds in the same way as in bacteria, the electrons come from the cofactor F 420 .

The CO dehydrogenase also uses the reducing power of the cofactor F 420 to reduce carbon dioxide .

meaning

This type of CO 2 fixation is probably the oldest and was used a billion years before the first oxygen formation. The acetyl-CoA formed is converted to triose phosphate to build up cell components. Another molecule of CO 2 is fixed, which catalyzes a ferredoxin- dependent pyruvate synthase. This is finally converted into triose phosphate using three molecules of ATP.

A total of four molecules of ATP are used to build one molecule of triose phosphate. From an energetic point of view, the reductive acetyl-CoA route is therefore the most favorable route to carbon dioxide fixation. However, it needs many coenzymes and many rare metals (Fe, Co, Ni, Mo or W).

As with homoacetogenic bacteria, the path can also be used to generate energy ( homoacetate fermentation ). This breaks down one molecule of glucose into three molecules of acetate . Here glucose is converted into two molecules of pyruvate in glycolysis . These are oxidatively decarboxylated by a pyruvate ferredoxin oxidoreductase to form two molecules of acetyl CoA . The high-energy thioester bond in acetyl-CoA is used to generate ATP through substrate chain phosphorylation (via acetyl phosphate). As a result, not only acetate but also reduction equivalents and CO 2 are formed . The latter can then be converted to acetate in the reductive acetyl-CoA route.

A proton gradient (e.g. in M. thermoacetica ) is also built up across the membrane. This is used by an ATPase for further proton-driven ATP synthesis. Some acetogens, such as 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 metabolic pathway can run in both directions. Either - as described above - acetate can be produced from C 1 compounds. However, some microorganisms can also grow on acetate through it, as this is broken down into two molecules of CO 2 in the Wood-Ljungdahl path ; in some cases this total oxidation of acetate to carbon dioxide even replaces the Krebs cycle.

The reversible machinery of the metabolic pathway is also used in methanogenesis in C 1 compounds, which produces both carbon dioxide and methane.

literature

  • Drake, HL. et al. (2008): Old acetogens, new light . In: Ann NY Acad Sci . 1125 ; 100-128; doi: 10.1196 / annals.1419.016 ; PMID 18378590 .
  • Katharina Munk (Ed.): Pocket textbook Biology: Microbiology . Thieme Verlag Stuttgart 2008; ISBN 978-3-13-144861-3 ; P. 413
  • Georg Fuchs (ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag Stuttgart; 8th edition 2007; ISBN 3-13-444608-1 ; P. 248f.
  • Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 3-8274-1107-6 ; P. 251ff.
  • Ragsdale, SW. and Pierce, E. (2008): Acetogenesis and the Wood-Ljungdahl pathway of CO (2) fixation . In: Biochim Biophys Acta . 1784 (12); 1873-1898; PMID 18801467 ; PMC 2646786 (free full text)
  • Ragsdale, SW. (2004): Life with carbon monoxide . In: Crit Rev Biochem Mol Biol. 39 (3); 165-195; PMID 15596550 ; PDF (free full text access)

Individual evidence

  1. ↑ M. thermoacetica in KEGG ; In 1994 the name was changed to Clostridium thermoaceticum
  2. ^ Berg, IA. et al . (2010): Study of the distribution of autotrophic CO2 fixation cycles in Crenarchaeota . In: Microbiology 156 (Pt 1); 256-269; PMID 19850614 ; doi: 10.1099 / mic.0.034298-0
  3. ↑ a b Ragsdale, SW. (2004): Life with carbon monoxide . In: Crit Rev Biochem Mol Biol. 39 (3); 165-195; PMID 15596550 ; PDF (free full text access)
  4. ↑ Wolfgang Fritsche: Microbiology . Spectrum Academic Publishing House; 3rd edition 2001; ISBN 3-8274-1107-6 ; P. 252.
  5. ↑ Ragsdale, SW. (1997): The eastern and western branches of the Wood / Ljungdahl pathway: how the east and west were won. In: Biofactors 6 (1); 3-11; PMID 9233535
  6. ↑ Borrel G, Adam PS, Gribaldo S. Methanogenesis and the Wood-Ljungdahl Pathway: An Ancient, Versatile, and Fragile Association. Genome Biol Evol. 2016 Jun 13; 8 (6): 1706-11. doi: 10.1093 / gbe / evw114. PMID 27189979
  7. ↑ Martin, W. and Russell, MJ. (2007): On the origin of biochemistry at an alkaline hydrothermal vent . In: Philos Trans R Soc Lond B Biol Sci . 362 (1486); 1887-1925; PMID 17255002 ; PMC 2442388 (free full text).
  8. ↑ Hu, SI. et al (1984): Acetate synthesis from carbon monoxide by Clostridium thermoaceticum. Purification of the corrinoid protein . In: J Biol Chem . 259 (14); 8892-8897; PMID 6746629 ; PDF (free full text access)
  9. ↑ Ragsdale, SW. (2008): Enzymology of the wood-Ljungdahl pathway of acetogenesis . In: Ann NY Acad Sci. 1125; 129-136; doi: 10.1196 / annals.1419.015 ; PMID 1837859 .
  10. ↑ Georg Fuchs: Alternative pathways of carbon dioxide fixation: insights into the early evolution of life? Annu Rev Microbiol. 2011; 65: 631-58. doi: 10.1146 / annurev-micro-090110-102801 , PMID 21740227

See also