Dicarboxylat/4-Hydroxybutyratzyklus - Dicarboxylat/4-Hydroxybutyratzyklus
The dicarboxylate / 4 Hydroxybutyratzyklus is a biochemical substance cycle, it some Archaea allowed, carbon dioxide (CO 2 ), in the form of bicarbonate (HCO 3 - to,) assimilate . The cycle can be divided into two halves: First, a molecule of succinyl-CoA is formed from acetyl-CoA , a molecule of carbon dioxide and bicarbonate . In the second part, 4-hydroxybutyrate then creates two molecules of acetyl-CoA, one of which is used for the next run. Hence the cycle owes its name to being the first intermediateDicarboxylic acids and, later, 4-hydroxybutyrate are formed.
The metabolic pathway was originally demonstrated in the hyperthermophilic Archaeon Ignicoccus hospitalis of the Crenarchaeota department . I. hospitalis is the host of the obligatory parasite Nanoarchaeum equitans , also an archaeon .  I. hospitalis lives chemolitoautotroph under anaerobic conditions.
The dicarboxylate / 4-hydroxybutyrate cycle has also been detected in Thermoproteus neutrophilus , a strictly anaerobic archaeon, and Pyrolobus fumarii , an optional aerobic archaeon.   The former belongs to the order Thermoproteales , the latter to the Desulfurococcales , both representatives of the Crenarchaeota.
Starting from acetyl-CoA , CO 2 is assimilated to pyruvate by an oxygen-sensitive pyruvate synthase , whereby reduction equivalents are required. Probably these come from reduced ferredoxins . Pyruvate is then converted to phosphoenolpyruvate (PEP) using ATP , which catalyzes a pyruvate: water dikinase. A molecule of bicarbonate then reacts with PEP to form oxaloacetate through an archean PEP carboxylase.
Oxaloacetate is converted to succinyl-CoA analogously in the reductive citric acid cycle via L - malate , fumarate , succinate . This consumes ATP and other reduction equivalents. The cycle owes half of its name to these isolable dicarboxylic acids. Succinyl-CoA is converted into 4-hydroxybutyrate by a succinyl-CoA reductase and a succinate semialdehyde reductase with the consumption of further reduction equivalents.
4-Hydroxybutyrate is then converted to 4-hydroxybutyryl-CoA through several enzymatic reactions. From this, through the key enzyme of the cycle, 4-hydroxybutyryl-CoA dehydratase, crotonyl-CoA is created . The hydratase, a 4Fe-4S and FAD -containing enzyme catalyzes the release of water through a Ketylradikalmechanismus.  Further catalytic reactions finally transform this into acetoacetyl-CoA via 3-hydroxybutryryl-CoA . An acetoacetyl-CoA β- ketothiolase finally splits acetoacetyl-CoA into two molecules of acetyl-CoA, thereby closing the cycle.
The balance for the formation of one acetyl-CoA molecule is as follows:
Acetyl-CoA is then usually fixed by fixing another molecule of CO 2 and consuming reduction equivalents, e.g. B. Ferredoxin (Fd), as well as ATP converted to glyceraldehyde-3-phosphate . This can then be further metabolized in the carbohydrate metabolism.
As a result, the overall balance for the formation of one molecule of glyceraldehyde-3-phosphate (GAP) is:
This metabolic pathway is the last discovered way to fix CO 2 and is similar to the 3-hydroxypropionate / 4-hydroxybutyrate cycle . However, the formation of succinyl-CoA takes place via dicarboxylic acids; from 4-hydroxybutyrate onwards, the two cycles correspond again to a large extent.
It should be noted, however, that with the Calvin cycle, some energy (and reduction equivalents) are always lost due to the photorespiration that occurs . As a result, the ATP and NAD (P) consumption given in the equation applies under optimal and not under the prevailing conditions in our oxygen-rich atmosphere.
Because of the numerous oxygen-sensitive iron-sulfur proteins and ferredoxins involved, this cycle only takes place under strictly anaerobic or microaerobic conditions. The metabolic pathway is possibly restricted to only a few Crenarchaeota . Whether this is one of the first autotrophic metabolic pathways is still being discussed.
- Huber, H. et al. (2008): A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis. In: Proc Natl Acad Sci USA 105(22); 7851–7856; PMID 18511565; PMC 2409403 (freier Volltext)
- Podar, M. et al. (2008): A genomic analysis of the archaeal system Ignicoccus hospitalis-Nanoarchaeum equitans. In: Genome Biol 9(11); R158; PMID 19000309; PMC 2614490 (freier Volltext).
- Ramos-Vera, WH. et al. (2009): Autotrophic carbon dioxide assimilation in Thermoproteales revisited. In: J Bacteriol 191(13); 4286–4297; PMID 19411323; PMC 2698501 (freier Volltext).
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