dicarboxylate/4-hydroxybutyrate cycle

Edge evidence

• carbon dioxide fixed by dicarboxylate/4-hydroxybutyrate cycle METPO:2007404

  CO2 is fixed by the anaerobic archaeal DC/4HB cycle.

  • DOI:10.1128/AEM.02473-10 Berg describes the DC/4HB cycle as the anaerobic counterpart of the 3HP/4HB cycle.
• dicarboxylate/4-hydroxybutyrate cycle contributes to carbon fixation RO:0002326

  The DC/4HB cycle is one of the recognized autotrophic CO2 pathways.

  • DOI:10.1126/science.1149976 Berg et al. establish 4-hydroxybutyrate chemistry shared by the cycle in Crenarchaeota.
• molecular oxygen negatively regulates dicarboxylate/4-hydroxybutyrate cycle

  O2 constrains the cycle because key enzymes and electron carriers are oxygen-sensitive, restricting it to anoxic conditions.

  • DOI:10.1128/AEM.02473-10 Restricted to organisms growing under anoxic conditions due to oxygen sensitivity of key enzymes and electron carriers.
• carbon dioxide is cosubstrate for pyruvate synthase (pyruvate:ferredoxin oxidoreductase)

  CO2 serves as the carboxylation cosubstrate for pyruvate synthase in the cycle.

  • DOI:10.1002/9783527629916 CO2 as cosubstrate for pyruvate synthase.
• bicarbonate is cosubstrate for phosphoenolpyruvate carboxylase

  Bicarbonate is the inorganic carbon cosubstrate for PEP carboxylase, supporting mixed CO2/HCO3- usage.

  • DOI:10.1002/9783527629916 Bicarbonate as cosubstrate for PEP carboxylase.
• pyruvate synthase (pyruvate:ferredoxin oxidoreductase) catalyzes conversion of acetyl-CoA

  Pyruvate synthase reductively carboxylates acetyl-CoA to pyruvate, the first fixation step.

  • DOI:10.1002/9783527629916 Starts from acetyl-CoA, which is reductively carboxylated to pyruvate.
• pyruvate synthase (pyruvate:ferredoxin oxidoreductase) produces pyruvate METPO:2000202

  Reductive carboxylation of acetyl-CoA by pyruvate synthase yields pyruvate.

  • DOI:10.1002/9783527629916 Acetyl-CoA is reductively carboxylated to pyruvate.
• phosphoenolpyruvate carboxylase catalyzes conversion of phosphoenolpyruvate

  PEP carboxylase carboxylates phosphoenolpyruvate to oxaloacetate in the dicarboxylate stage.

  • DOI:10.1002/9783527629916 Pyruvate is converted to PEP and then carboxylated to oxaloacetate.
• phosphoenolpyruvate carboxylase produces oxaloacetate METPO:2000202

  Carboxylation of PEP by PEP carboxylase yields oxaloacetate.

  • DOI:10.1002/9783527629916 PEP is carboxylated to oxaloacetate.
• 4-hydroxybutyryl-CoA dehydratase catalyzes conversion of 4-hydroxybutyryl-CoA

  Hallmark radical 4-hydroxybutyryl-CoA dehydratase dehydrates 4-hydroxybutyryl-CoA to crotonyl-CoA.

  • DOI:10.1073/pnas.0801043105 Dehydrated by a radical 4-hydroxybutyryl-CoA dehydratase to crotonyl-CoA; core hallmark enzyme of the 4HB branch.
• 4-hydroxybutyryl-CoA dehydratase produces crotonyl-CoA METPO:2000202

  The radical dehydratase yields crotonyl-CoA in the 4-hydroxybutyrate branch.

  • DOI:10.1073/pnas.0801043105 Dehydrated to crotonyl-CoA.
• crotonyl-CoA is converted via beta-oxidation to acetyl-CoA

  Crotonyl-CoA is cleaved via beta-oxidation to regenerate two acetyl-CoA, closing the cycle.

  • DOI:10.1073/pnas.0801043105 Via beta-oxidation yields two acetyl-CoA.