3-hydroxypropionate/4-hydroxybutyrate cycle

Edge evidence

• hydrogencarbonate fixed by 3-hydroxypropionate/4-hydroxybutyrate cycle METPO:2007404

  Two molecules of bicarbonate are fixed per cycle turn.

  • DOI:10.1126/science.1149976 Berg et al. describe the 3HP/4HB cycle in Sulfolobales.
• 3-hydroxypropionate/4-hydroxybutyrate cycle contributes to carbon fixation RO:0002326

  The 3HP/4HB cycle is among the six recognized autotrophic pathways.

  • DOI:10.1128/AEM.02473-10 Berg situates the 3HP/4HB cycle among recognized carbon-fixation pathways.
• 3-hydroxypropionate/4-hydroxybutyrate cycle produces acetyl-CoA METPO:2000202

  A complete cycle turn fixes two bicarbonate to produce one acetyl-CoA.

  • DOI:10.1186/s40643-023-00705-9 A complete 3HP/4HB cycle fixes two HCO3- to produce one acetyl-CoA.
• 3-hydroxypropionate/4-hydroxybutyrate cycle forms intermediate succinyl-CoA

  Carboxylation of acetyl-CoA/propionyl-CoA yields succinyl-CoA from two bicarbonate.

  • DOI:10.1186/s40643-023-00705-9 The pathway carboxylates acetyl-CoA/propionyl-CoA to form succinyl-CoA from two HCO3-.
• succinyl-CoA is reduced to 4-hydroxybutyrate

  Succinyl-CoA is reduced to 4-hydroxybutyrate.

  • DOI:10.1186/s40643-023-00705-9 Succinyl-CoA is reduced to 4-hydroxybutyrate.
• 4-hydroxybutyryl-CoA dehydratase dehydrates 4-hydroxybutyryl-CoA

  4-hydroxybutyryl-CoA is dehydrated to crotonyl-CoA.

  • DOI:10.1038/s42003-024-06432-x The cycle dehydrates 4-hydroxybutyryl-CoA to crotonyl-CoA.
• 4-hydroxybutyryl-CoA dehydratase produces crotonyl-CoA METPO:2000202

  Dehydration of 4-hydroxybutyryl-CoA yields crotonyl-CoA.

  • DOI:10.1038/s42003-024-06432-x The pathway diagram shows dehydration to crotonyl-CoA.
• crotonyl-CoA regenerates acetyl-CoA

  Crotonyl-CoA is processed/cleaved to regenerate acetyl-CoA for further cycle turns.

  • DOI:10.1186/s40643-023-00705-9 The pathway finally cleaves to yield acetyl-CoA for further turns.
• NADPH is required by 3-hydroxypropionate/4-hydroxybutyrate cycle

  Reductive steps of the cycle consume NADPH (about four NADPH per turn).

  • DOI:10.1186/s40643-023-00705-9 Five reactions in the cycle require oxidation of NADPH; full cycle consumes four NADPH.
• acetyl-CoA/propionyl-CoA carboxylase mediates carboxylation in 3-hydroxypropionate/4-hydroxybutyrate cycle

  Acetyl-CoA/propionyl-CoA carboxylase is the primary carboxylase fixing inorganic carbon.

  • DOI:10.1038/s42003-024-06432-x The cycle fixes CO2 onto acetyl-CoA via an acetyl-CoA/propionyl-CoA carboxylase, the primary carboxylase.