Hydroxypropionyl-Coenzyme A Dehydratase and Acryloyl-Coenzyme A Reductase, Enzymes of the Autotrophic 3-Hydroxypropionate/4-Hydroxybutyrate Cycle in the Sulfolobales

A 3-hydroxypropionate/4-hydroxybutyrate cycle operates in autotrophic CO2 fixation in various Crenarchaea, as studied in some detail in Metallosphaera sedula. This cycle and the autotrophic 3-hydroxypropionate cycle in Chloroflexus aurantiacus have in common the conversion of acetyl-coenzyme A (CoA) and two bicarbonates via 3-hydroxypropionate to succinyl-CoA. Both cycles require the reductive conversion of 3-hydroxypropionate to propionyl-CoA. In M. sedula the reaction sequence is catalyzed by three enzymes. The first enzyme, 3-hydroxypropionyl-CoA synthetase, catalyzes the CoA- and MgATP-dependent formation of 3-hydroxypropionyl-CoA. The next two enzymes were purified from M. sedula or Sulfolobus tokodaii and studied. 3-Hydroxypropionyl-CoA dehydratase, a member of the enoyl-CoA hydratase family, eliminates water from 3-hydroxypropionyl-CoA to form acryloyl-CoA. Acryloyl-CoA reductase, a member of the zinc-containing alcohol dehydrogenase family, reduces acryloyl-CoA with NADPH to propionyl-CoA. Genes highly similar to the Metallosphaera CoA synthetase, dehydratase, and reductase genes were found in autotrophic members of the Sulfolobales. The encoded enzymes are only distantly related to the respective three enzyme domains of propionyl-CoA synthase from C. aurantiacus, where this trifunctional enzyme catalyzes all three reactions. This indicates that the autotrophic carbon fixation cycles in Chloroflexus and in the Sulfolobales evolved independently and that different genes/enzymes have been recruited in the two lineages that catalyze the same kinds of reactions.