FIG. 6.
The 4-hydroxybutyrate cycles of autotrophic CO2 fixation (14). (A) The dicarboxylate/4-hydroxybutyrate cycle functioning in Desulfurococcales and Thermoproteales; (B) the 3-hydroxypropionate/4-hydroxybutyrate cycle functioning in Sulfolobales. Note that succinyl-CoA reductase in Thermoproteales and Sulfolobales uses NADPH (13, 62, 81, 82) and probably reduced ferredoxin (Fdred) in Desulfurococcales (15, 46). Enzymes: 1, pyruvate synthase; 2, pyruvate:water dikinase; 3, PEP carboxylase; 4, malate dehydrogenase; 5, fumarate hydratase; 6, fumarate reductase (natural electron acceptor is not known); 7, succinyl-CoA synthetase; 8, acetyl-CoA/propionyl-CoA carboxylase; 9, malonyl-CoA reductase; 10, malonic semialdehyde reductase; 11, 3-hydroxypropionate-CoA ligase; 12, 3-hydroxypropionyl-CoA dehydratase; 13, acryloyl- CoA reductase; 14, methylmalonyl-CoA epimerase; 15, methylmalonyl-CoA mutase; 16, succinyl-CoA reductase; 17, succinic semialdehyde reductase; 18, 4-hydroxybutyrate-CoA ligase; 19, 4-hydroxybutyryl-CoA dehydratase; 20, crotonyl-CoA hydratase; 21, (S)-3-hydroxybutyryl-CoA dehydrogenase (NAD+); 22, acetoacetyl-CoA β-ketothiolase. Fd, ferredoxin. (Adapted from reference 14.)