Fig. 2.

Proposed dicarboxylate/4-hydroxybutyrate cycle for autotrophic CO₂ fixation in I. hospitalis. Enzymes: 1, pyruvate synthase (reduced MV); 2, pyruvate:water dikinase; 3, PEP carboxylase; 4, malate dehydrogenase (NADH); 5, fumarate hydratase; 6, fumarate reductase (reduced MV); 7, succinate thiokinase (ADP forming); 8, succinyl-CoA reductase (reduced MV); 9, succinate semialdehyde reductase (NADPH); 10, 4-hydroxybutyryl-CoA synthetase (AMP forming); 11, 4-hydroxybutyryl-CoA dehydratase; 12, crotonyl-CoA hydratase; 13, 3-hydroxybutyryl-CoA dehydrogenase (NAD⁺); 14, acetoacetyl-CoA β-ketothiolase. Label from [1,4-¹³C₂]succinate is indicated by filled circles.