FIG. 1.
The autotrophic dicarboxylate/hydroxybutyrate cycle in T. neutrophilus. Arrows indicate where organic substrates feed into intermediate pools. Circled numbers represent enzymes as follows (electron carriers are given in parentheses): 1, pyruvate synthase (reduced methyl viologen [MV]); 2, pyruvate:water dikinase; 3, PEP carboxylase; 4, malate dehydrogenase (NADH); 5, fumarate hydratase; 6, fumarate reductase (reduced MV); 7, succinyl-CoA synthetase (ADP forming); 8, succinyl-CoA reductase (NADPH); 9, succinic semialdehyde reductase (NADPH); 10, 4-hydroxybutyrate-CoA ligase (AMP forming); 11, 4-hydroxybutyryl-CoA dehydratase; 12, crotonyl-CoA hydratase; 13, (S)-3-hydroxybutyryl-CoA dehydrogenase (NAD+); 14, acetoacetyl-CoA β-ketothiolase. Fdred, reduced ferredoxin. Reactions 12 and 13 are catalyzed by a bifunctional enzyme. MV may replace ferredoxin as an electron carrier.