Autotrophic Carbon Dioxide Assimilation in Thermoproteales Revisited

For Crenarchaea, two new autotrophic carbon fixation cycles were recently described. Sulfolobales use the 3-hydroxypropionate/4-hydroxybutyrate cycle, with acetyl-coenzyme A (CoA)/propionyl-CoA carboxylase as the carboxylating enzyme. Ignicoccus hospitalis (Desulfurococcales) uses the dicarboxylate/4-hydroxybutyrate cycle, with pyruvate synthase and phosphoenolpyruvate carboxylase being responsible for CO₂ fixation. In the two cycles, acetyl-CoA and two inorganic carbons are transformed to succinyl-CoA by different routes, whereas the regeneration of acetyl-CoA from succinyl-CoA proceeds via the same route. Thermoproteales would be an exception to this unifying concept, since for Thermoproteus neutrophilus, the reductive citric acid cycle was proposed as a carbon fixation mechanism. Here, evidence is presented for the operation of the dicarboxylate/4-hydroxybutyrate cycle in this archaeon. All required enzyme activities were detected in large amounts. The key enzymes of the cycle were strongly upregulated under autotrophic growth conditions, indicating their involvement in autotrophic CO₂ fixation. The corresponding genes were identified in the genome. ¹⁴C-labeled 4-hydroxybutyrate was incorporated into the central building blocks in accordance with the key position of this compound in the cycle. Moreover, the results of previous ¹³C-labeling studies, which could be reconciled with a reductive citric acid cycle only when some assumptions were made, were perfectly in line with the new proposal. We conclude that the dicarboxylate/4-hydroxybutyrate cycle is operating in CO₂ fixation in the strict anaerobic Thermoproteales as well as in Desulfurococcales.Two new autotrophic carbon fixation cycles have recently been discovered in the Crenarchaea, one of the two subgroups of the Archaea. The 3-hydroxypropionate/4-hydroxybutyrate cycle functions in the aerobic autotrophic Sulfolobales (7) and the dicarboxylate/4-hydroxybutyrate cycle (Fig. ​(Fig.1)1) in the anaerobic autotrophic Ignicoccus hospitalis, belonging to the Desulfurococcales (27). These pathways have in common the synthesis of succinyl-coenzyme A (CoA) from acetyl-CoA and two inorganic carbons, although this is accomplished in quite different ways and using different carboxylases. In the 3-hydroxypropionate/4-hydroxybutyrate cycle, acetyl-CoA/propionyl-CoA carboxylase fixes two molecules of bicarbonate, and in the dicarboxylate/4-hydroxybutyrate cycle, pyruvate synthase and phosphoenolpyruvate (PEP) carboxylase are the two carboxylating enzymes. Yet, the regenerations of acetyl-CoA, the primary CO₂ acceptor, from succinyl-CoA are similar in the two pathways.Open in a separate windowFIG. 1.Dicarboxylate/4-hydroxybutyrate cycle for autotrophic CO₂ fixation, as proposed for T. neutrophilus. 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, 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. Fd_red, reduced ferredoxin.Acetyl-CoA regeneration is as follows. The CO₂ fixation product succinyl-CoA is reduced to 4-hydroxybutyrate, which is activated to 4-hydroxybutyryl-CoA and then dehydrated to crotonyl-CoA by 4-hydroxybutyryl-CoA dehydratase. This radical 4Fe-4S] and flavin adenine dinucleotide-containing dehydratase (11, 37) is considered a key enzyme of the 4-hydroxybutyrate part of each pathway. Its product, crotonyl-CoA, is further converted to acetoacetyl-CoA and then to two acetyl-CoA molecules, closing the cycle and generating an additional molecule of acetyl-CoA for biosynthesis. Therefore, two different autotrophic pathways in different crenarchaeal orders share many common enzymes and intermediates.In this context, the order Thermoproteales would constitute an exception within the Crenarchaea, since the reductive citric acid cycle was proposed for Thermoproteus neutrophilus (6, 48-50, 55) and Pyrobaculum islandicum (26). T. neutrophilus is a strictly anaerobic hyperthermophilic archaeon growing autotrophically by reducing sulfur with hydrogen at 85°C and neutral pH (19). It can also assimilate organic compounds, such as acetate or succinate, but only in the presence of CO₂ and H₂, i.e., in a mixotrophic way (48).In the reductive citric acid cycle, succinyl-CoA is further transformed with 2 CO₂ to citrate, followed by citrate cleavage to oxaloacetate and acetyl-CoA. This requires two characteristic enzymes, 2-oxoglutarate synthase (2-oxoglutarate-ferredoxin oxidoreductase) and ATP citrate lyase. The proposal of the functioning of the reductive citric acid cycle in T. neutrophilus was based on the results of a ¹³C retrobiosynthetic analysis of the central carbon metabolism, using ¹³C-labeled succinate and acetate as an additional carbon source, following its incorporation into cellular building blocks. The ¹³C enrichment data of, e.g., glutamate, which is directly derived from 2-oxoglutarate, were consistent with the operation of a reductive citric acid cycle only when further assumptions were made (55). The activities of the enzymes of this cycle were demonstrated with extracts of autotrophically grown cells. However, the measured 2-oxoglutarate synthase and ATP-citrate lyase activity levels were very low and could not support the reported growth rate under autotrophic conditions (6, 48).The recent sequencing of the genome of Pyrobaculum aerophilum, belonging to the Thermoproteales (20), revealed a surprising feature, the presence of a 4-hydroxybutyryl-CoA dehydratase gene without the presence of an ATP-citrate lyase gene. Similar gene patterns are found in the genomes of T. neutrophilus as well as Pyrobaculum calidifontis and P. islandicum, sequenced by the DOE Joint Genome Institute (http://www.jgi.doe.gov/). This indicates a possible functioning of the dicarboxylate/4-hydroxybutyrate cycle in Thermoproteales and brings into question the involvement of the reductive citric acid cycle in autotrophic CO₂ fixation. This study has reinvestigated the pathway of autotrophic CO₂ fixation in Thermoproteus neutrophilus. We provide different lines of evidence for the operation of the dicarboxylate/4-hydroxybutyrate cycle.