Journal Article

dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis

Jahn, Ulrike, Gallenberger, Martin, Eisenreich, Wolfgang, Huber, Harald, Kockelkorn, Daniel, Eylert, Eva, Berg, Ivan A, Fuchs, Georg

2008

Overview

Ignicoccus hospitalis is an anaerobic, autotrophic, hyperthermophilic Archaeum that serves as a host for the symbiotic/parasitic Archaeum Nanoarchaeum equitans. It uses a yet unsolved autotrophic CO₂ fixation pathway that starts from acetyl-CoA (CoA), which is reductively carboxylated to pyruvate. Pyruvate is converted to phosphoenol-pyruvate (PEP), from which glucogenesis as well as oxaloacetate formation branch off. Here, we present the complete metabolic cycle by which the primary CO₂ acceptor molecule acetyl-CoA is regenerated. Oxaloacetate is reduced to succinyl-CoA by an incomplete reductive citric acid cycle lacking 2-oxoglutarate dehydrogenase or synthase. Succinyl-CoA is reduced to 4-hydroxybutyrate, which is then activated to the CoA thioester. By using the radical enzyme 4-hydroxybutyryl-CoA dehydratase, 4-hydroxybutyryl-CoA is dehydrated to crotonyl-CoA. Finally, β-oxidation of crotonyl-CoA leads to two molecules of acetyl-CoA. Thus, the cyclic pathway forms an extra molecule of acetyl-CoA, with pyruvate synthase and PEP carboxylase as the carboxylating enzymes. The proposal is based on in vitro transformation of 4-hydroxybutyrate, detection of all enzyme activities, and in vivo-labeling experiments using [1-¹⁴C]4-hydroxybutyrate, [1,4-¹³C₂], [U-¹³C₄]succinate, or [1-¹³C]pyruvate as tracers. The pathway is termed the dicarboxylate/4-hydroxybutyrate cycle. It combines anaerobic metabolic modules to a straightforward and efficient CO₂ fixation mechanism.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: acetyl coenzyme A / Acetyl Coenzyme A - biosynthesis / Amino acids / Amino Acids - metabolism / Archaea / beta oxidation / Biological Sciences / carbon / Carbon cycle / Carbon dioxide / Carbon Dioxide - metabolism / Carbon Isotopes - analysis / Carbon Isotopes - metabolism / Desulfurococcaceae / Desulfurococcaceae - metabolism / Dicarboxylic Acids - metabolism / Enzymatic activity / enzyme activity / Enzymes / Ferredoxins / Hydroxybutyrates - metabolism / Ignicoccus / Microbiology / Molecules / Nanoarchaeum equitans / Oxidation / oxoglutarate dehydrogenase (succinyl-transferring) / Proteins - metabolism / pyruvate synthase / pyruvic acid / Pyruvic Acid - metabolism / Radioactive decay / succinic acid / Succinic Acid - metabolism / tracer techniques / Tricarboxylic acid cycle