title: Insights into the autotrophic CO2 fixation pathway of the archaeon Ignicoccus hospitalis: comprehensive analysis of the central carbon metabolism.
abstract: Ignicoccus hospitalis is an autotrophic hyperthermophilic archaeon that serves as a host for another parasitic/symbiotic archaeon, Nanoarchaeum equitans. In this study, the biosynthetic pathways of I. hospitalis were investigated by in vitro enzymatic analyses, in vivo (13)C-labeling experiments, and genomic analyses. Our results suggest the operation of a so far unknown pathway of autotrophic CO(2) fixation that starts from acetyl-coenzyme A (CoA). The cyclic regeneration of acetyl-CoA, the primary CO(2) acceptor molecule, has not been clarified yet. In essence, acetyl-CoA is converted into pyruvate via reductive carboxylation by pyruvate-ferredoxin oxidoreductase. Pyruvate-water dikinase converts pyruvate into phosphoenolpyruvate (PEP), which is carboxylated to oxaloacetate by PEP carboxylase. An incomplete citric acid cycle is operating: citrate is synthesized from oxaloacetate and acetyl-CoA by a (re)-specific citrate synthase, whereas a 2-oxoglutarate-oxidizing enzyme is lacking. Further investigations revealed that several special biosynthetic pathways that have recently been described for various archaea are operating. Isoleucine is synthesized via the uncommon citramalate pathway and lysine via the alpha-aminoadipate pathway. Gluconeogenesis is achieved via a reverse Embden-Meyerhof pathway using a novel type of fructose 1,6-bisphosphate aldolase. Pentosephosphates are formed from hexosephosphates via the suggested ribulose-monophosphate pathway, whereby formaldehyde is released from C-1 of hexose. The organism may not contain any sugar-metabolizing pathway. This comprehensive analysis of the central carbon metabolism of I. hospitalis revealed further evidence for the unexpected and unexplored diversity of metabolic pathways within the (hyperthermophilic) archaea.
authors.affiliation: Lehrstuhl Mikrobiologie und Archaeenzentrum, Universität Regensburg, Universitätsstrasse 31, D-93053 Regensburg, Germany.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Jahn
authors.forename: Harald
authors.initials: H
authors.lastname: Huber
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Eisenreich
authors.forename: Michael
authors.initials: M
authors.lastname: Hügler
authors.forename: Georg
authors.initials: G
authors.lastname: Fuchs
mesh_terms: D015074:2-Aminoadipic Acid / Q000737:chemistry / Q000378:metabolism; D000105:Acetyl Coenzyme A / Q000378:metabolism; D001105:Archaea / Q000378:metabolism*; D052818:Autotrophic Processes; D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes / Q000378:metabolism; D002950:Citrate (si)-Synthase / Q000378:metabolism; D019343:Citric Acid / Q000737:chemistry / Q000378:metabolism; D005634:Fructose-Bisphosphate Aldolase / Q000737:chemistry / Q000378:metabolism; D005943:Gluconeogenesis; D006019:Glycolysis; D006600:Hexosephosphates / Q000737:chemistry / Q000378:metabolism; D007532:Isoleucine / Q000378:metabolism; D008239:Lysine / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008293:Malates / Q000737:chemistry / Q000378:metabolism; D008954:Models, Biological; D015394:Molecular Structure; D062907:Oxaloacetic Acid / Q000737:chemistry / Q000378:metabolism; D010428:Pentosephosphates / Q000737:chemistry / Q000378:metabolism; D010728:Phosphoenolpyruvate / Q000737:chemistry / Q000378:metabolism; D010730:Phosphoenolpyruvate Carboxylase / Q000737:chemistry / Q000378:metabolism; D050743:Pyruvate Synthase / Q000378:metabolism; D011773:Pyruvates / Q000737:chemistry / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D006600:Hexosephosphates; D008293:Malates; D010428:Pentosephosphates; D011773:Pyruvates; D007532:Isoleucine; D002245:Carbon Dioxide; D015074:2-Aminoadipic Acid; D019343:Citric Acid; D062907:Oxaloacetic Acid; D000105:Acetyl Coenzyme A; D010728:Phosphoenolpyruvate; D002244:Carbon; D050743:Pyruvate Synthase; D002950:Citrate (si)-Synthase; D010730:Phosphoenolpyruvate Carboxylase; D005634:Fructose-Bisphosphate Aldolase; C011729:citramalate; D008239:Lysine