ABSTRACT
Introduction ................................................................................................................................ 99 Classical EM Pathway in Eukarya and Bacteria ....................................................................... 100 Modifi ed EM Pathways and Acetate Formation in Hyperthermophilic Archaea ..................... 101
Unusual Enzymes of Modifi ed EM Pathways in Hyperthermophilic Archaea .............. 103 Glucose Phosphorylation to Glucose-6-Phosphate .............................................. 103 Glucose-6-Phosphate Isomerization to Fructose-6-Phosphate ............................ 103 Fructose-6-Phosphate Phosphorylation ............................................................... 104 Cleavage of FBP .................................................................................................. 104 GAP Oxidation .................................................................................................... 105 Conversion of 3-Phosphoglycerate to PEP .......................................................... 105 PEP Conversion to Pyruvate in Hyperthermophilic Archaea ............................. 105
Enzymes of Pyruvate Conversion to Acetate ................................................................. 105 Pyruvate Conversion to Acetyl CoA .................................................................... 105 Acetyl-CoA Conversion to Acetate ...................................................................... 106
Energetics of Modifi ed EM Pathways ............................................................................ 106 Classical EM Pathway and Acetate Formation in the Hyperthermophilic
Bacterium Thermotoga maritima ................................................................................... 106 Enzymes of the Classical EM Pathway in Thermotoga maritima ................................. 107 Enzymes of Pyruvate Conversion to Acetate ................................................................. 107 Energetics of Glucose Degradation to Acetate in Thermotoga maritima ...................... 108
Conclusions ................................................................................................................................ 108 References .................................................................................................................................. 110
Hyperthermophiles with an optimal growth temperature above 80°C and a maximum at 113°C [1] are considered to represent the phylogenetic most ancestral living organisms. According to their position in the single-stranded unit (SSU) ribosomal RNA (rRNA)-based phylogenetic tree, hyperthermophiles show the deepest branching and shortest lineages of all organisms analyzed. Most hyperthermophiles belong to the domain of archaea, only few hyperthermophilic genera of bacteria are known, which include, for example, Thermotogales. The metabolism of hyperthermophiles is diverse including both chemolithoautotrophs dependent on compounds derived from vulcanic activities, for example, H2, CO2, and various sulfur compounds, and chemo-organoheterotrophs growing, for example, on peptides or sugars [2]. Many sugar utilizing hyperthermophiles are strictly anaerobic fermentative organisms degrading glucose or glucose polymers (starch, maltose, cellobiose) to acetate as main fermentation product. These include, for example, the euryarchaeaota
Pyrococcus furiosus, Thermococcus sp., and the crenarchaeon Desulfurococcus amylolyticus. Recently, the sulfate reducing hyperthermophilic archaeon Archaeoglobus fulgidus strain 7324 was found to degrade starch incompletely to acetate reducing sulfate to H2S [3]. The crenarchaeota Thermoproteus tenax and Pyrobaculum aerophilum completely oxidize sugars to CO2 with sulfur and O2 or nitrate, respectively, as external electron acceptors [4]. The best studied sugar utilizing hyperthermophilic bacterium is Thermotoga maritima; the organism ferments glucose and glucose polymers to acetate as main products as do fermentative hyperthermophilic archaea [5,6].
