Metabolism of hyperthermophiles.

Hyperthermophiles are characterized by a temperature optimum for growth between 80 and 110°C. They are considered to represent the most ancient phenotype of living organisms and thus their metabolic design might reflect the situation at an early stage of evolution. Their modes of metabolism are diverse and include chemolithoautotrophic and chemoorganoheterotrophic. No extant phototrophic hyperthermophiles are known. Lithotrophic energy metabolism is mostly anaerobic or microaerophilic and based on the oxidation of H2 or S coupled to the reduction of S, SO inf4 (sup2-) , CO2 and NO inf3 (sup-) but rarely to O2. the substrates are derived from volcanic activities in hyperthermophilic habitats. The lithotrophic energy metabolism of hyperthermophiles appears to be similar to that of mesophiles. Autotrophic CO2 fixation proceeds via the reductive citric acid cycle, considered to be one of the first metabolic cycles, and via the reductive acetyl-CoA/carbon monoxide dehydrogenase pathway. The Calvin cycle has not been found in hyperthermophiles (or any Archaea). Organotrophic metabolism mainly involves peptides and sugars as substrates, which are either oxidized to CO2 by external electron acceptors or fermented to acetate and other products. Sugar catabolism in hyperthermophiles involves non-phosphorylated versions of the Entner-Doudoroff pathway and modified versions of the Embden-Meyerhof pathway. The 'classical' Embden-Meyerhof pathway is present in hyperthermophilic Bacteria (Thermotoga) but not in Archaea. All hyperthermophiles (and Archaea) tested so far utilize pyruvate:ferredoxin oxidoreductase for acetyl-CoA formation from pyruvate. Acetyl-CoA oxidation in anaerobic sulphur-reducing and aerobic hyperthermophiles proceeds via the citric acid cycle; in the hyperthermophilic sulphate-reducer Archaeoglobus an oxidative acetyl-CoA/carbon monoxide dehydrogenase pathway is operative. Acetate formation from acetyl-CoA in Archaea, including hyperthermophiles, is catalysed by acetyl-CoA synthetase (ADP-forming), a novel prokarvotic enzyme involved in energy conservation. In Bacteria, including the hyperthermophile Thermotoga, acetyl-CoA conversion to acetate involves two enzymes, phosphate acetyltransferase and acetate kinase.