Sugar metabolism of hyperthermophiles

In recent years a number of hyperthermophiles with the ability to utilize sugars as source for carbon and energy have been isolated. Analysis of their central metabolism may reveal adaptations to the extreme environment, or give information about the evolution of the primary pathways involved. The best studied representative is Pyrococcus furiosus , which has become the model organism of the heterotrophic hyperthermophiles. This deeply branched archaeon utilizes a modified Embden‐Meyerhof Pathway, which involves a set of unprecedented ADP‐dependent kinases, and a unique glyceraldehyde‐3‐phosphate: ferredoxin oxidoreductase. Moreover, pyruvate is converted via acetyl‐CoA to acetate, involving an ADP‐forming acetyl‐CoA synthetase, which is not encountered in Bacteria. Reductant generated by ferrodoxin‐linked enzymes is released either by S 0 ‐reduction to H 2 S, by proton reduction to H 2 or by the formation of alanine. Yield studies suggest that in addition to ATP synthesis by substrate level phosphorylation in the ultimate acetate‐forming step, there are alternative energy conserving systems. The ADP‐dependent Embden‐Meyerhof pathway is probably shared by other members of the Thermococcales . In contrast, an ATP‐dependent Embden‐Meyerhof pathway is operating in the S 0 ‐respiring archaeon Thermoproteus tenax , although it involves a PP i ‐dependent phosphofructokinase. Finally, hyperthermophilic bacteria such as Thermotoga maritima utilize a classical Embden‐Meyerhof pathway. Thus, the presence of the different versions of the Embden‐Meyerhof pathway in these deeply rooted microbes indicates that the hypothesis that the Entner‐Doudoroff pathway is more primitive is not correct.