SUGAR METABOLISM OF HYPERTHERMOPHILES

In recent years a number of hyperthermophiles with the ability to util ize sugars as source for carbon and energy have been isolated. Analysi s of their central metabolism may reveal adaptations to the extreme en vironment, or give information about the evolution of the primary path ways involved. The best studied representative is Pyrococcus furiosus, which has become the model organism of the heterotrophic hyperthermop hiles. This deeply branched archaeon utilizes a modified Embden-Meyerh of Pathway, which involves a set of unprecedented ADP-dependent kinase s, 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 Bact eria. Reductant generated by ferredoxin-linked enzymes is released eit her by S-0-reduction to H2S; by proton reduction to H-2 or by the form ation of alanine. Yield studies suggest that in addition to ATP synthe sis by substrate level phosphorylation in the ultimate acetate-forming step, there are alternative energy conserving systems. The ADP-depend ent 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, altho ugh it involves a PPi-dependent phosphofructokinase. Finally, hyperthe rmophilic bacteria such as Thermotoga maritima utilize a classical Emb den-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 primitiv e is nor correct.