We have previously shown that the hyperthermophilic archaeon, Sulfolobus solfataricus, catabolizes D-glucose and D-galactose to pyruvate and glyceraldehyde via a non-phosphorylative version of the Entner-Doudoroff pathway. At each step, one enzyme is active with both C6 epimers, leading to a metabolically promiscuous pathway. On further investigation, the catalytic promiscuity of the first enzyme in this pathway, glucose dehydrogenase, has been shown to extend to the C5 sugars, D-xylose and L-arabinose. In the current paper we establish that this promiscuity for C6 and C5 metabolites is also exhibited by the third enzyme in the pathway, 2-keto-3-deoxygluconate aldolase, but that the second step requires a specific C5-dehydratase, the gluconate dehydratase being active only with C6 metabolites. The products of this pathway for the catabolism of D-xylose and L-arabinose are pyruvate and glycolaldehyde, pyruvate entering the citric acid cycle after oxidative decarboxylation to acetyl-coenzyme A. We have identified and characterized the enzymes, both native and recombinant, that catalyze the conversion of glycolaldehyde to glycolate and then to glyoxylate, which can enter the citric acid cycle via the action of malate synthase. Evidence is also presented that similar enzymes for this pentose sugar pathway are present in Sulfolobus acidocaldarius, and metabolic tracer studies in this archaeon demonstrate its in vivo operation in parallel with a route involving no aldol cleavage of the 2-keto-3-deoxy-pentanoates but direct conversion to the citric acid cycle C5-metabolite, 2-oxoglutarate.Sulfolobus solfataricus and Sulfolobus acidocaldarius are hyperthermophilic archaea that grow optimally at 78 -85°C, pH 2-4, and are able to utilize a variety of carbon sources, including the four most-commonly occurring sugars in nature, D-glucose, D-galactose, D-xylose, and L-arabinose (1).Metabolism of glucose in S. solfataricus and S. acidocaldarius proceeds via a non-phosphorylative variant of the Entner-Doudoroff pathway, which generates pyruvate with no net production of ATP (Fig. 1) (2-5). Glucose dehydrogenase catalyzes the conversion of glucose to gluconate, which is then dehydrated to 2-keto-3-deoxygluconate (KD-gluconate) 4 by gluconate dehydratase. KD-gluconate in turn is cleaved to pyruvate and glyceraldehyde via 2-keto-3-deoxygluconate aldolase (KDG-aldolase), the glyceraldehyde generating a second molecule of pyruvate via the actions of glyceraldehyde oxidoreductase, glycerate kinase, enolase, and pyruvate kinase.In vitro kinetic analyses of glucose dehydrogenase, gluconate dehydratase, and KDG-aldolase from S. solfataricus showed these enzymes are also capable of catalyzing the catabolism of galactose, the C4 epimer of glucose, to pyruvate and glyceraldehyde, leading to the suggestion that the pathway exhibits a metabolic promiscuity toward these two hexose sugars (5-7). Because recombinantly produced glucose dehydrogenase has good activity with the pentose sugars D-xylose and L-arabinose (6) and KDG-aldolase catalyze...

The lipid cores from Ignisphaera aggregans, a hyperthermophilic Crenarchaeon recently isolated from New Zealand hot springs, have been profiled by liquid chromatography-tandem mass spectrometry. The distribution revealed includes relatively high proportions of monoalkyl (also known as H-shaped) tetraether cores which have previously been implicated as kingdom-specific biomarkers for the Euryarchaeota. Such high expression of monoalkyl tetraether lipids is unusual in the archaeal domain and may indicate that formation of these components is an adaptive mechanism that allows I. aggregans to regulate membrane behaviour at high temperatures. The observed dialkyl tetraether and monoalkyl tetraether lipid distributions are similar but not fully concordant, showing differences in the average number of incorporated rings. The similarity supports a biosynthetic route to the ring-containing dialkyl and monoalkyl tetraether lipids via a dialkyl tetraether core containing zero rings, or a closely related structural relative, as an intermediate. Currently, however, the precise nature of the biosynthetic route to these lipids cannot be deduced.

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