Archaebacteria halophilic

This complete oxidative cycle is found in a number of archaebacteria. Halophiles can fulfil their energy requirements by metabolism of amino acids and other nitrogenous compounds, and therefore it is probable that they possess an oxidative citric acid cycle. Aitken and Brown [45] have reported the presence of the cycle s enzymes in Halobacterium halobium and we have found the key enzymes, citrate synthase and succinate thiokinase, in a range of classical and alkaliphilic halophiles [46], Thus, it is probable that the cycle is generally present in this group of archaebacteria, but exhaustive studies have not been carried out. 

Archaebacteria the third most recently recognized primary kingdom and are characterized as living in extreme environments, such as anaerobic methanogens and halophilic bacteria. 

The Halobacteriaceae, commonly referred to as the halobacteria, are a family of extremely halophilic archaebacteria [113]. As in other archaebacteria, their membranes contain ether-linked lipids. The primary lipids present are diphytanyl phospholipids [113]. Their cell walls are also unique in structure and lack muramic acid. There are several species of halobacteria that vary considerably in their physiological characteristics. The halobacteria are unicellular rods or cocci. More recently flat, square and box-shaped cells have been described. Halobacteria are found growing in salterns or natural salt lakes and on the surface of salted fish. They often form dense planktonic blooms and can form massive accumulations on solid substrates. They may be involved in mat communities in hypersaline environments. 

This modified Entner—DoudoroflF pathway has been found in other species of Halobacterium and in species of Haloferax and Halococcus [9- ] and may thus be common to the halophiles. Whilst not reported in thermophilic or methanogenic archaebacteria, it is not unique to halophiles, having been found in a few eubacterial genera (see ref. [1] and references therein). 

Whilst the majority of investigations into halophilic hexose metabolism has been concerned with the catabolism of glucose, it has been recently reported [104,105] that Haloarcula vallismortis catabolises fructose via a modified Embden-Meyerhof pathway. Fructose is phosphorylated to fructose 1-phosphate via a ketokinase, and is then converted to fructose 1,6-bisphosphate via 1-phosphofructokinase. Aldol cleavage generates dihydroxyacetone-phosphate and glyceraldehyde 3-phosphate, both of which can be further metabolised via the glycolytic sequence described earlier. It remains to be established whether other halophilic archaebacteria can also catabolise fructose in this manner. 

Fig. 3. Pathways of glucose catabolism in halophilic and thermophilic archaebacteria. The modified Entner-Doudoroff pathway of halophiles (solid lines) and the non-phosphorylated Entner-Doudoroff pathway of Sulfolobus sol/ataricus and Thermoplasma acidophilum (dashed lines) are shown in comparison with the classical Entner-Doudoroff pathway of eubacteria (heavy solid lines) from Fig. 1.

Compared with the investigations in the extreme halophiles, there is very little information on the operation of a pentose-phosphate pathway in other archaebacteria. The radiorespirometric analyses of glucose metabolism in Sulfolobus species [13], which established the Entner-Doudoroff type pathway (section 2.2), were also consistent with a non-cyclic pentose-phosphate pathway in S. brierleyi and a conventional oxidation cycle in Sulfolobus strain LM. Similarly, respiratory studies [15] provide evidence for a pentose phosphate cycle capable of glucose oxidation in Tp. acidophilum. No data are available for the methanogens. 

The conversion of pyruvate to acetyl-CoA is catalysed by pyruvate oxidoreductase in the archaebacteria. The enzyme has been detected and characterised in Halobacterium halobium[i, 2i2 Tp. acidophilum, S. acidocaldarius and Desulfurococcus mobilis[i i], Pyrococcus furiosus [34] and in Methanobacterium thermoautotrophicum [35]. In the halophiles and thermophiles, ferredoxin serves as electron acceptor, whereas the methanogens use the deazaflavin derivative F420. 

The presence of the enzyme and cofactor are co-incident, indicating that lipoic acid may indeed be the trae substrate of the archaebacterial dihydrolipoamide dehydrogenase. Interestingly, their presence can be correlated with the organisms phylogenetic positions within the archaebacteria. That is, the archaea comprise two main divisions - the methanogens, extreme halophiles. Thermoplasma and Thermococcus in one, and the remaining sulphur-dependent thermophiles in the other [66,72]. The enzyme and/or the cofactor have been detected in all the phenotypes of the former division but neither have yet been discovered in the latter. Further analyses are required to test this correlation as the data are incomplete. 

With regard to amino acid metabolism, the data are scarce but it is probable that these metabolites give rise to, or are derived from, the oxoacids of the citric acid cycle via transamination or analogous reactions. See the literature [60-62] for examples of such enzymes in the halophilic, thermophilic and methanogenic archaebacteria. 

Fig. 7. Proposed biosynthetic pathway for the formation of diphytanyl glycerol ether lipids from the pathways of central metabolism in halophilic archaebacteria. The scheme outlined is taken from ref. [63] and M. Kates (personal communication).

Type I and type 11 DNA topoisomerases have also been identified in archaebacteria (for a previous review see ref [71]). They exhibit both classical and novel features. Up to now, biochemical studies have been performed only with enzymes isolated from thermophiles, whereas physiological and genetic studies have been performed mainly with halophiles. 

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