Archaebacteria metabolism

Archaebacteria—Metabolism. 2. Archaebacteria—Molecular aspects. I. Kates, Morris. II. Kushner, Donn. III. Matheson, A. 

Fig. 2. An evolution diagram illustrating a suggestion of common ancestry of some present-day organisms. The essential features of present-day photosynthesis may have originated in the prebiotic era and is preserved in its most primitive form in (at least some) present-day phototrophs. The heterotrophs may have developed parallel with the aerobic nonphotosynthetic bacteria, some l to 1.5 x 109 years after the emergence of the cyanobacteria. The eukaryotic photosynthetic organisms developed much later, perhaps some 1.5 to 0.5 x 109 years ago. The archaebacteria are primitive organisms that seem to have no evolutionary relation with the present prokaryotes.21 Little is known about their energy metabolism. Tentatively, they are considered as a very early form of cellular life.

Marine methanogenic archaebacteria respond to osmotic stress by accumulating Ne-acetyl-/ -lysine and yff-Glu as metabolism complatible solutes (a) K. R. Sowers, D. E. Robertson, D. Noll, R. P. Gunsalus, M. F. Roberts, Proc. Natl. Acad. Sci. USA 1990, 87, 9083-9087 (b) D. E. Robertson, D. Noll, M. F. Roberts,... 

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. 

Finally, the need for further investigations into the metabolism of glucose in thermophilic archaebacteria should be stressed. Firstly, the fate of glyceraldehyde in Sulfolobus species needs to be established, and there is still eontroversy coneeming the pathways in Tp. acidophilum. That is, Searcy and Whatley [15] have provided evidence from respiratory studies for the operation of glycolysis in Tp. acidophilum but we have been unable to detect many of the enzymes of this pathway [14]. Secondly, there is a... 

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. 

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. 

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. 

In order to draw conclusions about the origin and evolution of central metabolism from a study of these pathways in archaebacteria, eubacteria and eukaryotes, a definitive phytogeny of the organisms involved is required. In particular, a knowledge of which organisms are primitive is essential. 

Studies on the pathways of central metabolism of the archaebacteria take on special significance when it is realised that, from the universal phylogenetic tree in rooted form, Woese et al. [66] have proposed that the domain of the archaebacteria be known as archaea to denote their apparently primitive nature, especially with respect to the eukaryotes. Furthermore, within the archaea, thermophily is regarded as the ancestral phenotype. 

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).

From the studies on archaebacterial metabolism, new impetus has been given to the field of comparative enzymology and, in particular, to our investigations of the structural basis of protein stability. Indeed, the fact that the archaebacteria inhabit the extremes of life s environments means that they have a unique contribution to make this area. To realise this potential, the study of archaebacterial enzymes remains a priority. 

It is the purpose of this review to describe and discuss the pathways of central metabolism in the archaebacteria however, for the reasons given above, this will not be done in isolation but as a comparative survey with those found in eubacteria and eukaryotes. Indeed, not only will the pathways be compared, but the comparison will be extended to the enzymes catalysing the reactions of these pathways. For previous reviews the reader is referred to Danson[l,2]. 

Fischer F., Zillig W., Stetter K.O., Schreiber G. (1983) Chemolithoautotrophic metabolism of anaerobic extremely thermophilic archaebacteria. Nature 301, 511-13. 

The majority of information on the chemical and physical properties of lipids comes from studies on the major phospholipid classes of eubacteria and eukaryotes with only limited information on the lipids from archaebacteria. The biosynthetic pathways and the genetics of lipid metabolism have also been extensively studied in eubacteria (Chapter 3) and eukaryotes (Chapter 8). Clearly, the archaeol lipids confer some advantage with respect to the environment of archaebacteria. Interestingly, the pathways for phospholipid biosynthesis in eubacteria and archaebacteria are very similar even though their lipids differ in chirality of the glycerol backbone. Many of these organisms exist in harsh environments that call for more chemically stable lipid bilayers that are afforded by the above lipids. How the physical properties of the more commonly studied lipids change with environment will be discussed later. 

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