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Escherichia coli and Bacillus subtilis: The Frontiers of Molecular Microbiology Revisited, 2012: 341-355

ISBN: 978-81-308-0492-7 Editors: Yoshito Sadaie and Kouji Matsumoto

11. Archaea and Bacteria

11-1. Role of membrane lipids in the first specific differentiation

Yosuke Koga

9-14-20 Hinosato, Munakata 811-3425 Japan

1. Abstract. All the living organisms are classified into three domains (the newly proposed highest rank of classification of organisms). Escherichia coli and Bacillus subtilis are belonging to one of the domains, Bacteria. Works on the two bacteria made a great deal of contribution to construct modern microbiology, new concept of biochemistry and molecular biology. Compared with research history on Bacteria, Archaea have been studied merely in these three decades, however, the uniformity and diversity in biochemical properties of Archaea seem to illustrate pre-existing knowledge of Bacteria from the back, that is, bacterial information that had been ignored was discovered as novel aspects of biochemistry and molecular biology. One of them is the “lipid divide”, that is, the difference of stereo structure of glycerophosphate of membrane lipid backbone and hydrocarbon chains. In this chapter, first, a quite unique structure and biosynthesis of archaeal membrane lipids are introduced and then a hypothesis on the differentiation of Archaea and Bacteria mainly driven by segregation of glycerophosphate enantiomers of lipid backbones and hydrocarbon chains in the universal common ancestor.

Correspondence/Reprint request: Dr. Yosuke Koga, 9-14-20 Hinosato, Munakata 811-3425 Japan E-mail: yokoga@ams.odn.ne.jp

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2. Arachaea and bacteria: Two descendants of the universal common ancestor Escherichia coli and Bacillus subtilis are classified in Bacteria, which is a domain of life newly proposed phylogenetic classification of all life [1]. Other domains are Archaea and Eukarya. Because cells of Eukarya are generally accepted as cells emerged by symbiosis of an ancient archaeal and a bacterial species, Archaea and Bacteria are two fundamental lineages of life. Microbiological knowledge has been accumulated mainly on the basis of researches on many species of Bacteria, because researches on Archaea have only far young history compared with that of Bacteria. However, recent rapidly growing archaeal studies revealed novel aspects of insight of history of life. Experimental results on Archaea accumulated during these several decades showed significant differences from and fundamental similarities to Bacteria in biochemical and molecular biological aspects. Significant differences between biochemical and molecular biological aspects in Archaea and Bacteria have been stressed. In fact, archaeal characteristics are very striking. However, it should be recalled that Archaea and Bacteria have fundamentally common chemistry of cellular constituents and biochemical mechanisms. Both lineages of organisms share DNA, RNA, protein, sugars, and glycerolipids that are identical in fundamental structures. Types of bases and pentoses of DNA or RNA, amino acids, primary hexoses, ATP, and coenzymes are common. Among biochemical mechanisms, membrane lipids show maximum difference in structure (see below). Though Archaea and Bacteria show diverse biosynthetic and energy-yielding metabolisms, fundamental uniformity can be seen such as glycolysis, gluconeogenesis, TCA cycle, DNA replication, transcription, translation, and chemiosmotic mechanism of ATP synthesis. The unity and diversity of chemistry and biochemistry in Archaea and Bacteria are certain evidence for the existence of a common ancestor of Archaea and Bacteria. That is, Archaea and Bacteria, which are basically fundamental lineages of life, were differentiated from one common ancestor. Comparison of biochemical features in Archaea and Bacteria may result in suggestion of the mechanism of differentiation. Although phylogenetic difference between Archaea and Bacteria is observed by comparison of base sequences of rRNA, this is merely difference in base sequence in the polymers that has been accumulated during their evolutionary history from a common ancestral molecule but not from different compounds. The most different compounds in the both domains of organisms that fulfill common function are membrane lipids. Correct insight of the difference between archaeal and bacterial membrane lipids could lead

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one to understand essence of two significant lineages of life on the Earth. Therefore, this chapter starts with a brief review of archaeal lipids. 3. Structure and biosynthesis of archaeal membrane lipids

Basic structure of polar lipids in Archaea Archaeal polar lipids are composed of sn-glycerol-1-phosphate as a backbone and two isoprenoid chains (usually C20 phytanyl group) bound at the sn-2 and 3 positions (see footnote 1) of the backbone (archaetidic acid). A polar group (such as myo-inositol, glycerol, or serine/ethanolamine) is linked to the free hydroxyl group of phosphate group to form a phosphodiester bonds (archaetidyl-X, where X is, for example, myo-inositol). Glycolipids have two or more hexose units at the sn-1-position where a phosphate-containing polar group is removed (2, 3-di-phytanyl-sn-glycerol is called archaeol). Two polar lipids are often coupled by head-to-head condensation at the methyl termini of phytanyl groups to give rise to a tetraether polar lipid with two glycerol units, two C40 biphytanediyl chains, and a phosphate-containing polar group and an oligoglycosyl group. A tetraether lipid derived from this type of polar tetraether lipid by removing both polar groups is caldarchaeol. Archaeol and caldarchaeol are core lipids (Fig. 1) [2,3].

Figure 1. Characteristics of archaeal membrane lipids. 1) The stereochemistry of glycerophosphate backbone is sn-glycerol-1-phosphate. 2) Ether bonds between isoprenoid chains and G-1-P backbone. 3) Isoprenoid hydrocarbon chains (mostly C20 phytanyl group). The bacterial counterparts are sn-glycerol-3-phosphate, ester bonds, and fatty acyl chains, respectively.

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The most critical distinction between archaeal and bacterial membrane lipids is stereo chemical difference in the glycerophosphate (GP) backbones of the phospholipids. This is called the “lipid divide”. Although there are exceptions in the type of hydrocarbon chains and ether/ester bonds, there is no exception reported for the stereochemistry of GP to date. Thus, the “lipid divide” may account for the fundamental difference in membrane lipids between Archaea and Bacteria. Varieties of archaeal polar lipids There are a wide variety of diether- and tetraether-types of core lipids and polar head groups. Well-known structural varieties of archaeal lipids are shown in Fig. 2. One species of archaeon possesses one to several kinds of core lipids and two or more polar head groups. Consequently, combination of various core lipids and polar groups give rise to tens of polar lipid species in one species of archaeon. Composition of polar lipids of an archaeal species is roughly related with phylogenetic relationship [4]. Figure 2A

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Figure 2B

Figure 2. Variation of core lipids in Archaea. A, archaeol analogs; and B, caldarchaeol analogs. Biosynthetic pathway of archaeal polar lipids Isoprenoid chains in archaeal polar lipids are synthesized by the well-known mevalonate pathway, which is slightly modified, in contrast with the 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway found in most bacterial

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species (for details, see ref. 5). The most significant feature of archaeal phospholipid biosynthetic mechanism is the formation of the enantiomeric backbone structure of G-1-P, which is formed from dihydroxyacetonephosphate (DHAP) by G-1-P dehydrogenase [6]. Because phosphorylation of glycerol forms G-3-P but not G-1-P, this reaction does not involve in G-1-P formation. By contrast, G-3-P backbone of bacterial phospholipids is formed from DHAP by G-3-P dehydrogenase or by phosphorylation of glycerol by glycerol kinase. The G-1-P dehydrogenase activity was detected in the cell homogenates of all the Archaea so far measured and the gene coding the enzyme was detected in all the Archaea of which whole genome sequence was published. Archaeal lipid biosynthesis (Fig. 3) starts with etherification at the positions of sn-2 and 3 with geranylgeranyl diphosphate. The first and second products are geranylgeranyl-G-1-P (GGGP) and digeranylgeranyl-G-1-P (DGGGP, unsaturated archaetidic acid), respectively. Archaetidic acid is reacted with CTP to form CDP-archaeol, which is an activated form of archaeol. This reaction mode is identical to bacterial phospholipid synthesis (CDP-diacylglycerol formation). CDP-archaeol is a central intermediate for the phospholipid biosynthesis, because it reacts with most polar head groups to form phospholipids with the corresponding polar group by replacing CMP moiety of CDP-archaeol. CDP-archaeol synthase uses specifically unsaturated archaetidic acid with geranylgeranyl chains as isoprenoid chains, while archaetidylserine synthase and archaetidylinositol phosphate synthase react with saturated or unsaturated CDP-archaeol with phytanyl or geranylgeranyl chains. An enzyme that reduces double bonds of geranylgeranyl chains bound in phospholipids is found in Archaea [7]. Because the substrate specificity of the enzyme is not so tight, the exact point of reduction (saturation) of unsaturated isoprenoid chains is not clear. Most archaeal polar lipids as end products have saturated isoprenoid chains. Major polar head groups in archaeal phospholipids are synthesized in cytoplasmic compartment of archaeal cells by soluble enzymes from the intermediates of glycolysis or gluconeogenesis. L-serine was synthesized from phosphoenolpyruvate, GP is formed directly from DHAP, and myo-inositol-1-phosphate is from D-glucose-6-phosphate. Enzymes for lipid biosynthesis Although little enzymes of lipid biosynthesis in Archaea have been purified, some properties of the enzymes were reported. Characteristics relevant with lipid-driven differentiation of Archaea and Bacteria are briefly described below.

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Figure 3. Biosynthetic pathway of archaeal membrane lipids (archaetidylserine [AS], archaetidylinositol [AI], and diglucosylarchaeol (DGA)). The mevalonate pathway by which geranylgeranyl diphosphate is synthesized from acetyl-CoA is partly abridged. DHAP, dihydroxyacetonephosphate; GAP, glyceraldehyde-3-phosphate; DMAPP, dimethylallylphosphate; PPi, pyrophosphoric acid.

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G-1-P-dehydrogenase Part of characteristics of synthetic enzymes of lipids in Archaea is relevant with consideration of differentiation of Archaea and Bacteria. Because G-1-P dehydrogenase has little amino acid sequence similarity with G-3-P dehydrogenase, the stereo specificity of G-1-P dehydrogenase and G-3-P dehydrogenase could not be interchangeable [8]. That is, the two enzymes are derived from different ancestral enzymes. G-1-P dehydrogenase is a member of an enzyme family including glycerol dehydrogenase, alcohol dehydrogenase and dehydroquinate synthase. G-1-P dehydrogenase specifically transfers pro-R hydrogen of NADH to reduce DHAP to form G-1-P, in contrast to G-3-P dehydrogenase, which transfers pro-S hydrogen of NADH. The two dehydrogenases would have symmetric 3D structures at least from the point of view of nicotinamide plane. Ether bond forming enzymes The first ether bond-forming enzyme (GGGP synthase) is specific to G-1-P. G-1-P dehydrogenase provides G-1-P and GGGP synthase determines the final stereochemistry of archaeal polar lipids by its substrate specificity. Because the enzymes hereafter in the lipid synthetic pathway of Archaea are not G-1-P-specific, the determinant of the stereo structure of the GP backbone is limited to G-1-P dehydrogenase and GGGP synthase. Enzymes in the super family of CDP-alcohol phosphatidyltransferase (archaetidyl-X synthases) These polar head-specific enzymes belong to the enzyme super family of CDP-alcohol phosphatidyltransferase. The enzyme that catalyzes synthesis of each phospholipid seems to be specific to a polar head group but its reactivity is not restricted to one type of core lipid. For example, archaetidylserine synthase shows reactivity with the unnatural substrates, CDP-activated ester types, sn-G-3-P types and fatty acid types analogs of CDP-archaeol and the natural substrate, CDP-archaeol [9]. By now, the enzyme activities of archaetidylserine synthase and archaetidylinositolphosphate synthase [10] have been characterized in vitro from a methanoarchaeon, Methanothermobacter thermautotrophicus, while a number of enzymes belonging to the enzyme superfamily CDP-alcohol phosphatidyltransferase have been detected on completely-sequenced genomes of Archaea. The reactions and enzymes up to archaetidic acid synthesis in the archaeal phospholipid biosynthetic pathway are donated to building for a core lipid,

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but the enzymes of CDP-alcohol phosphatidyltransferase family are contributing to construct an amphiphilic phospholipid that have both a hydrophobic core portion and hydrophilic polar head group. Because amphiphilic lipids are essential constituents of biologic membranes, these enzymes of the family are regarded as membrane-forming enzymes. The enzymes reacting with most polar head groups and dialkyl ether and diacyl ester G-1-P and G-3-P core lipids from all three domains of life are all included in this enzyme family. 4. Functional charateristics of archaeal lipid membrans Archaeal membranes made of archaeol core and caldarchaeol core and several polar groups reveal unique characteristics compared with bacterial fatty acyl ester of G-3-P lipid membranes. Because of isoprenoid hydrocarbon chains, archaeal lipid membranes have a property of quite low permeabibility of H+ and other low molecular-weight compounds, and the low permeability is rather insensitive to temperature. The same kind of lipid composition of archaeal membrane can adapt to rather wide range of temperature; i.e., a moderate temperature (37°C)-grown methanoarchaeon Methanobacterium formicicum and a higher temperature (65°C)-grown methanoarchaeon Methanothermobacter thermautotrophicus show the same lipid composition. Caldarchaeol lipids are seen both in hyperthermophilic Archaea and psychrophilic Archaea. By contrast, bacterial membrane lipid composition is minutely regulated. Although an ether-bond itself is hardly cleaved under mild acidic and heating conditions (for example, 1 M HCl at 100°C for 3 h, under which condition an ester bond is completely hydrolyzed), ether bond-containing lipids do not necessarily give thermotolerancy to an organism, since ether lipids are synthesized via geranylgeranyl ether intermediates, which are one kind of acid-labile allyl ethers. Because of a tiny amount of this labile allyl intermediate in the lipid biosynthetic pathway, the heat-stable final product of saturated ether bonds cannot be synthesized. However, the low permeability even at rather high temperature is one of the most significant features of Archaeal membranes (3). 5. How were the membrane lipids of common ancestor cells composed? The homologous nature and universal distribution of enzymes of the family CDP-alcohol phosphatidyltransferase among almost all of the organisms of the three domains suggest the ancestral enzyme must be present

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in the last common ancestor cells. Genetic and biochemical data suggest the existence of isoprenoid synthetic system, fatty acid synthase, enzymatic or pre-biotic synthesis of G-1-P and G-3-P. Because major polar groups are shared by organisms of all three domains, they are likely also to exist in the universal common ancestor. Consequently, polar lipids made from chemical combinations, for example, isoprenoid-G-1-P lipid (Ai), isoprenoid-G-3-P (Bi) lipid, fatty acid-G-1-P (Af) lipid, and fatty acid G-3-P (Bf) lipid, should have been present in the universal common ancestor cells. 6. How did differentiation of archaea and bacteria take place? A question is raised: how the universal common ancestor cells with at least four types of lipids in their membranes were differentiated into Archaea and Bacteria, which have solely Ai and Bf lipids, respectively. Several hypotheses or speculations have been proposed. I do not intend, here, to review the various arguments, but I summarize a hypothesis that postulates lipid-driven differentiation [11]. According to Wächtershäuser’s proposal [12], the enantiomeric difference in GP backbones was the driving force of differentiation of Archaea and Bacteria from the pre-cells (the common ancestor cells). The membranes of the pre-cells are composed of lipids with a backbone of racemic GP (a mixture of G-1-P and G-3-P in equal amounts). Wächtershäuser postulated that membranes made of mixed GP enantiomers (heterochiral membranes) are less stable than membranes with higher GP chirality (i.e., more homochiral membranes), and that the lipid membrane spontaneously segregated into more homochiral membrane patches. Pre-cells with more homochiral lipid membranes then underwent frequent collision, fusion, and division. Consequently, highly homochiral pre-cells (G-1-P-rich pre-cells and G-3-P-rich pre-cells) evolved. During this process, heterochiral pre-cells became extinct as a result of their instability. The enzymes, G-1-P dehydrogenase and G-3-P dehydrogenase, induced pre-cells to develop membranes with a pure enantiomeric GP backbone. These cells became the ancestors of Archaea and Bacteria, respectively. If four types core lipids existed in the universal common ancestor cells, the segregation of membrane phospholipids with the G-1-P backbone with either isoprenoid or fatty acids (Ai or Af) and the G-3-P lipid with either isoprenoid or fatty acids (Bi or Bf). To segregate and survive as Ai and Bf lipids, not only the GP enantiomer backbones but also the hydrocarbon chains must be involved in this process. A hypothetical mechanism for the segregation of the four core lipids was quite recently proposed [11].

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Figure 4. Diastereomers of diphytanylglycerol (archaeol). The stereochemical configuration of three chiral centers were established by Kates et al [13] as all R. The configuration of the C2 positions of G-1-P and G-3-P are S and R, respectively. Therefore, Ai has S,R,R,R,R,R,R configuration, and Bi has R,R,R,R,R,R,R configuration, which are diastereomers each other. Enantiomeric GP backbones are assumed to be the main driving force for membrane segregation. In addition, different types of hydrocarbon chains would prompt segregation of the two lipids. Isoprenoid chains and typical fatty acid chains have largely different molecular shapes; isoprenoid chains are characterized by four methyl branches on the main chain at every four carbon atoms, and most isoprenoid chains of archaeal phospholipids are saturated, whereas fatty acyl chains have no or only one methyl branch and often have a cis double bond, at which the chain is kinked. Thus, the physicochemical properties of these chains differ and the effect of the hydrocarbon interaction on segregation is significant. Since the stereo structure of the whole molecule of Ai and Bi are diastereomers (seven chiral centers of diphytanyl-G-1-P is S,R,R,R,R,R,R; of

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diphytanyl-G-3-P is R,R,R,R,R,R,R,), while Bf and Af are enantiomers (Only one chiral center of difatty acyl-G-1-P and difatty acyl-G-3-P are S and R, respectively), the tendency toward segregation of Ai:Bf seems to be higher than Af:Bi, because diastereomers have different physicochemical properties as different compounds, whereas enantiomers have identical properties except for their chiral properties (Fig. 4). Consequently, descendent cells with Ai lipid membranes became the ancestor of Archaea and descendent cells with Bf lipid membranes became the ancestor of Bacteria. The Ai and Bf lipids are clearly segregated. Auto (self) segregation of homochiral lipid membranes is one of the driving forces of the differentiation of Archaea and Bacteria, i.e., the “lipid divide” is caused by membrane-driven evolution. 7. Membrane-driven differentiation If Archaea and Bacteria were differentiated by causes other than the segregation of homochiral lipid patches, Archaea and Bacteria would have an equal mix of Ai type and Bf type lipids, because lipids are in a fluid state (Singer and Nicholson, 1972) and a variety of lipid species would be mixed in the membrane. This is not the case. The common distribution of phospholipid polar head groups may indicate that both polar head groups and their synthesizing systems were equally transferred to Archaea and Bacteria through the fission (division) of pre-cells. This, in turn, suggests that polar head groups are not a driving force in the differentiation of Archaea and Bacteria. The fact that Archaea and Bacteria possess common polar groups and enzymes suggests the presence of common ancestor cells in which polar head groups and their synthesizing systems already existed. The cytoplasm is equally divided into both daughter cells. This is the reason for the common occurrence of polar groups in archaeal and bacterial phospholipids. CDP-alcohol phosphatidyltransferase enzymes were equally distributed in the both domains of organisms even though they were membrane-bound enzymes but they were not a driving force of membrane segregation. 8. Archaea research illuminate the biochemistry concerning E. coli and B. subtilis by a backlight It is needless to say that studies of E. coli and B. subtilis of half a century duration achieved magnificent results, which constructed major principles of molecular biology applicable to all the living organisms. The both microorganisms contributed creation of general principle of modern biology beyond their individual peculiarity. Phenomena, or biochemical mechanisms

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even peculiar to E. coli and B. subtilis were, occasionally, unpremeditatedly believed to be applicable to other organisms. However, some of research results of Archaea, which have many features strikingly different from bacterial species, illuminate an important property among biochemical properties of Bacteria from the other side. For example, the significance of G-3-P backbone of glycerolipids was first recognized when the backbone of archaeal lipids was found to be enantiomeric G-1-P without exception. Before then, the lipid backbone of all the organisms was believed to be G-3-P. Since archaeal G-1-P was found, G-3-P is merely one of the two types of GP backbones. Archaeal study illuminates from the reverse side what is important. The other example is substrate specificity of CDP-archaeol (CDP-diacylglycerol):serine archaetidyl (phosphatidyl) transferase (archaetidyl(phosphatidyl)serine synthase = AS (PS) synthase). AS synthase was found to be active towards CDP-1, 2-digeranylgeranyl-sn-glycerol, CDP-2, 3- digeranylgeranyl-sn-glycerol, CDP-1, 2-diacyl-sn-glycerol, CDP-2, 3-diacyl-sn-glycerol as well as CDP-2, 3-digeranylgeranyl-sn-glycerol. This shows the archaeal enzyme was active to the bacterial-type lipid substrate. Next the bacterial counterpart enzyme (PS synthase) was tested for its substrate specificity. The enzyme was also active towards all the above substrates. This property of the enzyme was inconceivable until archaeal biochemical studies achieved a measure of success. Although E. coli and B. subtilis are thoroughly investigated, they are not typical examples of organisms. Phenomena or mechanisms observed in those microorganisms should not be imprudently generalized. A number of mechanisms of metabolic regulation are greatly developed in E. coli. This is due to the habitats of the organism; E. coli usually lives in human intestine, where is warm, anoxic, and rich in nutritious organic materials, but is often released from the intestine to the outside of the host body, where is less warm, oxic, and poor in nutrition. E. coli is usually going back and forth between the two environments. To adapt, survive, and thrive in the often changes of habitat conditions, the organism have developed unique regulatory mechanisms of metabolism. For example, repression and allosteric inhibition were also first discovered in E. coli. There are rarely found microorganisms that inhabit two interchangeable, different environments; many inhabit one constant environment. When E. coli grows low temperature (e.g., 17oC), its membrane lipid is composed of more unsaturated fatty acid compared with the membrane lipid from cells grown at higher temperature (e.g., 37oC). Misconception about this phenomenon is often seen by unreasonable application to other microorganisms of increase of double bonds in lipid hydrocarbon chains as adaptation to lower temperature. In B. subtilis, little unsaturated fatty acid is present in the membrane lipids, and

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their change in composition is insignificant for adaptation to different growth temperature. Instead of unsaturated fatty acid, anteiso methyl branched fatty acid increased. Archaeal isoprenoid lipids keep lower permeability of hydrogen ion and low molecular weight compounds throughout temperature organisms are living, while bacterial straight chain alkyl chains must be regulated to a narrow range of fatty acid composition that shows just above phase transition temperature. Therefore, Archaea, in general, does not change their isoprenoid composition during growth temperature change. Thus, response in membrane lipid composition to growth temperature is different depending on species. E. coli is not at all the typical representative of organism, but a unique species in Bacteria. There have been known PS synthase type II and I. The type I enzyme was first found in E. coli. The type II enzyme was later found in B. subtilis and many other Bacteria and Archaea. Studies on lipid biosynthesis begin to illuminate what is ubiquitous and what is peculiar. As a results, it is clarfied that ubiquity represents on the peculiarities. Advances in studies on Archaea, which are the most distantly related with Bacteria, would supply unexpected but significant development to bacterial science. References 1. Woese, C.R., Kandler, O., and Wheelis, M.L. 1990, Proc. Natl. Acad. Sci. USA,

87, 4586. 2. Koga, Y., Nishihara, M., Morii, H., and Akagawa-Matsushita, M. 1993,

Microbiol. Rev., 57, 164. 3. Koga, Y. and Morii, H., 2005, Biosci. Biotechnol. Biochem., 69, 2019. 4. Koga, Y. and Nakano, M., 2008, System. Appl. Microbiol., 31, 169. 5. Koga, Y., and Morii, H. 2007, Microbiol. Mol. Biol. Rev., 71, 97. 6. Nishihara, M. and Koga, Y., 1995, J. Biochem., 117, 933. 7. Nishimura, Y., and Eguchi, T. 2006, J. Biochem. 139, 1073. 8. Koga, Y., Kyuragi, M., Nishihara, M., and Sone, N., 1998, J. Mol. Evol., 46, 54. 9. Morii, H. and Koga, Y., 2003, J. Bacteriol., 185, 1181. 10. Morii, H., and Koga, Y., 2009, J. Biol. Chem., 284, 30766. 11. Koga, Y. 2011, J. Mol. Evol., 72, 274. 12. Wächtershäuser, G., 2003, Mol. Microbiol. 47, 13. 13. Kates, M., Joo, C.N., Palameta, B., and Shier, T. 1967, Biochemistry, 6, 3329. Footnote 1. sn, stereospecific numbering. One of the nomenclatures of stereo isomers of the glycerol backbone of glycerolipids. Glycerol is a non chiral molecule but pro-chiral. The pro-S carbon is designated sn-1. sn-Glycerol-3-phosphate

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corresponds to L-glycerol-3-phosphate, which is the backbone of bacterial and eukaryal membrane lipids. sn-Glycerol-3-phosphate is the enantiomer of sn-glycerol-1-phosphate, which is in the archaeal membranes. 2. Diastereomers: Compounds which have two or more chiral centers and are not enantiomers each other. While enantiomers show the same physicochemical properties except for stereochemical properties, diastereomers are different in nature as different compounds.