Archaeol

Archaeol is one of the main core membrane lipids of archaea, one of the three domains of life. One of the key features that distinguishes archaea from bacteria and eukarya is their membrane lipids, where archaeol plays an important role. Because of this, archaeol is also broadly used as a biomarker for ancient archaea, especially methanogens, activity.. Archaeol is one of the main core membrane lipids of archaea, one of the three domains of life. One of the key features that distinguishes archaea from bacteria and eukarya is their membrane lipids, where archaeol plays an important role. Because of this, archaeol is also broadly used as a biomarker for ancient archaea, especially methanogens, activity.. Archaeol is generally composed by linking two phytanyl chains to the sn-2 and sn-3 positions of a glycerol molecule. The highly branched side chains are speculated to account for the very low permeability of archaeol-based membrane, which may be one of the key adaptations of archaea to extreme environments. Archaeol is a diether lipid commonly found in archaea. Standard archaeol is 2,3-di-O-phytanyl-sn-glycerol, with two phytanyl chains binding to the position of sn-2 and sn-3 of glycerol by ether bonds. The 2,3-sn-glycerol structure and ether bond linkage are two key differences of archaea lipids from those of bacteria and eukarya that use 1,2-sn-glycerol, and mostly, ester bonds. Natural archaeol has 3R, 7R, 11R configurations for the three chiral centers in the isoprenoid chains. There are four structural variations, contributing to the complexity of the membrane lipids in function and properties. The two phytanyl chains can form a 36-member ring to yield macrocyclic archaeol. Hydroxylated archaeol has phytanyl chains hydroxylated at the first tertiary carbon atom, while sesterterpanyl archaeol have the phytanyl side chains with C25 sesterterpanyl chains, substituting at C2 of glycerol or at both carbons. Unsaturated archaeol, with the same carbon skeleton as standard archaeol but one or multiple double bonds in the phytanyl side chains is also discovered. Two archaeol molecules can undergo head-to-head linkage to form caldarchaeol (one typical glycerol dialkyl glycerol tetraether, GDGT), one of the most common tetraether lipid in archaea. Archaeol has been found in all analyzed archaea so far, at least trace amount. It represents 100% of the diether core lipids in most neutrophilichalophiles and sulfur-dependent thermophiles (though their most core lipids are tetraether lipids). Methanogens contain hydroxyarchaeol and macrocyclic other than the standard archaeol, and sesterterpanyl-chain-containing archaeol is characteristic of alkaliphilic extreme halophiles. It is noteworthy that tetraether lipids are also widely present in archaea. Liposomes (a spherical vesicle having at least at least one lipid bilayer) of lipids from archaea typically demonstrate extremely low permeability for molecules and ions, even including protons. The ion permeability induced by ionophores (ion transporters across the membranes) are also quite low, and only comparable to that of egg phosphatidylcholine (a very common biological membrane component) at 37˚C when the temperature rises up to c.a. 70˚C. Compared to bacteria and eukarya, the isoprenoid side chains of archaeol are highly branched. This structural difference is believed lower the permeability of archaea over the whole growth temperature range which enables archaea to adapt to extreme environments. Archaeol is usually found as phospholipid in archaea cells. The synthetic pathway of fully saturated archaeol phospholipid proceeds as follows: the synthesis of isoprenoid side chains by head-to-tail linkage of isoprenes, ether linkage to glycerol-1-phosphate backbone, CDP archaeol formation, polar head group attachment and saturation of double bonds. Following this, tetraether lipids may be synthesized afterwards by dimerization reaction via a head-to-head linkage. Archaea feature different biosynthetic pathways of isoprenoid chains compared to bacteria and eukarya. The precursors for isoprenoid are C5 units isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which are universal for all three domains of life. Generally, the two compounds are synthesized in bacteria via 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway (MEP/DOXP pathway), and are synthezised via mevalonate (MVA) pathway in most eukarya. The synthesis of IPP and DMAPP in archaea follows an alternate MVA pathway which differs from the classic MVA pathway in the last three steps and shares the remaining four steps. Though archaeol, featuring the ether linkage between isoprenoid chain to glycerol, has been considered as a cogent biomarker for archaea, ether membrane lipids have also been discovered in some aerobic and anaerobic bacteria, including lipids with one ester bond and one ether bond to alkyl chains. Many strictly anoxic bacteria and a few aerobic species contain plasmalogens (Pla), which has an alkyl chain bound to sn-1 position of the glycerol via a vinyl-ether bond. Similar to archaea, these lipids are thought to increase the resistivity of bacteria to adverse environments. More stunning is the discovery of nonisoprenoid dialkyl glycerol diether lipids(DGD) and branched dialkyl glycerol tetraether lipids (brGDGT), which are formed, in the similar way to archaeol, by binding alkyls chains (but not isoprenoid chains) to glycerol molecules via ether linkage. It's highly notable that these lipids are only different from archaea ether lipids in the side chains and binding positions on the glycerol. DGD is reported in thermophilic bacteria, a few mesophilic bacteria and aggregating myxobacteria.