Lipids hopanoids

The cell envelopes prevent rapid diffusion of 02 into the cells but do permit rapid enough entry of N2 to maintain the observed rate of fixation of N2.72 In actinomycetes of the genus Frankia, which forms root nodules with woody plants, nitrogen fixation occurs in vesicles that are sheathed by multiple layers of hopanoid lipids (see Chapter 22).73... 

Moreau used a ternary gradient including 0.04% triethylamine in water 23 different classes of both nonpolar and polar lipids, including glycolipids and phospholipids, were resolved within 1 hour (57). Besides NL, FFA, DPG, PE, PG, PI, PS, and PC, three major hopanoid classes were... 

RA Moreau, MJ Powell, SF Osman, BD Whitaker, WF Fett, L Roth, DJ O Brien. Analysis of intact hopanoids and other lipids from the bacterium Zymomonas mobilis by high performance liquid chromatography. Anal Biochem 224 293-301, 1995. 

The concentrations of the total alkylthiophenes (except thiophene hopanoids) in the bitumens show a depth profile somewhat similar to that of the total sulfur content (cf. Figures 6a and Id). The high concentrations of alkylthiophenes in Facies B compared to A and C are consistent with the proposed incorporation of inorganic sulfur species into specific funtionalised lipids in anoxic marine environments (6-12). 

The resin fractions of organic sulfur-rich bitumens are for a substantial part composed of monomers with linear, isoprenoid, steroid, hopanoid and carotenoid carbon skeletons connected to each other by (poly)sulfide linkages. These structural units may contain additional intramolecular sulfur linkages. This sulfur-rich geopolymer is also formed by sulfur incorporation into functionalised lipids in an intermolecular fashion during early diagenesis. 

Rohmer, M. (2008) From molecular fossils of bacterial hopanoids to the formation of isoprene units discovery and elucidation of the methylerythritol phosphate pathway. Lipids, 43,1095-107. 

Many coals contain extended chain (> 31 carbon atoms) hopanes and related compounds which appear to be diagenetically derived from the lipids of certain procaryotic organisms. Biologically produced hopanoids exclusively possess 173H, 213H stereochemistry. The... 

Hopanoids (the most common organic natural product on earth) must have been involved in the evolution of the biomembrane itself. All known membranes contain terpene derivatives, such as cholesterol or carotenoids, which belong to, or can be derived from, hopanoids. However, we still do not know their biological function. Their most commonly proposed mechanism is to regulate membrane fluidity. Another obvious effect is their influence on the lipid bilayer (or monolayer in the case of archaebacteria) curvature. The different types of hopanoids occurring will certainly favour the relative stability of either the planar or of the intrinsically curved membrane conformation. The ether lipids of archaebacteria, which are hopanoid derivatives, forming curved bilayers as discussed above, therefore provide evidence for cubosomes as the first organised form of life. 

Hopanoids are pentacyclic triterpenoids that make up a class of essential membrane lipids [42,43,44]. They are believed to represent the most abundant natural products on earth. They occur widespread in Gram-negative and Gram-positive bacteria, where they play an important role in membrane stability and rigidity. In particular, hopanoids are thought to function as surrogates of cholesterol. Some hopanoids represent glycolipids, some examples of which are depicted in O Fig. 9. 

It makes possible a selective concentration of lipophiles in the membranes. Vesicles made of n-acyl lipids or of polyprenyl phosphates extract selectively into the membrane any lipophilic substance lipophilic pigments (Nile Red), cholesterol, polyprenols, carotenoids, etc. This is particularly important for those lipophilic substances that play a role in stabilizing the membrane cholesterol, hopanoids, polyprenols, and carotenoids (Bisseret et al., 1983 Milon et al., 1986 Lazrak et uZ.,1988 Krajewski-Bertrand et al., 1990). 

In this simplified view of the plasma membrane, several classes of protein and lipid are illustrated. Many of these proteins and certain lipids are covalently bound to carbohydrate molecules. (Glycolipids contain carbohydrate groups.) Hopanoids are complex lipid molecules that stabilize bacterial membranes. 

By these model experiments, It was shown that hopanoids act In a manner similar to sterols on lipid membranes. Often the magnitude of the Influence Is about the same on condensation, suppression of phase transition, enhancement of viscosity and reduction of permeability. Thus nature has Invented at least one additional molecular type besides sterols exerting the above described properties. It should be mentioned that other molecules, e.g., i<>-dlhydroxycaroten-olds have a smlllar effect on membranes [50]. 

A few more complex systems should be mentioned. An interesting one is the very peculiar lipid A-type pentasaccharide 75 whieh has been identified in the Bradyrhizobium strain BTAil lipid eontent, showing a unique structure in which the lipopolysaccharide (LPS) lipid A is covalently substituted by an hopanoid structure. This eompound is built on a pentasaccharidic backbone, with several fatty ehains eonnected as amides on the aminodeo groups of two internal residues. One of the fatty chains is substituted at its other end with an hopanoid backbone through an ester bond. One galacturonic and one dimannosyl residue complete the structure, that we could consider as a SFCX system, for sure a very complex one. The discovery of this unique molecule by Molinaro and co-workers and the accompanying studies revealed the influence of structural variations in lipid A type eompounds on the properties of cell membranes, notably in the eontext of plant-microbe symbioses. ... 

Ourisson, G. and Albrecht, P. (1992b) Hopanoids. 2. Biohopanoids A novel class of bacterial lipids. Acc. Chem. Res., 25, 403-408. 

However, all anammox baderia also contain nonladder-ane lipids, including squalene, bacteriohopanetetrol, and some other hopanoids (Rattray et ah, 2008). 

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