Hopanoids structures

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. ... 

Structures of some hydrocarbons (a) methane, (b) a branohed and unsaturated C31 hydrocarbon, (c) cyclohexane, (d) benzene, (e) some diamondoids, and (f) some hopanoids. 

Currently there is no experimentally determined three-dimensional structural information available for OSCs, although studies with a related enzyme, squa-lene-hopene cyclase (SC EC 5.4.99.7) have proved informative. SCs are involved in the direct cyclisation of squalene to pentacyclic triterpenoids known as hopanoids, which play an integral role in membrane structure in prokaryotes [ 51 ]. A number of SC genes have been cloned from bacteria [52 - 54]. The SC and OSC enzymes have related predicted amino acid sequences, and so should have similar spatial structures [55]. The crystal structure of recombinant SC from the Gram-positive bacterium Alicyclobacillus acidocaldarius has established that the enzyme is dimeric [55]. Each subunit consists of two a-a barrel domains that assemble to form a central hydrophobic cavity [55,56]. 

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. 

Figure 9.10 The general chemical structure of hopane and the simplest C30 hopanoid, diploptene.

In bacteria, a family of molecules with a striking chemical similarity to cholesterol, the hopanoids, insert into the membrane hemilayer and stabilize membrane structure (figure 7.28 bacteriohopanetetrol). The effects of these prokaryotic cholesterol analogs are similar to those of cholesterol they broaden the gel-fluid phase transition, condense the bilayer, and reduce bilayer permeability. Contents of hopanoids in bacterial membranes may rise with acclimation temperature (Poralla et ah, 1984). 

Fig. 1.4 (a) Generalized membrane structure. Hydrophobic heads of amphipathic molecules are shown in black and hydrophilic tails as wavy lines. Membrane reinforcers are shown as boxes and can be likened to nails spanning half the membrane (e.g.sterols and hopanoids) or rivets spanning the entire membrane (e.g. carotenoids). Where they are absent, in archaebacterial thermophiles, the amphipathic molecules span the entire membrane and act as struts , (b) Possible steps in the self-assembly of primitive membranes (M2+ are metal ions). 

Sterols comprise a very important class of compounds in plant and animal systems. Bacteria and other organisms utilize cholesterol or related sterols in the synthesis and maintenance of their cell walls. True sterols are present in cyanobacteria to only a very small degree they are largely replaced with a steroid-like class of triterpenoid known as the hopanoids. The carbon frameworks of sterols and bacterial hopanoids are synthesized in essentially the same manner as other terpenes, however, the cyclization reactions to yield their polycyclic skeletons are unique. A variety of sterols and hopanoids are found in cyanobacteria and they are believed to have important cellular functions in cell wall structure and function. " As discussed in Section 2.06.5, many of the hopanoids also possess a polyalcohol chain derived from a sugar. ... 

The branched isoprene unit, which is also synthesized from the C, pool, is the basic structure of terpenoids. Less condensed structures are used as volatile pheromones, e.g., jasmonic acid, menthol, or camphor, or as natural rubber material. More condensed structures such as steroids and hopanoids are part of membranes, influencing their fluidity. They are also highly specific to their source organisms. Best known are cholesterol (in animals and plants), ergosterol (in fungi), and brassicasterol (in diatoms). Besides cellulose, hopanoids are the most abundant biomolecules. 

It will be shown that bacteria contain similar compounds - namely the hopanoids. These are structural and functional equivalents of sterols. 

Hopanoids are pentacyclic molecules that are found in bacteria and in some plants. A typical bacterial hopanoid, bacteriohopanetetrol, is shown in Figure 12.4. Compare the structure of this compound with that of cholesterol. What effect would you expect a hopanoid to have on a bacterial membrane ... 

Some T. have major physiological significance. Thus, lanosterol is converted biosynthetically to cholesterol, the precursor of all steroid hormones, bile acids, and vitamin D3. In fungi, lanosterol is converted to er-gosterol (see sterols), an essential component of the fungal cell membrane. Plant cell membranes also incorporate steroids (phytosterols). In prokaryotes, the hopanoids take over the functions of steroids in the cell membranes. As a component of animal and plant waxes T. strengthen the structures. They protect the plant surface from desiccation and attack by microorganisms (e.g., betulin, lupeol, oleanolic acid, and ursolic acid). 

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