Hopanoid

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Hopanoids Diploptene, hopanoic acids Prokaryotes, including cyanobacteria... 

Ourisson, G., M. Rohmer, and K. Poralla. 1987. Prokaryotic hopanoids and other polyisoprenoid sterol surrogates. Annual Review of Microbiology 41 310-333. 

Summons, R.E., L.L. Jahnke, J.M. Hope, and G.A. Logan. 1999. 2-methyl hopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400 554-557. 

Hopanoids Membrane stabilizing chemical components found in the cell membranes of bacteria. 

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

Hopanoids Hydrocarbons that have four six-membered carbon tings and one five-membered ring. They are synthesized by prokaryotes and are well preserved in sediments due to their low reactivity. 

Fig. 4. Scheme for the alteration of hopenoids to aromatic hopanoids and hopane (namtal product examples are boxed). 

Aromatization of hopanoids, when derived from bacterial detritus in sediments, is inferred to proceed via diagenetic alteration or bonding of the precursors to organic matter (e.g. diploptene to neohopene. 

Ourisson G, Albrecht P, Rohmet M, The hopanoids, paleochemistty and biochemistry of a group of natural products. Pure and Applied Chemistry 51 709— 729, 1979. 

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 active site lies in a large interior cavity. The properties of the hopene synthase are similar to those of oxidosqualene synthase, and it appears to function by a similar mechanism, which resembles that of Fig. 22-6 but does not depend upon 02. Hopene lacks polar groups, but these are provided in the hopanoids by a polyol side chain. One of these compounds, bacteiiohopanetetrol, may be one of the most abundant compounds on earth.160 165 166 Hopanoids appear to originate from mevalonate synthesized via the 1-deoxyxylulose pathway (Fig. 22-2). The polyol side chain is probably formed from ribose.166... 

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... 

Ourisson. G., P. Albrecht, and M. Rohmer The Hopanoids Paleochernistry and Biochemistry, Pure and Applied Chemistry, 51(4). 709- 729 (April 1979). Ourisson, G., P. Albrecht, and M. Rohmer Predictive Microbial Biochemistry From Molecular Fossils in Procaryotic Membranes. Trends in Biochemical... 

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 saturated hydrocarbon distributions of the marl samples are dominated by long-chain n-alkanes of higher land plant origin (21) with a strong odd-over-even carbon number predominance. Hopanoid hydrocarbons are the next most abundant constituents, but other hydrocarbons particularly abundant in the laminite samples described hereafter are also clearly recognizable. 

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). 

Thiophene hopanoids. Two C35 thiophene hopanoids are present in the A3 fraction of the bitumens. One of these isomers (XXVI, Figure 19) has been previously identified by Valisolalao et al (7) in black shales. The other isomer (XXVII, Figure 19) has been tentatively identified by Sinninghe Damste et al (6). Because of the lack of a suitable standard for this fraction it was impossible to quantify these compounds as precisely as the previously discussed alkylthiophenes. However, the relative abundance of these hopanoid thiophenes (see experimental section) varies considerably throughout the sequence (Figure 19a). 

---