Hopenes

Here we used Hopen ) - 0. With the same separation for 9,... 

FIGURE 5. Mass spectra of two C30H50 isomers. Squalene and hopene (compound X) are isolated from a sedimentary organic matter1415... 

Fatty acids (methyl esters) FAMES TMS fragment Hopanes and hopenes Sterols (TMS)... 

Plant. After 8 d following absorption in barley plants, no metabolites were detected (Splittstoesser and Hopen, 1968). 

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]. 

Alicyclobacillus Squalene-hopene cyclase (SC). . raveyllscEkdeByI.. mekirryllheBredBt. . ypdvBdBavvvwa.. rraveylkreBkpdBsB.. kaldwveqhBnpdBgB.. acidocaldarius... 

In green plants, which contain little or no cholesterol, cydoartenol is the key intermediate in sterol biosynthesis.161-1623 As indicated in Fig. 22-6, step c, cydoartenol can be formed if the proton at C-9 is shifted (as a hydride ion) to displace the methyl group from C-8. A proton is lost from the adjacent methyl group to close the cyclopropane ring. There are still other ways in which squalene is cyclized,162/163/1633 including some that incorporate nitrogen atoms and form alkaloids.1631 One pathway leads to the hop-anoids. These triterpene derivatives function in bacterial membranes, probably much as cholesterol does in our membranes. The three-dimensional structure of a bacterial hopene synthase is known.164 1643 Like glucoamylase (Fig. 2-29) and farnesyl transferase, the enzyme has an (a,a)6-barrel structure in one domain and a somewhat similar barrel in a second domain. 

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... 

A number of selected molecular parameters obtained from analysis of immature crude oils and sediment extracts are evaluated as indicators of palaeosalinity. The nature of these parameters is discussed taking into account the role of intermolecular and intramolecular incorporation of sulfur into specific functionalized lipids. Specific distribution patterns of methylated chromans and C20 isoprenoid thiophenes and the relative abundance of gammacerane are excellent indicators for palaeosalinity, whilst other parameters such as 14< (H),17a(H)/140(H),170(H) -sterane ratios, the pristane/phytane ratio, the even-over-odd carbon number predominance of n-alkanes and the relative abundance of C35 hopanes and/or hopenes may indicate palaeohypersalinity but are affected by environmental factors other than hypersalinity and by diagenesis. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

The relative amounts of the extended hop-17(21)-enes (Figure 3e) and, especially, the relative concentrations of the C35 homologue (Figure 3f) show some sudden changes at the facies boundaries. It has been proposed (33-35) that this relative abundance of C35 extended hopenes (Facies B) reflects a hypersaline palaeoenvironment. Others consider this the typical distribution of the extended hopenes in anoxic marine sediments (32). 

Fig. 14.13. Enzymatic transformation of acyclic squalene (A) into pentacyclic hopene (I). The squalene-hopene cyclase controls the conformation of the substrate so effectively that only one out of 512 possible stereoisomers is formed.

One of the more exciting and recent advances in the field of plant biochemistry has been the discovery of the mevalonate-independent pathway for the biosynthesis of isoprenoids (Fig. 10.4). This new pathway, referred to a the methyl-erythritol-phosphate or MEP pathway for the first intermediate committed solely to the biosynthesis of isoprenoids, was first discovered in prokaryotes capable of accumulating hopenes, the equivalent of eukaryotic sterols. 6,17 The MEP pathway has since been confirmed in plants and, not surprisingly, has been localized to chloroplasts.18 Operation of the MEP pathway is intimately related to the reactions of CO2 fixation and photosynthesis, as evidenced by the two immediate precursors pyruvate and phosphoglyceraldehyde for this pathway. Two important features of this pathway are that mevalonate is not an intermediate in the plastidic synthesis of isopentenyl (IPP) and dimethylallyl diphosphate, (DMAPP), and this pathway... 

In this context, Hoshino et al. have recently reported the cyclization of (3S)-2,3-oxidosqualene catalyzed by a Gly600-deletion mutant (AG600SHC) of the enzyme squalene-hopene cyclase (SHC) from Alicyclobacillus acidocal-darius (Scheme 31) [108]. The enzymatic biotransformation gave monocyclic (9) and tricyclic triterpenoids (38 and 44-47), but no detectable bicychc products [108]. Despite the authors biogenetic proposal of a conventional carbocationic pathway, the skeletal profile of products reported closely resembles that expected for a radical-type cyclization. 

Summons R. E. and Jahnke E. E. (1992) Hopenes and hopanes methylated in ring-A correlation of the hopanoids from extant methylotrophic bacteria with their fossil analogues. In Biological Markers in Sediments and Petroleum (eds. J. M. Moldowan, P. Albrecht, and R. P. Philp). Prentice Hall, Englewood Cliffs, NJ, pp. 182-200. 

Further support comes from comparison of the H n.m.r. and mass spectral data of the diols (183) and (184) obtained by oxidative cleavage, followed by reduction, of the 17(21)-double bond in the hopene and flavicene series/ ... 

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