Hopanoids bacteria

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

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]. 

Hopanoids subgroup of compounds within the triterpenoids that are very abundant in bacteria where they have generally replaced cholesterol. 

Neunlist, S., Bisseret, P., and Rohmer, M. (1988) The hopanoids of the purple non-sulfur bacteria Rhodopseudomonas palustris and Rhodopseudomonas acidophila and the absolute configuration of bacteriohopanetetrol. Eur. J. Biochem. 171, 245-252. 

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). 

It is often said that hopanoids are the most abundant natural products on Earth (Ourisson and Albrecht, 1992) and a major body of work exists on their distributions in sediments, in prokaryotes and in plants. Most commonly, hopanoids are found in select groups of Bacteria, all of which are aerobic (Farrimond et al., 1998 Rohmer et al., 1984). In fact. 

Although they are known to be synthesized by a wide variety of cultured aerobic bacteria there does not appear to be any obligate requirement for oxygen in their biosynthesis. The biosynthesis and cyclization of squalene to a pentacyclic triterpenoid with a hopane skeleton does not seem to require oxygen and, therefore, hopanoid synthesis might also be possible in anaerobes. For instance, analysis of microbial mats at methane seeps under anoxic Black Sea water revealed the presence of C-depleted (8 C = -lS%c) hopanoids with an unusual stereochemistry. This isotopic depletion indicates in situ production and, therefore, suggests that anaerobes are responsible (Thiel et ai, 2003). 

It also appears that further clues about hopanoid origins can be drawn from the polar side-chains which carry different numbers and types of substituents. This, in turn, affects their subsequent diagenesis and the types of hopane hydrocarbon, ketone, and other products that are recorded in sediments. In addition to the diagnostic 3j8-methyl substituents, hexafunctionalized side-chains are prevalent in Type 1 methanotrophic bacteria (Neunlist and Rohmer, 1985 Zundel and Rohmer, 1985a). The hydroxy substituent at C-31 of these compounds appears to assist oxidative loss of this carbon or the one at C-30, resulting in a predominance of C30 hopane and 30-norhopane products where methanotrophs are prevalent or even dominant (e.g., Burhan et aL, 2002 Rohmer et ai, 1992). Anomalous C-depletion of these hopanoids often observed in sediments and oils is quite consistent with this interpretation (e.g.. Summons et ai, 2002). 

NeunUst S. and Rohmer M. (1985) Novel hopanoids from the methylotrophic bacteria Methylococcus capsulatus and Methylomonas methanica. Biochem. J. 231, 635—639. 

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. 

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. 

Hopanoids are thought to represent surrogates of cholesterol in bacteria, acting as efficient membrane stabilizers. This is supported by the findings thaf bofh sferols and hopanoids are... 

Kannenberg E.L., Poralla K. (1999) Hopanoid biosynthesis and function in bacteria. Naturwissenschafien 86, 168—76. 

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. ... 

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

There exists ample evidence by physiological experiments and by different physico-chemical methods that hopanoids In bacteria possess membrane properties similar to sterols, especially cholesterol, In higher organisms. We are aware that one line of evidence In this context Is still missing. Until now mutant analysis on hopanoids has not been done. Also the Influence on enzymatic membrane processes has not been measured. 

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 ... 

---