Monday, May 22, 2006

Hopanoids- not well known, but important molecules for the microbial cognoscenti

Hopanoids have a chemical structure somewhat like cholesterol. They are produced by many different bacteria , have an interesting history and perform a range of functions in membranes.

Berry 1993

Hopanoids occur in cell membranes in a wide range of microorganisms, where these lipids contribute to membrane stability and alter phase transition properties. Hopanoids are particularly important for microbial survival in extreme thermal environments and have been identified as a major component of oil shales.

Archean molecular fossils and the early rise of eukaryotes.
Brocks JJ, Logan GA, Buick R, Summons RE.
Science. 1999 Aug 13;285(5430):1033-6.
Comment on: Science.
1999 Aug 13;285(5430):1025-6.
School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia.

Molecular fossils of biological lipids are preserved in 2700-million-year-old shales from the Pilbara Craton, Australia. Sequential extraction of adjacent samples shows that these hydrocarbon biomarkers are indigenous and syngenetic ton the Archean shales, greatly extending the known geological range of such molecules. The presence of abundant 2alpha-methylhopanes, which are characteristic of cyanobacteria, indicates that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. The presence of steranes, particularly cholestane and its 28- to 30-carbon analogs, provides persuasive evidence for the existence of eukaryotes 500 million to 1 billion years before the extant fossil record indicates that the lineage arose.

Hopanoids are formed during transition from substrate to aerial hyphae in Streptomyces coelicolor A3(2).
Poralla K, Muth G, Hartner T.
FEMS Microbiol Lett. 2000 Aug 1;189(1):93-5.
Microbiological Institute, Microbiology/Biotechnology,
University of Tubingen,
Auf der Morgenstelle 28, D-72076, Tubingen, Germany.

Streptomyces coelicolor A3(2) contains a cluster of putative isoprenoid and hopanoid biosynthetic genes. The strain does not produce the pentacyclic hopanoids in liquid culture but produces them on solid medium when sporulating.

Mutants defective in the formation of aerial mycelium and spores (bld), with the exception of bldB, do not synthesize hopanoids, whereas mutants, which form aerial mycelium but no spores (whi), do. The membrane condensing hopanoids possibly may alleviate stress in aerial mycelium by diminishing water permeability across the membrane.

Hopanoid Lipids Compose the Frankia Vesicle Envelope, Presumptive Barrier of Oxygen Diffusion to Nitrogenase
Berry, OT Harriott, RA Moreau, SF Osman, DR Benson and AD Jones
Proceedings of the National
Academy of Sciences, Vol 90, 6091-6094

Biological nitrogen fixation in aerobic organisms requires a mechanism for excluding oxygen from the site of nitrogenase activity. Oxygen exclusion in Frankia spp., members of an actinomycetal genus that forms nitrogen-fixing root-nodule symbioses in a wide range of woody Angiosperms, is accomplished within specialized structures termed vesicles, where nitrogen fixation is localized. The lipidic vesicle envelope is apparently a functional analogue of the cyanobacterial heterocyst envelope, forming an external gas-diffusion barrier around the nitrogen-fixing cells. We report here that purified vesicle envelopes consist primarily of two hopanoid lipids, rather than of glycolipids, as is the case in cyanobacteria. One envelope hopanoid, bacteriohopanetetrol phenylacetate monoester, is vesicle-specific. The Frankia vesicle envelope thus represents a layer specific to the locus of nitrogen fixation that is biosynthetically uniquely derived.


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