Hopanoids are natural pentacyclic compounds (containing five rings) based on the chemical structure of hopane. The first known triterpenoid of the class, hydroxyhopanone, was isolated by two chemists at The National Gallery, London working on the chemistry of dammar, a natural resin used as a varnish for paintings.  The name hopane was derived from the genus Hopea (a source of the resin) itself named after John Hope, the botanist. Since this first finding, however, hopanoids have been found to be present in nature in vast amounts as components of bacteria and other primitive organisms. Their primary function is to improve plasma membrane strength and rigidity in bacteria. In eukaryotes (including humans) sterols serve a similar function. This relationship between biochemical structure and cellular function can be seen in the similarity of the basic structures of diploptene, a hopanoid compound found in some bacterial cell membranes, and cholesterol, a sterol compound found in eukaryotic membranes (I, II, and III in images at right). Hopanoids are not detected in archaea.
In many bacteria hopanoids may play important roles in the adjustment of cell membrane permeability and adaptation to extreme environmental conditions. They are formed in the aerial hyphae—spore bearing structures—of the prokaryotic soil bacteria Streptomyces, where they are thought to minimise water loss across the membrane to the air. This is a physiological adaptation not faced by most bacteria which mainly live in water, but similar adaptations are needed by eukaryotic fungi that produce aerial spore bearing hyphae.
In the ethanol fermenting bacterium Zymomonas mobilis hopanoids may have a role in adaptation of cell membranes to ethanol accumulation and to temperature changes which influence membrane functions. In the actinomycete Frankia, the hopanoids in diazovesicle membranes likely restrict the entry of oxygen by making the lipid bilayer more tight and compact.
A range of hopanoids are found in petroleum reservoirs, where they are used as biological markers.
Hopanoids in paleobiology
Hopanoids, including 2-alpha-methylhopanes from photosynthetic bacteria (cyanobacteria), were discovered by Roger Summons and colleagues as molecular fossils preserved in 2.7 Gya shales from the Pilbara, Australia. The presence of abundant 2-alpha-methylhopanes preserved in these shales may indicate that oxygenic photosynthesis evolved 2.7 Gya, well before the atmosphere became oxidizing. However Fischer et al. have demonstrated that Geobacter sulfurreducens can synthesize diverse hopanols, although not 2-methyl-hopanols, when grown under strictly anaerobic conditions. Archean 2-methyl-hopanes also might have been produced by ancestral cyanobacteria that predated oxygenic photosynthesis.
Andrew H. Knoll, in Life on a Young Planet (2003), especially in Chapter 6, The Oxygen Revolution, has an authoritative and very readable account of the usefulness of hopanoid molecular fossil biomarkers in reconstruction of early evolution and geology.
Geohopanoids are the molecular fossils of a long overlooked family of bacterial metabolites. Geohopanoids are probably the most abundant natural products on earth. Geohopanoids are part of the pentacyclic triterpenoid series and are present in the organic matter of all sediments, independent of their age, origin or nature  Triterpenes of the hopane series are rarely present in plants. Triterpenes of the hopane series are more common in lichens, mosses and especially ferns. In contrast to the higher plant hopanoids, which come from the cyclization of oxidosqualene and possess an oxygenated function at C-3, hopanoids from lower eukaryotes mostly come from the direct cyclization of squalene which is why they lack an oxygen function.
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