Hopanoids are pentacyclic compounds similar to sterols, whose primary function is to improve plasma membrane fluidity in prokaryotes. Cholesterol serves a similar function in eukaryotes (including humans).

Note the similarity of the basic structures of diploptene, a hopanoid compound found in some prokaryotic cell membranes, and cholesterol, a sterol compound found in eukaryotic membranes (I, II, and III in images at right).

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.

Hopanoid molecules, including particular types of hopanoid (2-alpha-methylhopanes) from photosynthetic bacteria (cyanobacteria), were discovered by Roger Summons and colleagues as molecular fossils preserved in 2700-million-year-old shales from the Pilbara, Australia. The presence of abundant 2-alpha-methylhopanes preserved in these shales indicates that oxygenic photosynthesis evolved 2.7 billion years ago, well before the atmosphere became oxidizing.

In many bacteria hopanoids have important roles in adjustment of cell membrane permeability to extreme environmental conditions. They are formed in the aerial hyphae (spore bearing structures) of prokaryotic streptomycetes soil bacteria, 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 on the diazovesicle membranes restrict the entry of oxygen by making the lipid bilayer more tight and compact.

References

Poralla K, Muth G, Hartner T (2000) Hopanoids are formed during transition from substrate to aerial hyphae in Streptomyces coelicolor A3(2). FEMS Microbiol Lett. Aug 1;189(1):93-5.

Berry AM, Harriott OT, MoreauRA, Osman SF, Benson DR and AD Jones (1993) Hopanoid Lipids Compose the Frankia Vesicle Envelope, Presumptive Barrier of Oxygen Diffusion to Nitrogenase. Proceedings of the National Academy of Sciences, Vol 90, 6091-6094 ^*

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

Hopanoid