Hopanoids



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). 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 prokaryotic cell membranes, and cholesterol, a sterol compound found in eukaryotic membranes (I, II, and III in images at right).

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 2.7-billion-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 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.

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.