Sterols such as cholesterol are known to be present in functional membrane microdomains (commonly referred to as rafts) in eukaryotic membranes. They play an important role in membranes by regulating their fluidity and often colocalise with proteins involved in signal transduction. Although functional membrane microdomains have been recently identified in bacterial membranes, lipids equivalent to sterols have yet to be identified in prokaryotes.Hopanoids are a class of pentacyclic triterpenoids that are found in a variety of bacteria. They have long been hypothesised to be surrogates of sterols primarily because of their similar structure. However, the nature of the actual effect of the presence of hopanoids on membranes is indeed poorly documented.
Unbiased atomistic molecular dynamics simulations have been used to investigate the relatives properties of model lipid bilayers containing the hopanoids bacteriohopanetetrol, diplopterol or diploptene, compared to a cholesterol-containing lipid bilayer, thereby shedding light on the structural and dynamical properties of hopanoid-containing lipid bilayers1. It was found that unlike bacteriohopanetetrol, diplopterol and diploptene behaved in a manner that was reminiscent of that of ubiquinone and isoprene in the membrane of plants to protect against heat and chemical shocks. Furthermore, the effect of bacteriohopanetetrol on the lipid bilayer (thickness, packing and ordering of the tails and headgroups of the lipids) was also substantially different from that of cholesterol. The simulations suggest that hopanoids may fulfil various functions in bacteria, other than being simple substitutes for sterols in prokaryotic membranes. Such functions would include stabilisation of membranes in high temperature conditions in thermophilic bacteria.