Chapter 37 : Bacterial Hopanoids: The Lipids That Last Forever

Abstract:

The world of lipids does not always gets its due. Their oleaginous charm is not always appreciated. For example, have you heard of hopanoids? They are made by some bacteria and are an unusual kind of polycyclic lipids that resemble steroids, but with an extra ring. Just like cholesterol in eukaryotic membranes, they insert in bacterial membranes where they contribute to their stability. Both hopanoid and steroid molecules are almost planar, thus can intercalate into the lipid bilayers with relative ease and there interact with the adjacent fatty acid chains to enhance stiffness. That hopanoids and steroids play analogous roles can be readily shown in mycoplasma, the bacteria that have the unusual ability to incorporate exogenous steroids in their membranes. Mycoplasma mycoides, it turns out, can grow without steroids if provided with hopanoids. Not all bacteria make hopanoids, but they play a vital role in the ones that do. How do we know? Inhibitors that stop their synthesis also inhibit the growth of the organisms that make them. Hopanoids are found in trace amounts in some plants and not at all in the Archaea. For a review, see here (1.usa.gov/1H96TT8), and for previous appearances in this blog, here for a piece (bit.ly/1MrGNCg) by Tanja Bosak and here (bit.ly/1MrH4VO) for one by Paula Welander.