Hopanoid biosynthesis and function in bacteria

Hopanoids are pentacyclic triterpenoid lipids occurring in bacteria. They a re synthesized from isopentenyl units which are formed in a new biosyntheti c route leading to isopentenyl diphosphate. Six C-5 units are joined to for m squalene, the immediate precursor in hopanoid synthesis. In a highly comp lex cyclization reaction that shares considerable similarities with that of oxidosqualene to sterols, the hopane skeleton is formed from squalene by t he squalene-hopene cyclase. Recent elucidation of the X-ray structure of th is membrane-bound cyclase has shed some light on the properties of this unu sual enzyme. The active site is located in a cavity within the enzyme. The squalene substrate diffuses through a channel structure from the membrane i nto this cavity and is there transformed into hopene. Polar side chains are attached to hopene resulting in the amphiphilic molecular structure of man y hopanoids. These hopanoids are membrane components involved in regulating membrane fluidity and stability. However, the many structural variants of hopanoids indicate that they may have other interesting but as yet unknown functions.