Hopanoids

Hopanoids are a class of pentacyclic triterpenoid lipids produced by many bacteria. They are structurally analogous to sterols in eukaryotes and contribute to the stability and function of bacterial membranes. The core hopane skeleton is a C30 five-ring framework; simple hopanoids such as diploptene serve as base structures from which a variety of side chains are derived to form bacteriohopanepolyols (BHPs).

Biosynthesis and genetics: Hopanoid production begins from standard isoprenoid precursors and proceeds through the cyclization of squalene by the enzyme squalene-hopene cyclase (SHC), yielding the hopane skeleton. Further enzymatic modifications generate derivatives such as diplopterol and a range of BHPs. In bacteria, genes dedicated to hopanoid biosynthesis are commonly organized in clusters (often called hpn or related operons) that encode enzymes for ring formation and side-chain elaboration.

Biological role: In membranes, hopanoids help modulate fluidity and permeability, stabilizing membranes under heat, osmotic stress, acidity, and desiccation. They play a role similar to sterols in eukaryotes, contributing to membrane integrity and proton barrier function, thereby aiding survival in diverse and challenging environments.

Ecology and significance: Hopanoids are widespread across bacterial phyla, notably Proteobacteria and cyanobacteria. They persist in sediments and rocks after cells decay, making them useful molecular fossils and biomarkers for ancient bacterial life and environmental conditions. Notable derivatives include diploptene, diplopterol, and bacteriohopanetetrol (BHT).