Lipid A

Lipid A is the conserved hydrophobic anchor of lipopolysaccharide (LPS), the dominant lipid in the outer leaflet of the gram-negative bacterial outer membrane. Its structure — with saturated, amide-linked acyl chains and hydroxylations — bears striking similarity to sphingomyelin, the primary partner of cholesterol in eukaryotic membrane ordering.

Structural Analogy to Sphingomyelin

We exploited the structural parallels between lipid A and sphingomyelin to test whether Diplopterol interacts with lipid A the way cholesterol interacts with sphingolipids. Key shared features include:

• Saturated acyl chains
• Amide-linked backbone
• Hydroxyl groups

Interaction with Hopanoids

Model membranes. Using synthetic kdo-lipid A (Di[3-deoxy-D-manno-octulosonyl]-lipid A), we showed that both diplopterol and cholesterol condense lipid A in monolayers, inhibit pH-induced gel phase transitions, and buffer pH-induced changes in membrane order (Sáenz et al., 2012, PNAS).

Thermodynamic favorability. Gibbs excess free energy of mixing (ΔGex) measurements showed that the diplopterol-lipid A interaction is nearly identical in favorability to the cholesterol-sphingomyelin interaction — both negative (attractive). Diplopterol, unlike cholesterol, interacts repulsively with unsaturated phospholipids, making lipid A the preferred partner in the bacterial outer membrane (Sáenz et al., 2015, PNAS).

Physiological Relevance

Lipid A undergoes a pH-dependent change in order, becoming more rigid (approaching gel state) at low pH. This could be detrimental to outer membrane function. The ability of Hopanoids to moderate these pH-induced ordering changes may explain why hopanoid-deficient mutants are sensitive to low pH and why hopanoids are prominent in bacteria living in acidic or pH-variable environments (soils, acid mine drainage).