Hopanoids

Hopanoids are pentacyclic isoprenoidal membrane lipids produced by some bacteria. They share a semi-planar polycyclic ring structure with sterols (e.g., cholesterol) and have been called “bacterial sterol surrogates” since the 1970s (Rohmer et al., 1979, PNAS; Ourisson et al., 1987, Annual Review of Microbiology). Critically, hopanoid biosynthesis does not require molecular oxygen — unlike sterol synthesis — making them candidates for the earliest lipid-ordering molecules in Earth’s history.

Structure

The hopanoid ring system consists of five fused rings (A–E) forming a rigid, planar scaffold. The simplest hopanoid, Diplopterol, has a hydroxyl group at C-22. Extended side-chain hopanoids, known as Bacteriohopanepolyols (BHPs), carry polyhydroxylated or amino-functionalized C₅ side chains. Structural modifications include:

• Methylation at C-2 (2-methylhopanoids, associated with some cyanobacteria and α-proteobacteria)
• Methylation at C-3 (found in some methanotrophs)
• Various side-chain modifications (cyclitol ethers, guanidine substitution, amino groups)

Biological Function

Our work has established two key functional roles for hopanoids:

Membrane ordering. We demonstrated that Diplopterol orders saturated lipids and forms liquid-ordered (Lo) phases in model membranes, functionally equivalent to cholesterol (Sáenz et al., 2012, PNAS). In [[methylobacterium-extorquens|Methylobacterium extorquens]], hopanoids are localized to the outer membrane where they interact with lipid A to determine membrane order — analogous to sterol-sphingolipid interactions in eukaryotic plasma membranes (Sáenz et al., 2015, PNAS).

Multidrug efflux. Deletion of hopanoid synthesis in M. extorquens impairs energy-dependent multidrug transport, linking membrane order to active transport function in bacteria (Sáenz et al., 2015, PNAS).

Hopanoids have also been associated with detergent-resistant membrane fractions in [[crocosphaera-watsonii|Crocosphaera watsonii]], consistent with a role in membrane lateral organization (Sáenz, 2010, Organic Geochemistry).

Phylogenetic Distribution

Not all bacteria produce hopanoids. An estimated 5–10% of marine bacteria carry squalene-hopene cyclase (SHC) genes (Pearson et al., 2007, Environmental Microbiology; Pearson & Rusch, 2009, ISME Journal). Among marine cyanobacteria, hopanoid production in pure culture has been detected only in nitrogen-fixing species: C. watsonii, Trichodesmium erythraeum, and Cyanothece spp. The cosmopolitan picocyanobacteria Prochlorococcus and Synechococcus do not produce hopanoids (Sáenz et al., 2012, Geobiology).

As Biomarkers

Geologically stable hopanoid degradation products (hopanes) are among the most abundant molecular fossils in sedimentary rocks. The oldest uncontested hopanoid records date to ~1.64 Ga, though older reports (>2.5 Ga) exist but their syngenicity has been contested due to contamination concerns (French et al., 2015, PNAS). Our environmental surveys showed that:

• BHP abundance and structural diversity in marine suspended particles are highest in suboxic/anoxic waters (Sáenz et al., 2011, Organic Geochemistry)
• Terrigenous SPM has 10× greater BHP concentration and far higher structural diversity than marine SPM (Sáenz et al., 2011, Organic Geochemistry)
• BHT II, an isomer of bacteriohopanetetrol, is a candidate biomarker for water column suboxia and anoxia

Hopanoids in Plant-Associated and Symbiotic Bacteria

Collaborative work has demonstrated that hopanoids play important roles in the environmental fitness of plant-associated bacteria. Extended hopanoids confer robustness to physicochemical variability (osmotic stress, pH, detergent challenge) in the nitrogen-fixing soybean symbiont Bradyrhizobium diazoefficiens, explaining why hopanoid-deficient mutants show defects in planta but not in standard culture (Tookmanian et al., 2022, mBio). A review by Tookmanian et al. (2021, mLife) synthesized evidence that hopanoid-mediated membrane resilience may help rhizobia withstand the osmotic and temperature stresses intensified by climate change.

Evolutionary Significance

Because hopanoid cyclization by SHC does not require O₂, the capacity to order membranes through polycyclic isoprenoids could predate the Great Oxidation Event (~2.4 Ga). Our demonstration that diplopterol forms Lo phases suggests that membrane lateral organization — the basis for lipid raft-like domains — may be an ancient, pre-eukaryotic innovation.