produce 2-methylhopanoids. Additionally, following their identification of hpnR, the gene responsible for methylating hopanoids at the C3 position, Paula Welander and Roger Summons identified putative hpnR homologs in members of alpha-, beta-, and gammaproteobacteria, actinomycetota, nitrospirota, candidate phylum NC10, and an acidobacterium, as well as in three metagenomes. As such, Welander and Summons conclude that 3-methylhopanoids alone cannot constitute evidence of aerobic methanotrophy. Applications Industry The elegant mechanism behind the protonase activity of squalene-hopene cyclase was appreciated and adapted by chemical engineers at the University of Stuttgart, Germany. Active site engineering resulted in loss of the enzyme's ability to form hopanoids, but enabled Brønsted acid catalysis for the stereoselective cyclization of the monoterpenoids geraniol, epoxygeraniol, and citronellal. Agriculture The application of hopanoids and hopanoid-producing nitrogen fixers to soil has been proposed and patented as a biofertilizer technique that increases environmental resistance of plant-associated microbial symbionts, including nitrogen-fixing bacteria that are essential for transforming atmospheric nitrogen to soluble forms available to crops. Medicine During later studies of interactions between diplopterol and lipid A in Methylobacterium extorquens, multidrug transport was found to be a hopanoid-dependent process. Squalene-hopene cyclase mutants derived from a wild type capable of multidrug efflux, a drug-resistance mechanism mediated by integral transport proteins, lost the ability to perform both multidrug transport and hopanoid synthesis. Researchers indicate this could be due to direct regulation of transport proteins by hopanoids or indirectly by altering membrane ordering in a way that disrupts the transport system. References Triterpenes Membrane biology Bacteria Microbiology