User:Ssilverman16/sandbox/isoarborinol

Isoarborinol is a triterpenoid ubiquitously produced by angiosperms and is thus considered a biomarker for higher plants. Though no isoarborinol-producing microbe has been identified, isoarborinol is also considered a possible biomarker for marine bacteria, as its diagenetic end product, arborane, has been found in ancient marine sediments that predate the rise of plants. Importantly, isoarborinol may represent the phylogenetic link between hopanols and sterols.

=Background= Isoarborinol is a pentacyclic triterpenoid, a class of 30-carbon isoprenoid compounds commonly found in higher plants. It is structurally similar to plant cyclics in the lupenoid series (including lupeol, betulin and lupane), but its isobutyl functional group occupies a different position on the cyclopental ring. Isoarborinol likely serves as a fluidity-buffering component of biological membranes, similar to sterols and hopanols.

=Synthesis?= Need to finish this

=Earth history= The known distribution of isoarborinol in extant organisms is predominantly limited to a few angiosperms (e.g., the Gramineae family ), which has led many to view isoarborinol as a biomarker for higher plants. However, arborane, the diagenetic product of isoarborinol, has been reliably identified in Permian and Triassic sediments, which significantly predate the late-Jurassic first appearance of angiosperms and thus support a microbial origin for isoarborinol. Additionally, arborane has been isolated from lacustrine sediments, and has shown carbon isotopic signatures inconsistent with plant origin. Though no isoarborinol-producing microbe has been found, the marine heterotrophic bacterium Eudoraea adriatica was discovered to make adriaticol and eudoraenol, two isomers of isoarborinol.

=Biogeochemical significance= The enzyme responsible for making isoarborinol may represent the evolutionary link between the hopanol-producing enzymes in bacteria and the sterol-producing enzymes in eukaryotes. Whereas hopanoid cyclases fold squalene into a five-membered ring with an all-chair conformation, sterol cyclases fold oxidosqualene into a four-membered ring with a chair-boat-chair conformation. Isoarborinol cyclase uses a combination of these aspects, cyclizing oxidosqualene into a five-membered ring with a chair-boat-chair conformation. Steroid cyclases may have evolved from hopanoid cyclases, and isoarborinol cyclase could represent the enzymatic intermediate of this transition. Alternatively, steroid cyclases and hopanoid cyclases may have diverged from a common ancestor, in which case the phylogenetic significance of isoarborinol cyclase is unclear.

=References=