Sterols and other triterpenoids: source specificity and evolution of biosynthetic pathways

The source specificity and biosynthesis of cyclic isoprenoids such as hopanoids, sterols and pentacyclic triterpenoids, which are commonly used as biomarkers in organic geochemistry, is reviewed. Advances in molecular biology have provided much new information about the identity of isoprenoid lipid synthesis genes, which now makes it possible to confirm whether the biosynthetic capability to produce a particular biomarker exists in an organism or not. Such information will allow organic geochemists to use biomarkers to assign sources of organic matter with more confidence and opens up the possibility of using fossil rDNA in younger sediments as the ultimate biomarker. In ancient sediments the presence of a biomarker may only provide information about the existence of a biosynthetic pathway rather than the presence of a particular group of organisms. For such applications, it is vital that we understand how biochemical pathways have evolved over time, in parallel with the well-recognised evolution of species on earth as codified in the three kingdoms of life – Archaea, Bacteria and Eukaryota. It has become clear that the cyclases that produce hopanoids from squalene (so-called SHCs) and sterols and pentacyclic triterpenoids from oxidosqualene (OSCs) are closely related and reflect evolutionary changes in this biosynthetic pathway over time. There has been much debate whether the sterols that have been detected in some cyanobacterial cultures are produced by de novo synthesis, and there is growing evidence that they might be derived from contaminating organisms such as yeasts and fungi or in some cases result from laboratory contamination or sterols in the culture medium. The presence of 4-methylsterols in some bacteria shows that some of the necessary biochemical apparatus is present in a few prokaryotes. The genomes of several cyanobacteria have been fully sequenced and amongst the genes recognized are several that have a close sequence match to genes related to sterol biosynthesis. These include a putative Δ 24 sterol C-methyltransferase gene, another coding a sterol C-methyltransferase and another for a sterol C-5-desaturase (or C-4 methyl sterol oxidase), but this is only a part of the full complement of genes needed for sterol biosynthesis.