Sterols are vital components of all eukaryotic cells. This review describes the variety of sterol structures found in microalgae, yeasts, fungi, protozoans and microheterotrophs. Reports of the occurrence of sterols in prokaryotic cells are critically assessed. Methylotrophic bacteria contain unusual 4-methylsterols, but reports of 4-desmethyl sterols in cyanobacteria and other bacteria are limited and many of these seem dubious. Possible application areas for sterols derived from mass culture of microalgae and other microorganisms are highlighted.

Sterols in microorganisms.

Publisher Summary This chapter highlights endosymbiont hypothesis. All contemporary genomes (including those of plastids and mitochondria) ultimately derive from a single genome—the genome of a single, presumably cellular, entity which was the ancestor of all surviving forms of live. The construction and interpretation of phylogenetic trees based on small subunit (SSU, or 16S-like) and large subunit (LSU, or 23S-like) rRNA sequences have proven especially informative in instances where morphological diversity tends to confound traditional methods of phylogenetic analysis. In addition to ribosomal RNA (rRNA) sequence, there are a number of traits that characterize the archaebacteria as a distinct group of organisms, separate from all other prokaryotes. These include (1) cell walls that, when present, lack peptidoglycan and muramic acid; (2) the presence of lipids containing phytanyl side groups in ether linkage; and (3) the presence of RNA polymerases that are distinct from those of eubacteria in subunit composition, response to RNA synthesis inhibitors or stimulators, immunological reactivity, and gene sequence. Two major divisions of archaebacteria include (1) sulfur-dependent, extreme thermophiles; and (2) methane-producers (methanogens) and their relatives, including the extreme halophiles.

The endosymbiont hypothesis revisited.

On the origins of triterpenoid skeletal diversity.

Cholesterol and its relatives possessing the 1,2-cyclopentanoperhydrophenanthrene ring system (Figure ​(Figure1)1) form the sterolome, which comprises a chemical library of more than 1000 natural products found in all forms of eukaryotes and some prokaryotes that serve a myriad of biological functions.1,2 The structural and stereochemical commonality of these compounds derive in large part from the action of oxidosqualene synthases (formerly cyclases) that generate the parent sterol frame. In their 1985 Nobel lecture, Brown and Goldstein stated that “cholesterol is the most highly decorated small molecule in biology”, a comment supported by the many Nobel prizes awarded to individuals who devoted a large part of their lives to research one or more aspects of the chemistry and chemical biology of sterols, their metabolites, or other isoprenoids.2,3



Figure 1

Common tetracyclic steroid frame containing the 1,2-cyclopentanoperhydrophenanthrene ring skeleton.

Biosynthesis of cholesterol and other sterols.

Lipid particles of the yeast Saccharomyces cerevisiae were isolated at high purity, and their proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Major lipid particle proteins were identified by mass spectrometric analysis, and the corresponding open reading frames (ORFs) were deduced. In silicio analysis revealed that all lipid particle proteins contain several hydrophobic domains but none or only few (hypothetical) transmembrane spanning regions. All lipid particle proteins identified by function so far, such as Erg1p, Erg6p, and Erg7p (ergosterol biosynthesis) and Faa1p, Faa4p, and Fat1p (fatty acid metabolism), are involved in lipid metabolism. Based on sequence homology, another group of three lipid particle proteins may be involved in lipid degradation. To examine whether lipid particle proteins of unknown function are also involved in lipid synthesis, mutants with deletions of the respective ORFs were constructed and subjected to systematic lipid analysis. Deletion of YDL193w resulted in a lethal phenotype which could not be suppressed by supplementation with ergosterol or fatty acids. Other deletion mutants were viable under standard conditions. Strains with YBR177c, YMR313c, and YKL140w deleted exhibited phospholipid and/or neutral lipid patterns that were different from the wild-type strain and thus may be further candidate ORFs involved in yeast lipid metabolism.

Identification and Characterization of Major Lipid Particle Proteins of the Yeast Saccharomyces cerevisiae

Comparison of the chromatographic properties of sterols, select additional steroids and triterpenoids: gravity-flow column liquid chromatography, thin-layer chromatography, gas-liquid chromatography and high-performance liquid chromatography.

The stereochemistry of several sterol precursors and end products synthesized by two fungal-like micro-organisms Prototheca wickerhamii (I) and Dictyostelium discoideum (II) have been determined by chromatographic (TLC, GLC, and HPLC) and spectral (UV, MS, and 1H NMR) methods. From I and II the following sterols were isolated from the cells: cycloartenol, cyclolaudenol, 24(28)-methylenecycloartanol, ergosterol, protothecasterol, 4alpha-methylergostanol, 4alpha-methylclionastanol, clionastanol, 24beta-ethylcholesta-8,22-enol, and dictyosterol. In addition, the mechanism of C-24 methylation was investigated in both organisms by feeding to I [2-3H]lanosterol, [2-3H]cycloartenol, [24-3H]lanosterol, and [methyl-2H3]methionine and by feeding to II [methyl-2H3]methionine. The results demonstrate that the 24beta configuration is formed by different alkylation routes in I and II. The Delta25(27) route operates in I while the Delta24(28) route operates in II. Based on what is known in the literature regarding sterol distribution and phylogenesis together with our findings that the stereochemical outcome of squalene oxide cyclization leads to the production of cycloartenol rather than lanosterol (characteristic of the fungal genealogy) and the chirality of the C-24 alkyl group is similar in the two nonphotosynthetic microbes (beta oriented), we conclude that Prototheca is an apoplastic Chlorella (i.e., an alga) and that Dictyostelium as well as the other soil amoebae that synthesize cycloartenol evolved from algal rather than fungal ancestors.

https://www.pnas.org/content/pnas/87/19/7565.full.pdf

F. G. Crumley

Papers

Comparison of the chromatographic properties of sterols, select additional steroids and triterpenoids: gravity-flow column liquid chromatography, thin-layer chromatography, gas-liquid chromatography and high-performance liquid chromatography.

Sterol phylogenesis and algal evolution.