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.

The differences between the biosynthesis of sterols in higher plants and yeast/mammals are believed to originate at the cyclization step of oxidosqualene, which is cyclized to cycloartenol in higher plants and lanosterol in yeast/mammals. Recently, lanosterol synthase genes were identified from dicotyledonous plant species including Arabidopsis, suggesting that higher plants possess dual biosynthetic pathways to phytosterols via lanosterol, and through cycloartenol. To identify the biosynthetic pathway to phytosterol via lanosterol, and to reveal the contributions to phytosterol biosynthesis via each cycloartenol and lanosterol, we performed feeding experiments by using [6-13C2H3]mevalonate with Arabidopsis seedlings. Applying 13C-{1H}{2H} nuclear magnetic resonance (NMR) techniques, the elucidation of deuterium on C-19 behavior of phytosterol provided evidence that small amounts of phytosterol were biosynthesized via lanosterol. The levels of phytosterol increased on overexpression of LAS1, and phytosterols derived from lanosterol were not observed in a LAS1-knockout plant. This is direct evidence to indicate that the biosynthetic pathway for phytosterol via lanosterol exists in plant cells. We designate the biosynthetic pathway to phytosterols via lanosterol “the lanosterol pathway.” LAS1 expression is reported to be induced by the application of jasmonate and is thought to have evolved from an ancestral cycloartenol synthase to a triterpenoid synthase, such as β-amyrin synthase and lupeol synthase. Considering this background, the lanosterol pathway may contribute to the biosynthesis of not only phytosterols, but also steroids as secondary metabolites.

https://www.pnas.org/content/pnas/106/3/725.full.pdf

Dual biosynthetic pathways to phytosterol via cycloartenol and lanosterol in Arabidopsis.

Publisher Summary Plants are a diverse group of organisms that differ from animals by not possessing a nervous system. They can be grouped into two great categories: those with and those without photosynthetic capacity. The photosynthetic group is comprised by four subgroups: the autotrophic bacteria; the algae, including the phytoflagellates, which resemble protozoa; the bryophytes; and the tracheophytes. The non-photosynthetic group includes the heterotrophic bacteria, mycoplasmas, protozoa, slime molds, fungi, and the sponges. The steroidal structure is composed of two different units. They are the tetracyclic “nucleus” in which all the carbon atoms are in rings and three types of non-annular carbon atoms. The isolation and identification of a particular compound falls into the general realm of structural elucidation, which can become complicated, especially for an unusual sterol. However, for the common dominant structures, advances in chromatography and spectroscopy allow a simple procedure.

The Biochemistry of Plant Sterols

The great variety of plant sterols and related compounds have for many years been a source of interest to natural products chemists because of the challenges they have offered in structural determination and development of analytical techniques. Interest in the biosynthesis of animal sterols spans the past thirty years, but plant sterol biosynthesis has been extensively investigated only since about 1965. Apart from the additional mechanisms required for the introduction of the extra carbon atoms into the side of typical plant sterols, there was probably the expectation that the pathway in plants would be much the same as the animal route leading from acetyl-CoA through mevalonic acid, squalene, and lanosterol to cholesterol. It soon became apparent, however, that such is not the case, and the studies of phytosterol biosynthesis have been rewarded by the recognition of several intriguing mechanistic differences not only between animals and plants but also between particular classes of plants. Moreover some of these differences appear to correlate with other evidence showing phylo-genetic relationships of various classes of lower and higher plants.

The Biosynthesis of Plant Sterols

Lanosterol biosynthesis in plants.

Tissue cultures of Nicotiana tabacum were treated with [14C]cyeloartenol and [14C]lanosterol, and the 4,4-dimethylsterol 4-methylsterol and phytosterol fractions were examined for radioactivity. In both cases activity was incorporated into the phytosterols.



The distributions of radioactivity in the 4,4-dimethylsterols and 4-methylsterols were compared with those obtained by incorporation of [14C]acetic acid. It is concluded that cycloartenol is a true phytosterol precursor in the tissue cultures, whereas the possible role of lanosterol (not detectable in the cultures, even by isotopic dilution experiments) is not proven.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1969.tb00513.x

M. J. E. Hewlins

Papers

The conversion of [14C]cycloartenol and [14C)lanosterol into phytosterols by cultures of Nicotiana tabacum.