Phytosterol Profiles, Genomes and Enzymes - An Overview

Abstract

The remarkable diversity of sterol biosynthetic capacities described in living organisms is enriched at a fast pace by a growing number of sequenced genomes. Whereas analytical chemistry has produced a wealth of sterol profiles of species in diverse taxonomic groups including seed and non-seed plants, algae, phytoplanktonic species and other unicellular eukaryotes, functional assays and validation of candidate genes unveils new enzymes and new pathways besides canonical biosynthetic schemes. An overview of the current landscape of sterol pathways in the tree of life is tentatively assembled in a series of sterolotypes that encompass major groups and provides also peculiar features of sterol profiles in bacteria, fungi, plants, and algae.

Keywords: algae; cholesterol; eukaryote; phytosterol; plant; prokaryote.

FIGURE 1

Synthetic view of sterol pathways. Dashed lines discriminate mammalian (blue), fungal (red), and plants (green) canonical pathways. The so-called non-canonical pathways shown for algae is framed in mixed red and green colors to point out Δ^5,7-sterols in both fungal and algal groups, and to recall common side chain modifying enzymes SSR2 and SMTs. Substrate-product enzymatic conversion is visualized with a single arrow. The dashed line between campesterol and castasterone (brassinosteroids) represents multiple steps. Sterol names are non-systematic names. The comprehensive IUPAC recommendations and conventional system for sterol nomenclature based on cholestane, ergostane, and stigmastane scaffolds, and the associated stereochemistry is given in Moss (1989) and also commented in Nes (2011). LAS, lanosterol synthase; CAS, cycloartenol synthase; C14DM, sterol-14-demethylase; C14R, sterol-14-reductase; SMO, sterol methyl oxidase; βHSD, 3β-hydroxysteroid dehydrogenase/C4-decarboxylase; SR, sterone ketoreductase; 7ISO, sterol-7(8)-isomerase; C5D, sterol-5(6)-desaturase; C7R, sterol-7-reductase; C24R, sterol-24-reductase; SMT1, sterol-C24-methyltransferase; SMT2, sterol-C28-methyltransferase; CPI, cyclopropyl isomerase; and C22D, sterol-22-desaturase.

FIGURE 2

Major sterolotypes defined in living organisms. Names in black boxes are chosen according to species or taxonomic groups for which a sterol pathway is well documented. Green boxes indicated whether a CAS (cycloartenol synthase) or LAS (lanosterol synthase) defines a sterol pathway; brackets are for minor contribution of a pathway as currently understood. Orange boxes are marking an AltSQE, alternative squalene epoxidase (canonical SQE otherwise not indicated in sterolotypes). Blue and magenta boxes are for sterol side chain modifications by SSR2 (sterol side chain reductase 2), SMT, sterol-C-methyltransferases. Major or representative sterols are represented for each sterolotype. Nomenclature as in Figure 1. Arabido-type and Solanum-type exhibit the epimeric pair of campesterol (C24α-methylcholesterol) and 22(23)-dihydrobrassicasterol (C24β-methylcholesterol) represented by 24-methylcholesterol.

FIGURE 3

Diverse sterols found in algae and plants. Side chain C22, C23, and C24 positions are in red. Chiral substituents at C24 of the sterol side chain are C24α for a methyl or ethyl group in front: campesterol, sitosterol, crinosterol, stigmasterol, chondrillasterol; or C24β for a methyl or ethyl group in back: dinosterol, clionasterol, gorgosterol, poriferasterol. Nomenclature as in Figure 1.

FIGURE 4

Sterol profiles in algae and plants. Sterol compositions of twenty species are plotted on a schematic diagram, which indicates proportions in percent and molecular structures by color codes. C₂₇, sterols bearing a C₈-side chain (e.g., cholesterol); C₂₈, C₉-side chain (e.g., ergosterol); and C₂₉, C₁₀-side chain (e.g., fucosterol). C₃₀^a, C₃₀H₅₀O direct cyclization products of (3S)-2,3-oxidosqualene (e.g., tetrahymanol). C₃₀^b, sterol products with additional methyl groups on the tetracyclic moiety and on the side chain (e.g., dinosterol). C₃₀^c, sterol products with a C₁₁-side chain (e.g., propylidene sterols). Sterol compositions as percent of the total were found in published references indicated by upper case numbering of species. Sterols contributing less than 2% of a total were not considered for clarity of this schematic representation. 1, Leblond et al. (2011); 2, Miller et al. (2012); 3, Akihisa et al. (1992); 4, Raederstorff and Rohmer (1987); 5, Chiu et al. (1985); 6, Morikawa et al. (2009); 7, Schaller et al. (1998); 8, Schaeffer et al. (2000); 9, Mitova et al. (1999); 10, Bhatt and Bhatt (1984); 11, Tasende (2000); 12, Lu et al. (2014); 13, Mercer et al. (1974); 14, Lu et al. (2020); 15, Conner et al. (1982); 16, Rampen et al. (2010); 17, Patterson and Nes (1991); 18, Mikami et al. (2018); 19, Giner et al. (2009); 20, Lamacka and Sajbidor (1997); and 21, Chakrabarti et al. (2017). Tetrahymena thermophila, Saccharomyces cerevisiae, and Homo sapiens are shown as single pathway end-product species. Sterols tagged with an asterisk * are C-24 epimeric mixtures.

FIGURE 5

Cyclization products of squalene and (3S)-2,3-oxidosqualene in species and taxonomic groups. THC, tetrahymanol synthase; SHC, squalene hopene cyclase; SQE, squalene epoxidase; AltSQE, alternative squalene epoxidase; PAS, parkeol synthase; LAS, lanosterol cyclase; and CAS, cycloartenol cyclase.

FIGURE 6

Sterol metabolization reactions in the ciliate Tetrahymena. The conversion of sitostanol is shown [adapted from Najle et al. (2013)]. C5-DES, sterol-5(6)-desaturase; C7-DES, sterol-7-desaturase; C22-DES, sterol-22-desaturase; and C24DET, sterol-24-de-ethylase enzyme abbreviations from Najle et al. (2013).

FIGURE 7

Sterolotype distribution over a tree of eukaryotes. A selection of species is tagged with sterolotypes presented in Figure 2. References about genomes and gene functions, and sterol profiles, are indicated by G1 to G8. G1, (Lu et al., 2014); G2, (Brumfield et al., 2017); G3, (Gold et al., 2016); G4, (Morikawa et al., 2009); G5, (Schaller, 2010); G6, (Sonawane et al., 2016); G7, (Lu et al., 2020); and G8, (Fügi et al., 2014). Species with no sterolotype indications are awaiting for a comprehensive mining of corresponding genomes. Gene nomenclature for Arabidopsis thaliana, Saccharomyces cerevisiae, Homo sapiens, and Chlamydomonas reinhardtii is given in Supplementary Table 1.