Claereboudt EJS et al. (2019), Triterpenoids in Echinoderms: Fundamental Diffe...

ECB-ART-47262

Mar Drugs 2019 Jun 13;176:. doi: 10.3390/md17060352.

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Triterpenoids in Echinoderms: Fundamental Differences in Diversity and Biosynthetic Pathways.

Claereboudt EJS , Caulier G , Decroo C , Colson E , Gerbaux P , Claereboudt MR , Schaller H , Flammang P , Deleu M , Eeckhaut I .

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Echinoderms form a remarkable phylum of marine invertebrates that present specific chemical signatures unique in the animal kingdom. It is particularly the case for essential triterpenoids that evolved separately in each of the five echinoderm classes. Indeed, while most animals have Δ5-sterols, sea cucumbers (Holothuroidea) and sea stars (Asteroidea) also possess Δ7 and Δ9(11)-sterols, a characteristic not shared with brittle stars (Ophiuroidea), sea urchins (Echinoidea), and crinoids (Crinoidea). These particular Δ7 and Δ9(11) sterols emerged as a self-protection against membranolytic saponins that only sea cucumbers and sea stars produce as a defense mechanism. The diversity of saponins is large; several hundred molecules have been described in the two classes of these saponins (i.e., triterpenoid or steroid saponins). This review aims to highlight the diversity of triterpenoids in echinoderms by focusing on sterols and triterpenoid glycosides, but more importantly to provide an updated view of the biosynthesis of these molecules in echinoderms.

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Species referenced: Echinodermata
Genes referenced: cse1l cyp51a1 fdft1 hmgcs1 irak1bp1 LOC100887844 LOC115919910 LOC115925415 LOC579804 LOC586184 lss mvk prpf8 sc5d

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	Figure 1. A. Structure of cholesterol (with carbon numbers and stereo position of bonds), the primary sterol in most metazoan cell membranes. B. 5α-Cholest-7-en-3β-ol, the most abundant free sterol of Holothuria scabra. C. 4α,14α-dimethyl-5α-cholest-9(11)-en-3β-ol, the second most abundant free sterol in H. scabra [36]. D,E,F. 3D structure of the sterols [19].
	Figure 2. A. Structure of the hypothesized precursor of holothuroid triterpenoids; parkeol. B. The most abundant aglygone moiety in holothuroid triterpene glycosides; holostanol [53,60].
	Figure 3. Structures of Holostanol characterized by an 18(20)-lactone fragment. Most sea cucumber triterpene glycosides possess such a type of holostane aglycones. Holotoxin A1 is an example of non-sulfated holostane glycoside. Holothurin A is an example of sulfated holostane glycoside [60].
	Figure 4. Example structures of A. Steroid aglycone saponin backbone from the sea star, Asterias rubens [14] B. Triterpene aglycone backbone from the sea cucumber, Holothuria scabra [13]. R1 represents the position of the glycoside moiety of both saponin types. The position of R2, R3, and R4 can vary and are illustrated here as examples.
	Figure 5. Scheme to summarize the current understanding of triterpenoid biosynthesis in metazoans. The metazoan mevalonate (MVA) pathway according to the published literature is common to all metazoans (gray box) [84]. The subsequent biosynthetic routes to sterols and triterpenes are group dependent. Enzyme abbreviations: AACT, acetoacetyl- CoA thiolase; HMGS, hydroxymethylglutaryl-CoA synthase; HMGR, hydroxymethylglutaryl-CoA reductase; MVK, mevalonate kinase; PMK, phosphomevalonate kinase; MDC, mevalonate-5-decarboxylase; IDI1, Isopentenyl diphosphate isomerase; FPPS, fanesyl diphosphate synthase; SQS, squalene synthase; FPPP, fanesyl diphosphate phosphatase; LAS, lanosterol synthase; CAS, cycloartenol synthase; BAS, β-amyrin synthase; HSC, holothuroid squalene cyclase. Other abbreviations: CBC, chair-boat-chair; CCC, chair-chair-chair [83].
	Figure 6. Hypothetical scheme of biosynthesis (solid arrows) and metabolism (dashed arrow) of steroids and triterpenoids in sea cucumbers. Adapted from Stonik et al. (1999) [96].
	Figure 7. Scheme of the post squalene pathway for cholesterol biosynthesis. SQE: Squalene epoxydase; LAS: Lanosterol synthase; CYP51: Lanosterol-14α-demethylase; 14RED: Sterol-14-reductase; SMO: Sterol-4α-methyl-oxidase; C4D: C4 decarboxylase; SKR: sterone ketoreductase; SI: Sterol-8-isomerase; C5DES: Sterol-C5-desaturase; 24RED: Sterol-24-reductase; 7RED: Sterol-7-reductase. Reproduced with permission from Marijanovic et al., Molecular Endocrinology, published by Oxford University Press (2003), enzymes names were simplified.
	Figure 8. Summary of the diversity of oxido-squalene cyclases (OSCs) in echinoderms. LAS: Lanosterol squalene cyclase is the principal cyclase of the animal kingdom. HSC1 and HSC2: Holothuroid squalene cyclases 1 and 2 are recently discovered cyclase isoforms identified in S. horrens [88] and A. japonicus [89]. In order to unify nomenclature across the literature, holothuroid OSCs were labeled HSC (instead of LAS [104] or OSC [105]). Pictograms underneath the structure illustrate the five classes of echinoderms: Crinoidea, Ophiuroidea, Echinoidea, Asteroidea, and Holothuroidea.

References [+] :

Abe, Enzymatic synthesis of cyclic triterpenes. 2007, Pubmed