Triterpenoid saponin biosynthesis genes and their expression patterns during the development of sea cucumber Apostichopus japonicus

Sea cucumbers are echinoderm species with a leathery skin and an elongated body. Natural compounds, including saponins, have been previously isolated from sea cucumber. The majority of saponins derived from this organism typically belong to the triterpene glycosides, which exhibit various biological effects (e.g., antifungal, cytotoxic, hemolytic, and cytostatic immunomodulatory activities). In the present study, 30 saponin biosynthesis enzymes, namely 2 AACTs, 2 FPSs, 2 HMGSs, 2 OSCs, 1 SS, 2 SEs, and 19 UGTs, were found in the genome of sea cucumber Apostichopus japonicus . Compared with upstream saponin biosynthesis enzymes (i.e., AACTs, FPSs, HMGSs, OSCs, and SSs), the downstream saponin biosynthesis enzymes (SEs and UGTs) shared lower amino acid sequence similarity with the corresponding genes in other echinoderms. It was proposed that the enzyme structures comprised both α-helices and β-sheets. Notably, only a low percentage of β-sheets were present in FPSs, OSCs, and SS. The saponin biosynthesis enzymes showed dynamic expression patterns during five critical developmental stages of A. japonicus (fertilized oocytes, blastula, gastrula, doliolaria, and penractula). The present study involved elucidation of putative saponin biosynthesis pathways in sea cucumber and provides a valuable platform for further investigation of saponin biosynthesis in echinoderms.