The Structure of Storage Triacylglycerols of Mature Seeds of Lunaria rediviva L., a Hyperaccumulator of Very Long-Chain Monounsaturated Fatty Acids, from the Perspective of Statistical Distribution Theories and New Insights Based on Simple Calculations

Abstract

This article represents the first consideration of the peculiarities of the fatty acid (FAs) composition and structure of storage triacylglycerols (TAGs) of the relict plant Lunaria rediviva L. The composition of storage TAGs was found to comprise 21 individual FAs, with an unsaturated FA content of 96.8%. Additionally, monounsaturated acids with a very long chain (VLCFAs), specifically C20:1-C24:1, constituted over 60% of the total FAs. The ethylene bond position isomers of unsaturated FAs were accurately identified and the presence of unusual isomers, including 20:1Δ13, 22:1Δ15, and 24:1Δ17 acids. Furthermore, the unusual minor 24:2Δ15,18 acid was identified and characterised for the first time. The pathways of the mentioned VLCFA's biosynthesis have been proposed. The distribution of FA acyls between the sn positions of triacylglycerols was found to be highly specific. Thus, VLCFAs exclusively acylate the α positions of the carbon atoms of the glycerol residue of the TAG molecule (sn-1 and sn-3 positions), while unsaturated C18 acids exclusively acylate the β-carbon atom (sn-2 position). The composition of the molecular species of TAGs was analysed using a calculation method based on the Vander Wal model and by RP-HPLC-ESI-MS. A significant discrepancy from the statistical model was observed, indicating a preference for the formation of symmetrical TAGs, such as sn-1,3-dierucoyl-2-oleoyl-glycerol and related molecular species. This observation led to the formulation of a hypothesis regarding the potential existence of at least two specialised enzyme isoforms involved in the biosynthesis of such TAGs via the Kennedy pathway, exhibiting unusual substrate specificity. Consequently, this plant can be regarded not only as a producer of unusual molecular types of triacylglycerols but also as a source of genetic material for the search of genes encoding the aforementioned enzymes with unusual substrate specificity.

Keywords: Lunaria rediviva; RP-HPLC-PDA/ESI-MS; Vander Wal model; mature seeds; positional-species composition; storage oils; triacylglycerols; very long-chain unsaturated fatty acids.

Figure 1

A gas chromatography–mass spectrometry (GC-MS) chromatogram of fatty acid methyl esters derived from triacylglycerols of mature seeds of Lunaria rediviva L. (total ion current).

Figure 2

Mass spectra of DMOX derivatives of unsaturated C24 fatty acids. (Panel A) depicts the spectrum of cis-17-tetracosenoic acid, while (panel B) depicts the spectrum of cis,cis,-15,18-tetracosadienoic acid.

Figure 3

HPLC-UV $\lambda$ = 205 nm chromatogram of separation of a mixture of triacylglycerol molecular species from mature seeds of Lunaria rediviva and the corresponding predominant compound masses in each peak (by ESI-MS analysis).

Figure 4

Mass profile of individual peaks of different molecular species of TAGs belonging to ABC and ABA types (A)—rac-20:1–18:1–22:1, ECN = 54; (B)—20:1–18:1–20:1, ECN = 52; (C)—rac-22:1–18:1–24:1, ECN = 58.

Figure 5

Fractional composition of triacylglycerols grouped by ECN value found experimentally and calculated using the Vander Wal model, where mol.% (n = 9).

Figure 6

Positional-species composition of triacylglycerols of Lunaria rediviva determined by the NA-RP-HPLC method and calculated using the Vander Wal model.