Plastidial acyl carrier protein Δ9‐desaturase modulates eicosapentaenoic acid biosynthesis and triacylglycerol accumulation in Phaeodactylum tricornutum

Abstract

SUMMARY The unicellular marine diatom Phaeodactylum tricornutum accumulates up to 35% eicosapentaenoic acid (EPA, 20:5n3) and has been used as a model organism to study long chain polyunsaturated fatty acids (LC‐PUFA) biosynthesis due to an excellent annotated genome sequence and established transformation system. In P. tricornutum, the majority of EPA accumulates in polar lipids, particularly in galactolipids such as mono‐ and di‐galactosyldiacylglycerol. LC‐PUFA biosynthesis is considered to start from oleic acid (18:1n9). EPA can be synthesized via a series of desaturation and elongation steps occurring at the endoplasmic reticulum and newly synthesized EPA is then imported into the plastids for incorporation into galactolipids via an unknown route. The basis for the flux of EPA is fundamental to understanding LC‐PUFA biosynthesis in diatoms. We used P. tricornutum to study acyl modifying activities, upstream of 18:1n9, on subsequent LC‐PUFA biosynthesis. We identified the gene coding for the plastidial acyl carrier protein Δ9‐desaturase, a key enzyme in fatty acid modification and analyzed the impact of overexpression and knock out of this gene on glycerolipid metabolism. This revealed a previously unknown role of this soluble desaturase in EPA synthesis and production of triacylglycerol. This study provides further insight into the distinctive nature of lipid metabolism in the marine diatom P. tricornutum and suggests additional approaches for tailoring oil composition in microalgae.

Key Enzymes in Fatty Acid Synthesis Pathway for Bioactive Lipids Biosynthesis

“…Thus, homology modeling and docking are the essential methods to analyze the structure of bacterial membrane fatty acid desaturases and elongase in the current state (21). Herein, we summarized the research approach of homology modeling and docking based on the previous work (21)(22)(23)55). First, after homology analyzes the amino acid sequence, the scientist can determine templates with higher homology and resolution in the protein database (e.g., Protein Data Bank, PDB).…”

“…Another study of homology modeling and docking also showed that four α-helices constitute the catalytic site and transmembrane domain in Δ9-desaturase from Arthrospira platensis ( 55 ). Beyond that, 3D structure modeling studies revealed that three variant amino acids (F160, A223, and L156) in domain Δ9-desaturase encoded by gene PtAAD narrow the space of substrate binding center, explaining the reason of substrate preference of this especial Δ9-desaturase for palmitic acid ( 22 , 23 ) ( Table 2 ).…”

Diatoms and Plants Acyl-CoA:lysophosphatidylcholine Acyltransferases (LPCATs) Exhibit Diverse Substrate Specificity and Biochemical Properties

“…Using pulse-chased experiments with [ 14 C] fatty acids, Arao and Yamada [7] proposed the predominant route of EPA biosynthesis in P. tricornutum. The first step in this pathway is ∆6-desaturation of linoleic acid (LA; 18:2 ∆9,12 ) to γ-linolenic acid (GLA; 18:3 ∆6,9,12 ), which is then ∆15-desaturated to produce stearidonic acid (SDA; 18:4 ∆6, 9,12,15 ). SDA is elongated by two carbons and desaturated to yield eicosapentaenoic acid (EPA; 20:5 ∆5, 8,11,14,17 ).…”

“…However, P. tricornutum produces DHA in trace levels only [6]. After biosynthesis in the endoplasmic reticulum, EPA can be transported to the plastid and incorporated into galactolipids, especially MGDG and DGDG [8,9]. The mechanism of trafficking EPA between the endoplasmic reticulum and plastid is unclear and requires further investigations [1,9].…”

“…After biosynthesis in the endoplasmic reticulum, EPA can be transported to the plastid and incorporated into galactolipids, especially MGDG and DGDG [8,9]. The mechanism of trafficking EPA between the endoplasmic reticulum and plastid is unclear and requires further investigations [1,9]. It is speculated that direct contact of the outer plastidial membrane and ER membrane can facilitate EPA translocation [1].…”