Hydrophobic Mismatch in the Thylakoid Membrane Regulates Photosynthetic Light Harvesting.

The ability to harvest light effectively in a changing environment is necessary to ensure efficient photosynthesis and crop growth. One mechanism, known as qE, protects photosystem II (PSII) and regulates electron transfer through the harmless dissipation of excess absorbed photons as heat. This process involves reversible clustering of the major light-harvesting complexes of PSII (LHCII) in the thylakoid membrane and relies upon the ΔpH gradient and the allosteric modulator protein PsbS. To date, the exact role of PsbS in the qE mechanism has remained elusive. Here, we show that PsbS induces hydrophobic mismatch in the thylakoid membrane through dynamic rearrangement of lipids around LHCII leading to observed membrane thinning. We found that upon illumination, the thylakoid membrane reversibly shrinks from around 4.3 to 3.2 nm, without PsbS, this response is eliminated. Furthermore, we show that the lipid digalactosyldiacylglycerol (DGDG) is repelled from the LHCII-PsbS complex due to an increase in both the pKa of lumenal residues and in the dipole moment of LHCII, which allows for further conformational change and clustering in the membrane. Our results suggest a mechanistic role for PsbS as a facilitator of a hydrophobic mismatch-mediated phase transition between LHCII-PsbS and its environment. This could act as the driving force to sort LHCII into photoprotective nanodomains in the thylakoid membrane. This work shows an example of the key role of the hydrophobic mismatch process in regulating membrane protein function in plants.

Monitoring galactolipid digestion and simultaneous changes in lipid-bile salt micellar organization by real-time NMR spectroscopy.

The use of Nuclear Magnetic Resonance spectroscopy for studying lipid digestion in vitro most often consists of quantifying lipolysis products after they have been extracted from the reaction medium using organic solvents. However, the current sensitivity level of NMR spectrometers makes possible to avoid the extraction step and continuously quantify the lipids directly in the reaction medium. We used real-time 1H NMR spectroscopy and guinea pig pancreatic lipase-related protein 2 (GPLRP2) as biocatalyst to monitor in situ the lipolysis of monogalactosyl diacylglycerol (MGDG) in the form of mixed micelles with the bile salt sodium taurodeoxycholate (NaTDC). Residual substrate and lipolysis products (monogalactosyl monoacylglycerol (MGMG); monogalactosylglycerol (MGG) and octanoic acid (OA) were simultaneously quantified throughout the reaction thanks to specific proton resonances. Lipolysis was complete with the release of all MGDG fatty acids. These results were confirmed by thin layer chromatography (TLC) and densitometry after lipid extraction at different reaction times. Using diffusion-ordered NMR spectroscopy (DOSY), we could also estimate the diffusion coefficients of all the reaction compounds and deduce the hydrodynamic radius of the lipid aggregates in which they were present. It was shown that MGDG-NaTDC mixed micelles with an initial hydrodynamic radius rH of 7.3 ± 0.5 nm were changed into smaller micelles of NaTDC-MGDG-MGMG of 2.3 ± 0.5 nm in the course of the lipolysis reaction, and finally into NaTDC-OA mixed micelles (rH of 2.9 ± 0.5 nm) and water soluble MGG. These results provide a better understanding of the digestion of galactolipids by PLRP2, a process that leads to the complete micellar solubilisation of their fatty acids and renders their intestinal absorption possible.

In situ monitoring of galactolipid digestion by infrared spectroscopy in both model micelles and spinach chloroplasts.

Galactolipids are the main lipids from plant photosynthetic membranes and they can be digested by pancreatic lipase related protein 2 (PLRP2), an enzyme found in the pancreatic secretion in many animal species. Here, we used transmission Fourier-transform infrared spectroscopy (FTIR) to monitor continuously the hydrolysis of galactolipids by PLRP2, in situ and in real time. The method was first developed with a model substrate, a synthetic monogalactosyl diacylglycerol with 8-carbon acyl chains (C8-MGDG), in the form of mixed micelles with a bile salt, sodium taurodeoxycholate (NaTDC). The concentrations of the residual substrate and reaction products (monogalactosylmonoglyceride, MGMG; monogalactosylglycerol, MGG; octanoic acid) were estimated from the carbonyl and carboxylate vibration bands after calibration with reference standards. The results were confirmed by thin layer chromatography analysis (TLC) and specific staining of galactosylated compounds with thymol and sulfuric acid. The method was then applied to the lipolysis of more complex substrates, a natural extract of MGDG with long acyl chains, micellized with NaTDC, and intact chloroplasts isolated from spinach leaves. After a calibration performed with α-linolenic acid, the main fatty acid (FA) found in plant galactolipids, FTIR allowed quantitative measurement of chloroplast lipolysis by PLRP2. A full release of FA from membrane galactolipids was observed, that was not dependent on the presence of bile salts. Nevertheless, the evolution of amide vibration band in FTIR spectra suggested the interaction of membrane proteins with NaTDC and lipolysis products.

Structural Analysis and Anti-Inflammatory Effect of a Digalactosyldiacylglycerol-Monoestolide, a Characteristic Glycolipid in Oats.

Digalactosyldiacylglycerol- (DGDG-) monoestolide is a characteristic glycolipid in oats. DGDG-monoestolides possess a unique structure whereby a fatty acid of DGDG is replaced by a fatty acid ester of hydroxy fatty acid (FAHFA). While the physiological effects of DGDG and FAHFA have been reported previously, the effects of DGDG-monoestolides are unknown. Hence, we isolated a major DGDG-monoestolide molecular species from oats, analyzed its structure, and evaluated its anti-inflammatory effect. Based on GC-MS, MS/MS, and NMR analyses, the isolated compound was identified as a DGDG-monoestolide that contains the linoleic acid ester of 15-hydroxy linoleic acid (LAHLA) and linoleic acid (i.e., DGDG-LAHLA). The isolated DGDG-LAHLA was evaluated for its anti-inflammatory effect on LPS-stimulated RAW264 cells. The production of nitric oxide and cytokines (IL-6, TNF-α, and IL-10) were significantly decreased by DGDG-LAHLA, suggesting the anti-inflammatory effect of DGDG-LAHLA for the first time. In addition, our data showed a pronounced uptake of DGDG-LAHLA by cells. Some compounds corresponding to the predicted DGDG-LAHLA metabolites were also detected, suggesting that both intact DGDG-LAHLA and its metabolites may contribute to the above anti-inflammatory activities. These results are expected to expand the availability of oats as a functional food.

Interfacial organization and phase behavior of mixed galactolipid-DPPC-phytosterol assemblies at the air-water interface and in hydrated mesophases.

The structural behavior of model assemblies composed of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), the two main galactolipids found in plants, was investigated at the air/water interface and in aqueous dispersion. To approach the composition of the natural photosynthetic membranes, tunable Langmuir model membrane of galactolipids (GL) were used, and were complexified to form either heterogenous binary or ternary assemblies of GL, phospholipids (PL), and phytosterols (pS). The impact of pS, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or both on the structural properties of GL membrane was studied. The nature of the interactions between the different molecules was investigated using biophysical characterizations (ellipsometry, tensiometry, atomic force microscopy). In addition, the phase behavior was determined by SAXS analysis on the model assemblies in aqueous dispersions. Results revealed the good interfacial stability of these specific plant membrane lipids. The morphology of the GL film was characteristic of a fluid phase, with an interfacial roughness induced by the intercalation of monogalactosyl and digalactosyl polar heads of MGDG and DGDG, respectively. A phase heterogeneity in the monolayer was induced by the addition of DPPC and/or pS, which resulted in the modification of galactolipid organization and headgroup interactions. These structural changes were confirmed by SAXS analysis, showing more favorable interactions between MGDG and DPPC than between DGDG and DPPC in aqueous dispersion. This phenomenon was exacerbated in the presence of pS.

Lyme Disease, Borrelia burgdorferi, and Lipid Immunogens.

The human immune system detects potentially pathogenic microbes with receptors that respond to microbial metabolites. While the overall immune signaling pathway is known in considerable detail, the initial molecular signals, the microbially produced immunogens, for important diseases like Lyme disease (LD) are often not well-defined. The immunogens for LD are produced by the spirochete Borrelia burgdorferi, and a galactoglycerolipid (1) has been identified as a key trigger for the inflammatory immune response that characterizes LD. This report corrects the original structural assignment of 1 to 3, a change of an α-galactopyranose to an α-galactofuranose headgroup. The seemingly small change has important implications for the diagnosis, prevention, and treatment of LD.

Reversible changes in galactolipid saturation level and head group composition are associated with tolerance to postharvest chilling in tomato fruit.

BACKGROUND: Chilling injury (CI) is a physiological disorder that results in a limitation for cold storage (CS) of many fruits and vegetables. The low temperature-induced changes in the properties and composition of cell membranes are involved in the response to chilling temperature and in the mechanism of CI and tolerance. RESULTS: We compared the changes in the lipid composition by gas chromatography-mass spectrometry before, immediately after CS, as well as during a 3-day subsequent period, of tomato fruits with different chilling-sensitivity: Micro-Tom (tolerant) and Minitomato (susceptible). The changes in linolenic acid content, double bond index and digalactosyldiacylglycerol/monogalactosyldiacylglycerol ratio (DGDG/MGDG) showed membrane fluidity adjustment, depending on the temperature. By a database search, we identified 18 membrane-bound fatty acid desaturase (FAD) genes and five DGDG synthases (DGD) genes that phylogenetically clustered into four and two subfamilies, respectively. The FAD and DGD genes were differentially expressed in response to CS, as determined by quantitative reverse transcriptase-polymerase chain reaction analysis. CONCLUSION: The data strongly suggest that reversion of CS-induced changes during the recovery period is important for the proper function of the membrane and tolerance to postharvest CI in tomato fruit. © 2021 Society of Chemical Industry.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. I. 31P-NMR Spectroscopy.

Build-up of the energized state of thylakoid membranes and the synthesis of ATP are warranted by organizing their bulk lipids into a bilayer. However, the major lipid species of these membranes, monogalactosyldiacylglycerol, is a non-bilayer lipid. It has also been documented that fully functional thylakoid membranes, in addition to the bilayer, contain an inverted hexagonal (HII) phase and two isotropic phases. To shed light on the origin of these non-lamellar phases, we performed 31P-NMR spectroscopy experiments on sub-chloroplast particles of spinach: stacked, granum and unstacked, stroma thylakoid membranes. These membranes exhibited similar lipid polymorphism as the whole thylakoids. Saturation transfer experiments, applying saturating pulses at characteristic frequencies at 5 °C, provided evidence for distinct lipid phases-with component spectra very similar to those derived from mathematical deconvolution of the 31P-NMR spectra. Wheat-germ lipase treatment of samples selectively eliminated the phases exhibiting sharp isotropic peaks, suggesting easier accessibility of these lipids compared to the bilayer and the HII phases. Gradually increasing lipid exchanges were observed between the bilayer and the two isotropic phases upon gradually elevating the temperature from 5 to 35 °C, suggesting close connections between these lipid phases. Data concerning the identity and structural and functional roles of different lipid phases will be presented in the accompanying paper.

Head-Group Acylation of Chloroplast Membrane Lipids.

Head group-acylated chloroplast lipids were discovered in the 1960s, but interest was renewed about 15 years ago with the discovery of Arabidopsides E and G, acylated monogalactosyldiacylglycerols with oxidized fatty acyl chains originally identified in Arabidopsis thaliana. Since then, plant biologists have applied the power of mass spectrometry to identify additional oxidized and non-oxidized chloroplast lipids and quantify their levels in response to biotic and abiotic stresses. The enzyme responsible for the head-group acylation of chloroplast lipids was identified as a cytosolic protein closely associated with the chloroplast outer membrane and christened acylated galactolipid-associated phospholipase 1 (AGAP1). Despite many advances, critical questions remain about the biological functions of AGAP1 and its head group-acylated products.

Enhanced Production of Photosynthetic Pigments and Various Metabolites and Lipids in the Cyanobacteria Synechocystis sp. PCC 7338 Culture in the Presence of Exogenous Glucose.

Synechocystis strains are cyanobacteria that can produce useful biomaterials for biofuel and pharmaceutical resources. In this study, the effects of exogenous glucose (5-mM) on cell growth, photosynthetic pigments, metabolites, and lipids in Synechocystis sp. PCC 7338 (referred to as Synechocystis 7338) were investigated. Exogenous glucose increased cell growth on days 9 and 18. The highest production (mg/L) of chlorophyll a (34.66), phycocyanin (84.94), allophycocyanin (34.28), and phycoerythrin (6.90) was observed on day 18 in Synechocystis 7338 culture under 5-mM glucose. Alterations in metabolic and lipidomic profiles under 5-mM glucose were investigated using gas chromatography-mass spectrometry (MS) and nanoelectrospray ionization-MS. The highest production (relative intensity/L) of aspartic acid, glutamic acid, glycerol-3-phosphate, linolenic acid, monogalactosyldiacylglycerol (MGDG) 16:0/18:1, MGDG 16:0/20:2, MGDG 18:1/18:2, neophytadiene, oleic acid, phosphatidylglycerol (PG) 16:0/16:0, and PG 16:0/17:2 was achieved on day 9. The highest production of pyroglutamic acid and sucrose was observed on day 18. We suggest that the addition of exogenous glucose to Synechocystis 7338 culture could be an efficient strategy for improving growth of cells and production of photosynthetic pigments, metabolites, and intact lipid species for industrial applications.

Isolation and compositional analysis of galactoglycerolipids from perilla [Perilla frutescens (L.) Britton] leaves and comparison to the galactoglycerolipids from spinach and parsley.

The aim of this study was to isolate monogalactosyldiacylglycerols (MGDGs) and digalactosyldiacylglycerols (DGDGs) from perilla [Perilla frutescens (L.) Britton] and to investigate their fatty acid profiles. Perilla displayed the greatest total MGDG and DGDG content among the three types of leaf vegetables tested, that is, spinach, parsley, and perilla, containing 0.16 g/100 g MGDG and 0.04 g/100 g DGDG (on wet weight basis). High purity MGDG (approximately 97 g/100 g) and DGDG (approximately 86 g/100 g) were isolated from perilla chloroform/methanol (2:1, v/v) extracts by two-step silica gel column chromatography. MGDGs were primarily composed of 18:3n-3 and 16:3n-3, predominantly located at the sn-1 and sn-2 positions, respectively. In DGDG, 18:3n-3 and 16:0 were the most abundant fatty acids and were primarily found at the sn-1 and sn-2 positions, respectively. PRACTICAL APPLICATION: MGDGs and DGDGs are the most prevalent forms of galactoglycerolipids found in leaf vegetables including perilla and have been shown to exert health-beneficial effects, such as antitumor, anti-inflammatory, anticancer, and appetite-suppressing activities. Both MGDGs and DGDGs possess emulsifying properties. The present study may help better understand the health-beneficial effects of MGDG and DGDG from perilla, by providing total composition and positional distribution of the fatty acids. The present study also successfully established a protocol to isolate high purity MGDG and DGDG from perilla, thereby increasing their possible use as an ingredient in foods and nutraceuticals.

Lipid sponge droplets as programmable synthetic organelles.

Living cells segregate molecules and reactions in various subcellular compartments known as organelles. Spatial organization is likely essential for expanding the biochemical functions of synthetic reaction systems, including artificial cells. Many studies have attempted to mimic organelle functions using lamellar membrane-bound vesicles. However, vesicles typically suffer from highly limited transport across the membranes and an inability to mimic the dense membrane networks typically found in organelles such as the endoplasmic reticulum. Here, we describe programmable synthetic organelles based on highly stable nonlamellar sponge phase droplets that spontaneously assemble from a single-chain galactolipid and nonionic detergents. Due to their nanoporous structure, lipid sponge droplets readily exchange materials with the surrounding environment. In addition, the sponge phase contains a dense network of lipid bilayers and nanometric aqueous channels, which allows different classes of molecules to partition based on their size, polarity, and specific binding motifs. The sequestration of biologically relevant macromolecules can be programmed by the addition of suitably functionalized amphiphiles to the droplets. We demonstrate that droplets can harbor functional soluble and transmembrane proteins, allowing for the colocalization and concentration of enzymes and substrates to enhance reaction rates. Droplets protect bound proteins from proteases, and these interactions can be engineered to be reversible and optically controlled. Our results show that lipid sponge droplets permit the facile integration of membrane-rich environments and self-assembling spatial organization with biochemical reaction systems.

Simple liquid chromatography-electrospray ionization ion trap mass spectrometry method for the quantification of galacto-oxylipin arabidopsides in plant samples.

A simple and sensitive method to quantify five different arabidopsides by HPLC-ion trap mass spectrometry in complex plant samples was developed and validated. Arabidopsides are oxidized galactolipids first described in Arabidopsis thaliana but also produced by other plant species under stress conditions. External calibration was performed using arabidopsides purified from freeze-thawed Arabidopsis leaves. Lipids were extracted and pre-purified on an SPE silica column before HPLC-MS analysis. Arabidopsides were separated on a C18 column using a gradient of mQ water and acetonitrile:mQ water (85:15) supplemented with formic acid (0.2%) and ammonium formate (12 mM). The method was validated according to European commission decision 2002/657/CE. LOD, LOQ, linearity, intra-day and inter-day precision and accuracy, selectivity, matrix effects and recoveries were determined for the five metabolites. The established method is highly selective in a complex plant matrix. LOD and LOQ were, respectively, in the range 0.098-0.78 and 0.64-1.56 µM, allowing the arabidopside quantification from 25.6-62.4 nmol/g fresh weight. Calibration curve correlation coefficients were higher than 0.997. Matrix effects ranged from -2.09% to 6.10% and recoveries between 70.7% and 109%. The method was successfully applied to complex plant matrixes: Arabidopsis thaliana and Nasturtium officinale.

A proteoliposome-based system reveals how lipids control photosynthetic light harvesting.

Integral membrane proteins are exposed to a complex and dynamic lipid environment modulated by nonbilayer lipids that can influence protein functions by lipid-protein interactions. The nonbilayer lipid monogalactosyldiacylglycerol (MGDG) is the most abundant lipid in plant photosynthetic thylakoid membranes, but its impact on the functionality of energy-converting membrane protein complexes is unknown. Here, we optimized a detergent-based reconstitution protocol to develop a proteoliposome technique that incorporates the major light-harvesting complex II (LHCII) into compositionally well-defined large unilamellar lipid bilayer vesicles to study the impact of MGDG on light harvesting by LHCII. Using steady-state fluorescence spectroscopy, CD spectroscopy, and time-correlated single-photon counting, we found that both chlorophyll fluorescence quantum yields and fluorescence lifetimes clearly indicate that the presence of MGDG in lipid bilayers switches LHCII from a light-harvesting to a more energy-quenching mode that dissipates harvested light into heat. It is hypothesized that in the in vitro system developed here, MGDG controls light harvesting of LHCII by modulating the hydrostatic lateral membrane pressure profile in the lipid bilayer sensed by LHCII-bound peripheral pigments.

Monogalactosyldiacylglycerols with High PUFA Content From Microalgae for Value-Added Products.

Monogalactosyldiacylglycerols (MGDGs) from microalgae containing a high proportion of polyunsaturated fatty acids (PUFAs) have enormous potential applications in various industries. In this study, the productivity and fatty acid composition of MGDGs from three microalgae in commercialized production, Chlorella sorokiniana GT, Nannochloropsis oceanica IMET1, and Arthrospira platensis, were evaluated under nitrogen-sufficient (+N) and nitrogen-deficient (-N) conditions. Under +N conditions, higher productivities of MGDGs from C. sorokiniana GT1, N. oceanica IMET1, and A. platensis were obtained with 16.3, 4.3, and 1.3 mg/L/day, respectively. In agreement with a high ratio of PUFAs in MGDGs, they accounted for 56%, 66%, and 47% of the total fatty acids in MGDG correspondingly. α-linoleic acid (ALA), eicosapentaenoic acid (EPA), and γ-linoleic acid (GLA) were the specific PUFAs of the three microalgae. The proportions of the specific PUFAs in MGDG accounted for 26%, 44%, and 63% of total specific PUFAs, respectively. Considering the production, cost of microalgae biomass and price of MGDG, the commercialized microalgae C. sorokiniana GT1 and A. platensis showed great potential to produce MGDGs with a high content of PUFAs, in contrast to N. oceanica IMET1, with its high biomass cost and low production. This study provides basic data for the extraction and separation of microalgae lipids from commercialized microalgae, which is helpful to promote the commercialization of microalgae.

In vitro digestion of galactolipids from chloroplast-rich fraction (CRF) of postharvest, pea vine field residue (haulm) and spinach leaves.

The removal of intact chloroplasts from their cell wall confinement offers a novel way to obtain lipophilic nutrients from green biomass, especially carotenoids and galactolipids. These latter are the main membrane lipids in plants and they represent a major source of the essential α-linolenic acid (18:3; ALA). Nevertheless, knowledge on their digestion is still limited. We have developed a physical method of recovering a chloroplast-rich fraction (CRF) from green biomass and tested its digestibility in vitro under simulated gastrointestinal conditions. Using a two-step static model, CRF from both spinach leaves and postharvest, pea vine field residue (haulm) were first exposed to enzymes from rabbit gastric extracts and then either to pancreatic enzymes from human pancreatic juice (HPJ) or to porcine pancreatic extracts (PPE). The lipolysis of monogalactosyldiacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG) was monitored by thin layer chromatography and gas chromatography of fatty acid methyl esters. For both CRF preparations, MGDG and DGDG were converted to monogalactosylmonoacylglycerol (MGMG) and digalactosylmonoacylglycerol (DGMG), respectively, during the intestinal phase and ALA was the main fatty acid released. Galactolipids were more effectively hydrolysed by HPJ than by PPE, and PPE showed a higher activity on MGDG than on DGDG. These findings may be explained by the higher levels of galactolipase activity in HPJ compared to PPE, which mainly results from pancreatic lipase-related protein 2. Thus, we showed that CRF galactolipids are well digested by pancreatic enzymes and represent an interesting vehicle for ALA supplementation in human diet.

Lipases in wheat flour bread making: Importance of an appropriate balance between wheat endogenous lipids and their enzymatically released hydrolysis products.

Lipids are only minor wheat flour constituents but play major roles in bread making (BM). Here, the importance of a well-balanced lipid population in BM was studied by applying a lipase from Fusarium oxysporum in the process. Monogalactosyldiacylglycerols and N-acyl phosphatidylethanolamines were the most accessible lipase substrates. Hydrolysis thereof into their corresponding lysolipids was largely if not entirely responsible for loaf volume increases upon lipase application. Degradation of endogenously present lipids and enzymatically released lysolipids caused loaf volume to decrease, confirming that an appropriate balance between different types of lipids is crucial in BM. For optimal dough gas cell stability, the level of lipids promoting lamellar mesophases and, thus, liquid condensed monolayers needs to be maximal while maintaining an appropriate balance between lipids promoting hexagonal I phases, non-polar lipids and lipids promoting hexagonal II or cubic phases.

The effect of carotenoids on the concentration of singlet oxygen in lipid membranes.

An effect of ß-carotene and its polar derivative, zeaxanthin, on a concentration of singlet oxygen in lipid membranes was studied in a model system. The carotenoids were incorporated into the membranes of small unilamellar liposomes at a concentration of 0.15 mol% with respect to lipid. Singlet oxygen was generated in a liposome suspension via photosensitization of toluidine blue, and its concentration in a membrane was detected with application of a specific fluorescence probe (singlet oxygen sensor green reagent) located in the lipid bilayer. The results show the carotenoid-dependent decrease in the concentration of singlet oxygen in the membranes formed with unsaturated lipids (egg yolk phosphatidylcholine and digalactosyldiacylglycerol) but not in the case of the membranes formed with a saturated lipid (dimyristoylphosphatidylcholine). The effect of carotenoids was about twice as high as in the case of cholesterol present in liposomes at the same concentration. The results suggest that carotenoids protect membranes formed with unsaturated lipids against singlet oxygen through combined activity of different mechanisms: modification of structural properties of the lipid bilayers, physical quenching of singlet oxygen and chemical reactions leading to the pigment oxidation. The latter conclusion is based on the analysis of the absorption spectra of liposomes before and after light exposure. An importance of the different modes of protection by carotenoids against single oxygen toxicity towards biomembranes is discussed.

The Spatiotemporal Deposition of Lysophosphatidylcholine Within Starch Granules of Maize Endosperm and its Relationships to the Expression of Genes Involved in Endoplasmic Reticulum-Amyloplast Lipid Trafficking and Galactolipid Synthesis.

The presence of lipids within starch granules is specific to cereal endosperm starches. These starch lipids are composed of lysophospholipids, especially lysophosphatidylcholine (LysoPC) and free fatty acids that strongly impact the assembly and properties of cereal starches. However, the molecular mechanisms associated with this specific lipid routing have never been investigated. In this study, matrix-assisted laser desorption ionization mass spectrometry imaging revealed decreasing gradients in starch LysoPC concentrations from the periphery to the center of developing maize endosperms. This spatiotemporal deposition of starch LysoPC was similar to that previously observed for endoplasmic reticulum (ER)-synthesized storage proteins, i.e. zeins, suggesting that LysoPC might originate in the ER, as already reported for chloroplasts. Furthermore, a decrease of the palmitate concentration of amyloplast galactolipids was observed during endosperm development, correlated with the preferential trapping of palmitoyl-LysoPC by starch carbohydrates, suggesting a link between LysoPC and galactolipid synthesis. Using microarray, the homologous genes of the Arabidopsis ER-chloroplast lipid trafficking and galactolipid synthesis pathways were also expressed in maize endosperm. These strong similarities suggest that the encoded enzymes and transporters are adapted to managing the differences between chloroplast and amyloplast lipid homeostasis. Altogether, our results led us to propose a model where ER-amyloplast lipid trafficking directs the LysoPC towards one of two routes, the first towards the stroma and starch granules and the other towards galactolipid synthesis.

Enzymatic Synthesis of Structured Monogalactosyldiacylglycerols Enriched in Pinolenic Acid.

We enzymatically prepared structured monogalactosydiacylglycerols (MGDGs) enriched in pinolenic acid (PLA). PLA-enriched free fatty acids (FFAs) containing ∼86 mol % PLA were produced from an FFA fraction obtained from pine nut oil (PLA content, ∼13 mol %) by urea crystallization. Commercial MGDGs (5 mg) were acidolyzed with PLA-enriched FFAs using four commercial immobilized lipases as biocatalysts. The reaction was performed in acetone (4 mL) in a stirred-batch reactor. Lipozyme RM IM (immobilized Rhizomucor miehei lipase) was the most effective biocatalyst for the reaction. Structured MGDGs containing 42.1 mol % PLA were obtained under optimal reaction conditions: temperature, 25 °C; substrate molar ratio, 1:30 (MGDGs/PLA-enriched FFAs); enzyme loading, 20 wt % of total substrates; and reaction time, 36 h. The structured MGDGs were separated from the reaction products at a purity of 96.6 wt % using silica column chromatography. The structured MGDGs could be possibly used as emulsifiers with appetite-suppression effects.