Solubilisation, partial purification and properties of acyl-CoA: glycerol-3-phosphate acyltransferase from avocado (Persea americana) fruit mesocarp

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Sunflower HaGPAT9-1 is the predominant GPAT during seed development

In oil crops, triacylglycerol biosynthesis is an important metabolic pathway in which glycerol-3-phosphate acyltransferase (GPAT) performs the first acylation step. Mass spectrometry analysis of developing sunflower (Helianthus annuus) seed membrane fractions identified an abundant GPAT, HaGPAT9 isoform 1, with a N-terminal peptide that possessed two phosphorylated residues with possible regulatory function. HaGPAT9-1 belongs to a broad eukaryotic GPAT family, similar to mammalian GPAT3, and it represents one of the two sunflower GPAT9 isoforms, sharing 90% identity with HaGPAT9-2. Both sunflower genes are expressed during seed development and in vegetative tissues, with HaGPAT9-1 transcripts accumulating at relatively higher levels than those for HaGPAT9-2. Green fluorescent protein tagging of HaGPAT9-1 confirmed its subcellular accumulation in the endoplasmic reticulum. Despite their overall sequence similarities, the two sunflower isoforms displayed significant differences in their enzymatic activities. For instance, HaGPAT9-1 possesses in vivo GPAT activity that rescues the lethal phenotype of the cmy228 yeast strain, while in vitro assays revealed a preference of HaGPAT9-1 for palmitoyl-, oleoyl- and linoleoyl-CoAs of one order of magnitude, with the highest increase in yield for oleoyl- and linoleoyl-CoAs. By contrast, no enzymatic activity could be detected for HaGPAT9-2, even though its over-expression modified the TAG profile of yeast.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

sn-Glycerol-3-phosphate acyltransferases in plants

sn-Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the acylation at sn-1 position of glycerol-3-phosphate to produce lysophosphatidic acid (LPA). LPA is an important intermediate for the formation of different types of acyl-lipids, such as extracellular lipid polyesters, storage and membrane lipids. Three types of GPAT have been found in plants, localizing to the plastid, endoplasmic reticulum, and mitochondria. These GPATs are involved in several lipid biosynthetic pathways and play important biological roles in plant development. In the present review, we will focus on the recent progress in studying the physiological functions of GPATs and their metabolic roles in glycerolipid biosynthesis.

Characterization and partial purification of acyl-CoA:glycerol 3-phosphate acyltransferase from sunflower (Helianthus annuus L.) developing seeds

The glycerol 3-phosphate acyltransferase (GPAT, EC 2.3.1.15) from sunflower (Helianthus annuus L.) microsomes has been characterised and partially purified. The in vitro determination of activity was optimized, and the maximum value for GPAT activity identified between 15 and 20 days after flowering. The apparent Michaelis-Menten K(m) for the glycerol 3-phosphate was 354 muM. The preferred substrates were palmitoyl-CoA = linoleoyl-CoA > oleoyl-CoA with the lowest activity using stearoyl-CoA. High solubilisation was achieved using 0.75% Tween80 and the solubilised GPAT was partially purified by ion-exchange chromatography using a Hi-Trap DEAE FF column, followed by gel filtration chromatography using a Superose 12 HR column. The fraction containing the GPAT activity was analysed by SDS-PAGE and contained a major band of 60.1 kDa. Finally, evidence is provided which shows the role of GPAT in the asymmetrical distribution, between positions sn-1 and sn-3, of saturated fatty acids in highly saturated sunflower triacylglycerols. This work provides background information on the sunflower endoplasmic reticulum GPAT which may prove valuable for future modification of oil deposition in this important crop.

Molecular identification of microsomal acyl-CoA:glycerol-3-phosphate acyltransferase, a key enzyme in de novo triacylglycerol synthesis

Acyl-CoA:glycerol-3-phosphate acyltransferase (GPAT) catalyzes the first step during de novo synthesis of triacylglycerol. It has been well recognized that mammals possess multiple enzymatically distinct proteins with GPAT activity. Although the mitochondrial-associated GPAT has been cloned and extensively characterized, the molecular identity of the endoplasmic reticulum (ER)-associated GPAT, which accounts for the majority of total GPAT activity in most tissues, has remained elusive. Here we report the identification of genes encoding human and mouse ER-associated GPAT (termed GPAT3). GPAT3 is a member of the acyltransferase family predominantly expressed in tissues characterized by active lipid metabolism, such as adipose tissue, small intestine, kidney, and heart. Ectopic expression of GPAT3 leads to a significant increase in N-ethylmaleimide-sensitive GPAT activity, whereas acyltransferase activity toward a variety of other lysophospholipids, as well as neutral lipid substrates, is not altered. Overexpression of GPAT3 in mammalian cells results in increased triacylglycerol, but not phospholipid, formation. GPAT3 is localized to the ER when overexpressed in COS-7 cells. GPAT3 mRNA is dramatically up-regulated during adipocyte differentiation, is reciprocally regulated in adipose tissue and liver of ob/ob mice, and is up-regulated in mice treated with a peroxisome proliferator-activated receptor gamma (PPARgamma) agonist. A substantial loss of GPAT activity in 3T3-L1 adipocytes was achieved by reducing GPAT3 mRNA levels through GPAT3-specific siRNA knockdown. These findings identify GPAT3 as a previously uncharacterized triacylglycerol biosynthetic enzyme. Similar to other lipogenic enzymes, GPAT3 may be useful as a target for the treatment of obesity.

Metabolic control analysis reveals an important role for diacylglycerol acyltransferase in olive but not in oil palm lipid accumulation

We applied metabolic control analysis to the Kennedy pathway for triacylglycerol formation in tissue cultures from the important oil crops, olive (Olea europaea L.) and oil palm (Elaeis guineensis Jacq.). When microsomal fractions were incubated at 30 degrees C rather than 20 degrees C, there was an increase in triacylglycerol labelling. This increase was accompanied by a build up of diacylglycerol (DAG) radioactivity in olive but not in oil palm, suggesting that the activity of DAG acyltransferase (DAGAT) was becoming limiting in olive. We used 2-bromooctanoate as a specific inhibitor of DAGAT and showed that the enzyme had a flux control coefficient under the experimental conditions of 0.74 in olive but only 0.12 in oil palm. These data revealed important differences in the regulation of lipid biosynthesis in cultures from different plants and suggest that changes in the endogenous activity of DAGAT is unlikely to affect oil accumulation in oil palm crops.

Limnanthes douglasii lysophosphatidic acid acyltransferases: immunological quantification, acyl selectivity and functional replacement of the Escherichia coli plsC gene

Antibodies were raised against the two membrane-bound lysophosphatidic acid acyltransferase (LPAAT) enzymes from Limnanthes douglasii (meadowfoam), LAT1 and LAT2, using the predicted soluble portion of each protein as recombinant protein antigens. The antibodies can distinguish between the two acyltransferase proteins and demonstrate that both migrate in an anomalous fashion on SDS/PAGE gels. The antibodies were used to determine that LAT1 is present in both leaf and developing seeds, whereas LAT2 is only detectable in developing seeds later than 22 daf (days after flowering). Both proteins were found exclusively in microsomal fractions and their amount was determined using the recombinant antigens as quantification standards. LAT1 is present at a level of 27 pg/microg of membrane protein in leaf tissue and <or=12.5 pg/microg of membrane protein in developing embryos. The amount of LAT2 reaches a peak at 305 pg/microg of membrane protein 25 daf and is not expressed 20 daf or before. This is the first study to quantify these membrane-bound proteins in a plant tissue. The maximal level of LAT2 protein coincides with the maximal level of erucic acid synthesis in the seeds. Both full-length proteins were expressed in the Escherichia coli LPAAT mutant JC201, and membranes from these strains were used to investigate the substrate selectivity of these two enzymes, demonstrating that they are different. Finally, we report that LAT2 and a maize LPAAT enzyme (MAT1) can functionally replace the E. coli plsC gene after its deletion in the chromosome, whereas LAT1 and a coconut LPAAT (Coco1) cannot. This is probably due to differences in substrate utilization.

Purification and characterization of thiol-reagent-sensitive glycerol-3-phosphate acyltransferase from the membrane fraction of an oleaginous fungus

Glycerol-3-phosphate acyltransferase (GPAT), responsible for the first committed, rate-limiting, step of glycerolipid synthesis, was purified to homogeneity from the membrane fraction of an oleaginous fungus, Mortierella ramanniana var. angulispora. The enzyme was solubilized from the membrane fraction by pretreatment with 0.05% Triton X-100 and treatment of the resulting pellet with 0.3% Triton X-100. The enzyme was subsequently purified by column chromatography on heparin-Sepharose, Yellow 86 agarose, a second heparin-Sepharose column, Superdex-200 and hydroxylapatite Bio-Gel. Enzyme activity was finally enriched 1308-fold over that of the starting membrane fraction. SDS/PAGE of the purified fraction revealed a single band with a molecular mass of 45 kDa. Native PAGE showed a major band that corresponded to GPAT activity. Enzyme activity was inhibited by thiol reagents, suggesting that it originated from microsomes rather than mitochondria. Purified GPAT depended on exogenous oleoyl-CoA and sn-glycerol-3-phosphate, with the highest activity at approx. 50 and 250 microM, respectively, and preferred oleoyl-CoA 5.4-fold over palmitoyl-CoA as an acyl donor. Anionic phospholipids, such as phosphatidic acid and phosphatidylserine, were absolutely required for activity of the purified enzyme, and their ability to activate GPAT was influenced by the purity of the GPAT preparation. Bivalent cations, such as Mg(2+) and Ca(2+), inhibited purified GPAT activity, whereas 5 mM Mn(2+) elevated activity approx. 2-fold. These results provide new insights into the molecular characterization of microsomal GPAT, which has not been well characterized compared with mitochondrial and plastidic GPAT.

Phosphatidic acid, a key intermediate in lipid metabolism

Phosphatidic acid (PtdOH) is a key intermediate in glycerolipid biosynthesis. Two different pathways are known for de novo formation of this compound, namely (a) the Gro3P (glycerol 3-phosphate) pathway, and (b) the GrnP (dihydroxyacetone phosphate) pathway. Whereas the former route of PtdOH synthesis is present in bacteria and all types of eukaryotes, the GrnP pathway is restricted to yeast and mammalian cells. In this review article, we describe the enzymes catalyzing de novo formation of PtdOH, their properties and their occurrence in different cell types and organelles. Much attention has recently been paid to the subcellular localization of enzymes involved in the biosynthesis of PtdOH. In all eukaryotic cells, microsomes (ER) harbour the complete set of enzymes catalyzing these pathways and are thus the usual organelle for PtdOH formation. In contrast, the contribution of mitochondria to PtdOH synthesis is restricted to certain enzymes and depends on the cell type. In addition, chloroplasts of plants, lipid particles of the yeast, and peroxisomes of mammalian cells are significantly involved in PtdOH biosynthesis. Redundant systems of acyltransferases, the interplay of organelles, regulation of the pathway on the compartmental level, and finally the contribution of alternative pathways (phosphorylation of diacylglycerol and cleavage of phospholipids by phospholipases) to PtdOH biosynthesis appear to be required for the balanced formation of this important lipid intermediate. Dysfunction of enzymes involved in PtdOH synthesis can result in severe defects of various cellular processes. In this context, the possible physiological role(s) of PtdOH and its related metabolites, lysophosphatidic acid and diacylglycerol, will be discussed.

Glycerol-3-phosphate acyltransferase in plants

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the transfer of an acyl group from an acyl donor to the sn-1 position of glycerol 3-phosphate. The plant cell contains three types of GPAT, which are located in the chloroplasts, mitochondria and cytoplasm, respectively. The enzyme in chloroplasts is soluble and uses acyl-(acyl-carrier protein) as the acyl donor, whereas the enzymes in the mitochondria and the cytoplasm are bound to membranes and use acyl-CoA as the acyl donor. cDNAs for GPAT of chloroplasts have been cloned from several plants, and the gene for the enzyme has been cloned from Arabidopsis thaliana. The amino acid sequences deduced from the nucleotide sequences of cDNAs indicate that the product of translation is a precursor of about 460 amino acid residues, which consists of a leader sequence of about 70 amino acid residues and a mature protein of about 400 residues, with a molecular mass of about 42 kDa. Genetic engineering of the unsaturation of fatty acids has been achieved by manipulation of the cDNA for the GPAT found in chloroplasts and has allowed modification of the ability of tobacco to tolerate chilling temperatures.

Phosphatidate phosphatases of mammals, yeast, and higher plants

Phosphatidate phosphatase (EC 3.1.3.4) catalyzes the hydrolysis of phosphatidate to yield sn-1,2-diacylglycerol and inorganic phosphate. In mammalian systems, forms of phosphatidate phosphatase involved in glycerolipid synthesis and signal transduction have been identified. Forms of the enzyme involved in signal transduction have been purified and partially characterized. In yeast, phosphatidate phosphatases associated with the endoplasmic reticulum and mitochondria have also been purified and partially characterized. Information on phosphatidate phosphatases from mammals and yeast is useful in characterizing the enzyme from plant systems. This review examines progress on the characterization of phosphatidate phosphatases from mammals, yeast, and higher plants. The purification and characterization of the phosphatidate phosphatase involved in glycerolipid synthesis in developing oilseeds may lead to the identification of the encoding gene. Increasing our understanding of the enzymes of lipid synthesis in developing seeds will aid in the development of biotechnological strategies for seed oil modification.