Mutants of Saccharomyces cerevisiae defective in sn-glycerol-3-phosphate acyltransferase. Simultaneous loss of dihydroxyacetone phosphate acyltransferase indicates a common gene

Phosphatidic acid biosynthesis in the model organism yeast Saccharomyces cerevisiae - a survey

Phosphatidic acid biosynthesis represents the initial part of de novo formation of all glycerophospholipids (membrane lipids) as well as triacylglycerols (storage lipids), and is thus the centerpiece of glycerolipid metabolism. The universal route of phosphatidic acid biosynthesis starts from the precursor glycerol-3-phosphate and comprises two consecutive acylation reactions which are catalyzed by a glycerol-3-phosphate acyltransferase and a 1-acyl glycerol-3-phosphate acyltransferase. In addition, yeast and mammals harbor a set of enzymes which can synthesize phosphatidic acid from the precursor dihydroxyacetone phosphate. In the present review our current knowledge about enzymes contributing to phosphatidic acid biosynthesis in the invaluable model organism yeast, Saccharomyces cerevisiae, is summarized. A special focus is laid upon the regulation and the localization of these enzymes. Furthermore, research needs for a deeper insight into the high complexity of phosphatidic acid biosynthesis and consequently the entire lipid metabolic network is presented.

A review of phosphatidate phosphatase assays

Phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol and regulates the synthesis of membrane phospholipids. There is much interest in this enzyme because it controls the cellular levels of its substrate, phosphatidate (PA), and product, DAG; defects in the metabolism of these lipid intermediates are the basis for lipid-based diseases such as obesity, lipodystrophy, and inflammation. The measurement of PAP activity is required for studies aimed at understanding its mechanisms of action, how it is regulated, and for screening its activators and/or inhibitors. Enzyme activity is determined through the use of radioactive and nonradioactive assays that measure the product, DAG, or P_i However, sensitivity and ease of use are variable across these methods. This review summarizes approaches to synthesize radioactive PA, to analyze radioactive and nonradioactive products, DAG and P_i, and discusses the advantages and disadvantages of each PAP assay.

Potential of a Saccharomyces cerevisiae recombinant strain lacking ethanol and glycerol biosynthesis pathways in efficient anaerobic bioproduction

Saccharomyces cerevisiae shows high growth activity under low pH conditions and can be used for producing acidic chemicals such as organic acids as well as fuel ethanol. However, ethanol can also be a problematic by-product in the production of chemicals except for ethanol. We have reported that a stable low-ethanol production phenotype was achieved by disrupting 6 NADH-dependent alcohol dehydrogenase genes of S. cerevisiae. Moreover, the genes encoding the NADH-dependent glycerol biosynthesis enzymes were further disrupted because the ADH-disrupted recombinant strain showed high glycerol production to maintain intracellular redox balance. The recombinant strain incapable producing ethanol and glycerol could have the potential to be a host for producing metabolite(s) whose biosynthesis is coupled with NADH oxidation. Indeed, we successfully achieved almost 100% yield for L-lactate production using this recombinant strain as a host. In addition, the potential of our constructed recombinant strain for efficient bioproduction, particularly under anaerobic conditions, is also discussed.

A family of eukaryotic lysophospholipid acyltransferases with broad specificity

The budding yeast ALE1 gene encodes a lysophospholipid acyltransferase (LPLAT) with broad specificity. We show that yeast LPLAT (ScLPLAT) belongs to a distinct protein family that includes human MBOAT1, MBOAT2, MBOAT4, and several closely related proteins from other eukaryotes. We further show that two plant proteins within this family, the Arabidopsis proteins AtLPLAT1 and AtLPLAT2, possess lysophospholipid acyltransferase activities similar to ScLPLAT. We propose that other members of this protein family, which we refer to as the LPLAT family, also are likely to possess LPLAT activity. Finally, we show that ScLPLAT differs from the specific lysophosphatidic acid acyltransferase that is encoded by SLC1 in that it cannot efficiently use lysophosphatidic acid produced by acylation of glycerol-3-phosphate in vitro.

Substrate specificity of Arabidopsis 3-ketoacyl-CoA synthases

The very long chain fatty acids (VLCFA) incorporated into plant lipids are derived from the iterative addition of C2 units provided by malonyl-CoA to an acyl-CoA by the 3-ketoacyl-CoA synthase (KCS) component of a fatty acid elongase (FAE) complex. Mining of the Arabidopsis genome sequence database revealed 20 genes with homology to seed-specific FAE1 KCS. Eight of the 20 putative KCSs were cloned, expressed in yeast, and isolated as (His)6 fusion proteins. Five of the eight (At1g71160, At1g19440, At1g07720, At5g04530, and At4g34250) had little or no activity with C16 to C20 substrates while three demonstrated activity with C16, C18, and C20 saturated acyl-CoA substrates. At1g01120 KCS (KCS1) and At2g26640 KCS had broad substrate specificities when assayed with saturated and mono-unsaturated C16 to C24 acyl-CoAs while At4g34510 KCS was specific for saturated fatty acyl-CoA substrates.

The initial step of glycerolipid metabolism inLeishmania majorpromastigotes involves a single glycerol-3-phosphate acyltransferase enzyme important for the synthesis of triacylglycerol but not essential for virulence

The synthesis of the major phospholipids, including those that play an essential role in Leishmania virulence, initiates with the acylation of glycerol-3-phosphate and dihydroxyacetonephosphate at the sn-1 position by glycerol-3-phosphate and dihydroxyacetonephosphate acyltransferases respectively. In this study, we show that Leishmania major promastigotes express a single glycerol-3-phosphate acyltransferase activity important for triacylglycerol synthesis but not essential for virulence. The encoding gene, LmGAT, expressed in yeast results in full complementation of the lethality of a mutant, gat1Deltagat2Delta, lacking glycerol-3-phosphate activity. Biochemical analyses revealed that LmGAT is a low-affinity glycerol-3-phosphate acyltransferase and exhibits higher specific activity with unsaturated long fatty acyl-CoA donors. A L. major null mutant, Deltalmgat/Deltalmgat, was created and a thorough analysis of its lipid composition was performed. Deletion of LmGAT resulted in a complete loss of Leishmania glycerol-3-phosphate acyltransferase activity and a major reduction in triacylglycerol synthesis. Consistent with the specificity of LmGAT for glycerol-3-phosphate but not dihydroxyacetonephosphate, Deltalmgat/Deltalmgat mutant expressed normal levels of the ether-lipid derivatives and virulence factors, lipophosphoglycan and GPI-anchored proteins, gp63, and its virulence was not affected in mice.

A root-specific condensing enzyme from Lesquerella fendleri that elongates very-long-chain saturated fatty acids

The LfKCS45 gene with a high sequence similarity to known 3-ketoacyl-CoA synthases of the membrane-bound fatty acid elongase was isolated from Lesquerella fendleri. The LfKCS45 gene has a 1464 bp open reading frame without introns, and is predicted to encode a polypeptide of 487 amino acids with an estimated molecular mass of 54.6 kD. High-stringency DNA blot analysis indicated that there were no closely related genes to LfKCS45 in the L. fendleri genome. Analysis of the fatty acid composition of transformed yeast revealed that expression of the LfKCS45 protein results in the synthesis of two novel very-long-chain fatty acids identified as C28:0 and C30:0. LfKCS45 was found to be not active with acyl-CoA substrates C16 to C24 in length. Reverse transcription-PCR experiments showed that the LfKCS45 gene is expressed only in L. fendleri root tips. Histochemical assays for GUS activity in Arabidopsis transformed with the LfKCS45 promoter-GUS fusion construct confirmed this expression pattern and demonstrated that LfKCS45 transcription is restricted to the cells of the lateral root cap.

The Plasmodium falciparum PfGatp is an Endoplasmic Reticulum Membrane Protein Important for the Initial Step of Malarial Glycerolipid Synthesis

During its 48-h asexual life cycle within human erythrocytes, Plasmodium falciparum grows to many times its own size and divides to produce 16-32 new parasites. This rapid multiplication requires active synthesis of new membranes and is fueled by phospholipid precursors and fatty acids that are scavenged from the human host. Plasmodium membrane biogenesis relies heavily on the expression of parasite enzymes that incorporate these precursors into phospholipids. However, little is known about the genes involved in membrane biogenesis or where this process takes place within the parasite. Here, we describe the analysis in P. falciparum of the first step of phospholipid biosynthesis that controls acylation of glycerol 3-phosphate (GPAT) at the sn-1 position. We show that this activity is of parasite origin and is specific for glycerol 3-phosphate substrate. We have identified the gene, PfGAT, encoding this activity in P. falciparum and reconstituted its codon composition for optimal expression in the yeast Saccharomyces cerevisiae. PfGAT complements the lethality of a yeast double mutant gat1Deltagat2Delta, lacking GPAT activity. Biochemical analysis revealed that PfGatp is a low affinity GPAT enzyme with a high specificity for C16:0 and C16:1 substrates. PfGatp is an integral membrane protein of the endoplasmic reticulum expressed throughout the intraerythrocytic life cycle of the parasite but induced mainly at the trophozoite stage. This study, which describes the first protozoan GPAT gene, reveals an important role for the endoplasmic reticulum in the initial step of Plasmodium membrane biogenesis.

Triacylglycerol biosynthesis in yeast

Triacylglycerol (TAG) is the major storage component for fatty acids, and thus for energy, in eukaryotic cells. In this mini-review, we describe recent progress that has been made with the yeast Saccharomyces cerevisiae in understanding formation of TAG and its cell biological role. Formation of TAG involves the synthesis of phosphatidic acid (PA) and diacylglycerol (DAG), two key intermediates of lipid metabolism. De novo formation of PA in yeast as in other types of cells can occur either through the glycerol-3-phosphate- or dihydroxyacetone phosphate-pathways-each named after its respective precursor. PA, formed in two steps of acylation, is converted to DAG by phosphatidate phosphatase. Acylation of DAG to yield TAG is catalyzed mainly by the two yeast proteins Dga1p and Lro1p, which utilize acyl-CoA or phosphatidylcholine, respectively, as acyl donors. In addition, minor alternative routes of DAG acylation appear to exist. Endoplasmic reticulum and lipid particles (LP), the TAG storage compartment in yeast, are the major sites of TAG synthesis. The interplay of these organelles, formation of LP, and enzymatic properties of enzymes catalyzing the synthesis of PA, DAG, and TAG in yeast are discussed in this communication.

Differential partitioning of lipids metabolized by separate yeast glycerol-3-phosphate acyltransferases reveals that phospholipase D generation of phosphatidic acid mediates sensitivity to choline-containing lysolipids and drugs

In this study we demonstrate that the GAT1 and GAT2 genes encode the major glycerol-3-phosphate acyltransferase activities in Saccharomyces cerevisiae. Genetic inactivation of either GAT1 or GAT2 did not alter cell growth but inactivation of both resulted in growth cessation. Metabolic analyses of gat1 and gat2 yeast detected that the major differences were: (i) a 50% increase in the rate of triacylglycerol synthesis in gat1 yeast and a corresponding 50% decrease in gat2 yeast, and (ii) a 5-fold increase in glycerophosphocholine production through deacylation of phosphatidylcholine synthesized through the CDP-choline pathway in gat1 yeast, whereas gat2 yeast displayed a 10-fold decrease. To address why we observed alterations in phospholipid turnover specific to phosphatidylcholine produced through the CDP-choline pathway in gat1 and gat2 yeast we tested their sensitivity to various cytotoxic lysolipids and observed that gat2 cells were more sensitive to lysophosphatidylcholine, but not other lysolipids. To pursue the mechanism we analyzed their sensitivity to choline-containing lysolipids or drugs that could not be deacylated and/or reacylated. Our data showed that gat1 and gat2 yeast were resistant and sensitive to lysoplatelet activating factor, platelet activating factor, and the anti-tumor lipid edelfosine, respectively, indicating that their sensitivity to these compounds was not because of differences in rates of phosphatidylcholine deacylation. As growth of gat2 cells was impaired in the presence of ethanol, a phospholipase D (Spo14p) inhibitor, we inferred that phospholipase D may play important biologic and metabolic roles in phenotypes observed in gat yeast. Genetic inactivation of the SPO14 gene resulted in increased susceptibility, whereas expression of Escherichia coli diacylglycerol kinase relieved growth inhibition, to choline-containing lysolipids and drugs. Our results are consistent with a model whereby phosphatidic acid generated from phosphatidylcholine hydrolysis by Spo14p regulates susceptibility to choline-containing lysolipid analogs and drugs.

Studies into factors contributing to substrate specificity of membrane-bound 3-ketoacyl-CoA synthases

We are interested in constructing a model for the substrate-binding site of fatty acid elongase-1 3-ketoacyl CoA synthase (FAE1 KCS), the enzyme responsible for production of very long chain fatty acids of plant seed oils. Arabidopsis thaliana and Brassica napus FAE1 KCS enzymes are highly homologous but the seed oil content of these plants suggests that their substrate specificities differ with respect to acyl chain length. We used in vivo and in vitro assays of Saccharomyces cerevisiae-expressed FAE1 KCSs to demonstrate that the B. napus FAE1 KCS enzyme favors longer chain acyl substrates than the A. thaliana enzyme. Domains/residues responsible for substrate specificity were investigated by determining catalytic activity and substrate specificity of chimeric enzymes of A. thaliana and B. napus FAE1 KCS. The N-terminal region, excluding the transmembrane domain, was shown to be involved in substrate specificity. One chimeric enzyme that included A. thaliana sequence from the N terminus to residue 114 and B. napus sequence from residue 115 to the C terminus had substrate specificity similar to that of A. thaliana FAE1 KCS. However, a K92R substitution in this chimeric enzyme changed the specificity to that of the B. napus enzyme without loss of catalytic activity. Thus, this study was successful in identifying a domain involved in determining substrate specificity in FAE1 KCS and in engineering an enzyme with novel activity.

The initial step of the glycerolipid pathway: identification of glycerol 3-phosphate/dihydroxyacetone phosphate dual substrate acyltransferases in Saccharomyces cerevisiae

The initial step of phospholipid biosynthesis in yeast is carried out through the acylation of glycerol 3-phosphate (G-3-P) and dihydroxyacetone phosphate by stereospecific sn-1 acyltransferases. Here we report the identification of two key fatty acyltransferases of the glycerolipid biosynthesis pathway in Saccharomyces cerevisiae. Disruption of the open reading frame YBL011w, corresponding to a gene previously identified as a choline transporter suppressor (SCT1), resulted in a substantial decrease of total cellular G-3-P acyltransferase activity. A yeast strain disrupted at the open reading frame YKR067w, which encodes a protein closely related to Sct1p, also exhibited a dramatic reduction in G-3-P acyltransferase activity. Molecular characterizations of the genes revealed that a missense mutation in YKR067w accounted for a defect in the activities of the G-3-P acyltransferase in the yeast mutant strain TTA1. Heterologous expression of YKR067w in Escherichia coli further confirmed its enzyme activity. These results indicate that YKR067w and YBL011w, designated herein as GAT1 and GAT2(SCT1), respectively, are yeast G-3-P acyltransferase genes. Furthermore, biochemical results are presented to show that both Gat1p and Gat2p(Sct1p) are G-3-P/dihydroxyacetone phosphate dual substrate-specific sn-1 acyltransferases. The fatty acyl specificity of Gat1p is similar to that of the mammalian microsomal G-3-P acyltransferase, as it can effectively utilize a broad range of fatty acids as acyl donors. In contrast, Gat2p(Sct1p) displayed preference toward 16-carbon fatty acids. The most notable of the altered phospholipid compositions of the gat1Delta and gat2(sct1)Delta strains are a decreased phosphatidic acid pool and an increased phosphatidylserine/phosphatidylinositol ratio. This did not appear to affect the mutants as no growth defect was found. However, null mutations of both GAT1 and GAT2(SCT1) are synthetically lethal to yeast.

Active-site residues of a plant membrane-bound fatty acid elongase beta-ketoacyl-CoA synthase, FAE1 KCS

The fatty acid elongase-1 beta-ketoacyl-CoA synthase, FAE1 KCS, a seed-specific elongase condensing enzyme from Arabidopsis, is involved in the production of eicosenoic (C20:1) and erucic (C22:1) acids. Alignment of the amino acid sequences of FAE1 KCS, KCS1, and five other putative elongase condensing enzymes (KCSs) revealed the presence of six conserved cysteine and four conserved histidine residues. Each of the conserved cysteine and histidine residues was individually converted by site-directed mutagenesis to both alanine and serine, and alanine and lysine respectively. After expression in yeast cells, the mutant enzymes were analyzed for their fatty acid elongase activity. Our results indicated that only cysteine 223 is an essential residue for enzyme activity, presumably for acyl chain transfer. All histidine substitutions resulted in complete loss of elongase activity. The loss of activity of these mutants was not due to their lower expression level since immunoblot analysis confirmed each was expressed to the same extent as the wild type FAE1 KCS.

Microaerobic glycerol formation in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae produces large amounts of glycerol as an osmoregulator during hyperosmotic stress and as a redox sink at low oxygen availability. NAD(+)-dependent glycerol-3-phosphate dehydrogenase in S. cerevisiae is present in two isoforms, coded for by two different genes, GPD1 and GPD2. Mutants for either one or both of these genes were investigated under carefully controlled static and dynamic conditions in continuous cultures at low oxygen transfer rates. Our results show that S. cerevisiae controls the production of glycerol in response to hypoxic conditions by regulating the expression of several genes. At high demand for NADH reoxidation, a strong induction was seen not only of the GPD2 gene, but also of GPP1, encoding one of the molecular forms of glycerol-3-phosphatase. Induction of the GPP1 gene appears to play a decisive role at elevated growth rates. At low demand for NADH reoxidation via glycerol formation, the GPD1, GPD2, GPP1, and GPP2 genes were all expressed at basal levels. The dynamics of the gene induction and the glycerol formation at low demand for NADH reoxidation point to an important role of the Gpd1p; deletion of the GPD1 gene strongly altered the expression patterns of the GPD2 and GPP1 genes under such conditions. Furthermore, our results indicate that GCY1 and DAK1, tentatively encoding glycerol dehydrogenase and dihydroxyacetone kinase, respectively, may be involved in the redox regulation of S. cerevisiae.

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.

Redundant systems of phosphatidic acid biosynthesis via acylation of glycerol-3-phosphate or dihydroxyacetone phosphate in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae lipid particles harbor two acyltransferases, Gat1p and Slc1p, which catalyze subsequent steps of acylation required for the formation of phosphatidic acid. Both enzymes are also components of the endoplasmic reticulum, but this compartment contains additional acyltransferase(s) involved in the biosynthesis of phosphatidic acid (K. Athenstaedt and G. Daum, J. Bacteriol. 179:7611-7616, 1997). Using the gat1 mutant strain TTA1, we show here that Gat1p present in both subcellular fractions accepts glycerol-3-phosphate and dihydroxyacetone phosphate as a substrate. Similarly, the additional acyltransferase(s) present in the endoplasmic reticulum can acylate both precursors. In contrast, yeast mitochondria harbor an enzyme(s) that significantly prefers dihydroxyacetone phosphate as a substrate for acylation, suggesting that at least one additional independent acyltransferase is present in this organelle. Surprisingly, enzymatic activity of 1-acyldihydroxyacetone phosphate reductase, which is required for the conversion of 1-acyldihydroxyacetone phosphate to 1-acylglycerol-3-phosphate (lysophosphatidic acid), is detectable only in lipid particles and the endoplasmic reticulum and not in mitochondria. In vivo labeling of wild-type cells with [2-3H, U-14C]glycerol revealed that both glycerol-3-phosphate and dihydroxyacetone phosphate can be incorporated as a backbone of glycerolipids. In the gat1 mutant and the 1-acylglycerol-3-phosphate acyltransferase slc1 mutant, the dihydroxyacetone phosphate pathway of phosphatidic acid biosynthesis is slightly preferred as compared to the wild type. Thus, mutations of the major acyltransferases Gat1p and Slc1p lead to an increased contribution of mitochondrial acyltransferase(s) to glycerolipid synthesis due to their substrate preference for dihydroxyacetone phosphate.

Genetic regulation of phospholipid metabolism: yeast as a model eukaryote

Baker's yeast, Saccharomyces cerevisiae, is an excellent and an increasingly important model for the study of fundamental questions in eukaryotic cell biology and genetic regulation. The fission yeast, Schizosaccharomyces pombe, although not as intensively studied as S. cerevisiae, also has many advantages as a model system. In this review, we discuss progress over the past several decades in biochemical and molecular genetic studies of the regulation of phospholipid metabolism in these two organisms and higher eukaryotes. In S. cerevisiae, following the recent completion of the yeast genome project, a very high percentage of the gene-enzyme relationships in phospholipid metabolism have been assigned and the remaining assignments are expected to be completed rapidly. Complex transcriptional regulation, sensitive to the availability of phospholipid precusors, as well as growth phase, coordinates the expression of the structural genes encoding these enzymes in S. cerevisiae. In this article, this regulation is described, the mechanism by which the cell senses the ongoing metabolic activity in the pathways for phospholipid biosynthesis is discussed, and a model is presented. Recent information relating to the role of phosphatidylcholine turnover in S. cerevisiae and its relationship to the secretory pathway, as well as to the regulation of phospholipid metabolism, is also presented. Similarities in the role of phospholipase D-mediated phosphatidylcholine turnover in the secretory process in yeast and mammals lend further credence to yeast as a model system.

Biosynthesis of phosphatidic acid in lipid particles and endoplasmic reticulum of Saccharomyces cerevisiae

Lipid particles of the yeast Saccharomyces cerevisiae harbor two enzymes that stepwise acylate glycerol-3-phosphate to phosphatidic acid, a key intermediate in lipid biosynthesis. In lipid particles of the s1c1 disruptant YMN5 (M. M. Nagiec et al., J. Biol. Chem. 268:22156-22163, 1993) acylation stops after the first step, resulting in the accumulation of lysophosphatidic acid. Two-dimensional gel electrophoresis confirmed that S1c1p is a component of lipid particles. Lipid particles of a second mutant strain, TTA1 (T. S. Tillman and R. M. Bell, J. Biol. Chem. 261:9144-9149, 1986), which harbors a point mutation in the GAT gene, are essentially devoid of glycerol-3-phosphate acyltransferase activity in vitro. Synthesis of phosphatidic acid is reconstituted by combining lipid particles from YMN5 and TTA1. These results indicate that two distinct enzymes are necessary for phosphatidic acid synthesis in lipid particles: the first step, acylation of glycerol-3-phosphate, is catalyzed by a putative Gat1p; the second step, acylation of lysophosphatidic acid, requires S1c1p. Surprisingly, YMN5 and TTA1 mutants grow like the corresponding wild types because the endoplasmic reticulum of both mutants has the capacity to form a reduced but significant amount of phosphatidic acid. As a consequence, an s1c1 gat1 double mutant is also viable. Lipid particles from this double mutant fail completely to acylate glycerol-3-phosphate, whereas endoplasmic reticulum membranes harbor residual enzyme activities to synthesize phosphatidic acid. Thus, yeast contains at least two independent systems of phosphatidic acid biosynthesis.

Dihydroxyacetone phosphate acyltransferase

In this article the properties, assay, distribution, subcellular localization, deficiency in congenital peroxisomal disorders, purification and physiological functions of dihydroxyacetone phosphate acyltransferase (EC 2.3.1.42) are reviewed.

The acyl dihydroxyacetone phosphate pathway enzymes for glycerolipid biosynthesis are present in the yeast Saccharomyces cerevisiae

The presence of the acyl dihydroxyacetone phosphate (acyl DHAP) pathway in yeasts was investigated by examining three key enzyme activities of this pathway in Saccharomyces cerevisiae. In the total membrane fraction of S. cerevisiae, we confirmed the presence of both DHAP acyltransferase (DHAPAT; Km = 1.27 mM; Vmax = 5.9 nmol/min/mg of protein) and sn-glycerol 3-phosphate acyltransferase (GPAT; Km = 0.28 mM; Vmax = 12.6 nmol/min/mg of protein). The properties of these two acyltransferases are similar with respect to thermal stability and optimum temperature of activity but differ with respect to pH optimum (6.5 for GPAT and 7.4 for DHAPAT) and sensitivity toward the sulfhydryl blocking agent N-ethylmaleimide. Total membrane fraction of S. cerevisiae also exhibited acyl/alkyl DHAP reductase (EC 1.1.1.101) activity, which has not been reported previously. The reductase has a Vmax of 3.8 nmol/min/mg of protein for the reduction of hexadecyl DHAP (Km = 15 microM) by NADPH (Km = 20 microM). Both acyl DHAP and alkyl DHAP acted as substrates. NADPH was the specific cofactor. Divalent cations and N-ethylmaleimide inhibited the enzymatic reaction. Reductase activity in the total membrane fraction from aerobically grown yeast cells was twice that from anaerobically grown cells. Similarly, DHAPAT and GPAT activities were also greater in aerobically grown yeast cells. The presence of these enzymes, together with the absence of both ether glycerolipids and the ether lipid-synthesizing enzyme (alkyl DHAP synthase) in S. cerevisiae, indicates that non-ether glycerolipids are synthesized in this organism via the acyl DHAP pathway.

Regulation of phosphatidate phosphatase activity by inositol in Saccharomyces cerevisiae

Regulation of phosphatidate phosphatase (EC 3.1.34) activity was examined in Saccharomyces cerevisiae cells supplemented with phospholipid precursors. Addition of inositol to the growth medium of wild-type cells resulted in a twofold increase in phosphatidate phosphatase activity. The increase in phosphatidate phosphatase activity was not due to soluble effector molecules, and inositol did not have a direct effect on enzyme activity. The phosphatidate phosphatase activity associated with the mitochondrial, microsomal, and cytosolic fractions of the cell was regulated by inositol in the same manner. Cells supplemented with inositol had elevated phospholipid levels and reduced triacylglycerol levels compared with unsupplemented cells. Serine, ethanolamine, and choline did not significantly affect the phosphatidate phosphatase activity of cells grown in the absence or presence of inositol. Enzyme activity was not regulated in inositol biosynthesis regulatory mutants, suggesting that regulation by inositol is coupled to regulation of inositol biosynthesis. Phosphatidate phosphatase activity was pleiotropically expressed in structural gene mutants defective in phospholipid biosynthesis. These results suggested that phosphatidate phosphatase was regulated by inositol at a genetic level.