Spinach leaf acetyl-coenzyme a synthetase: purification and characterization

Enzymatic characterization of two acetyl-CoA synthetase genes from Populus trichocarpa

The acetyl-CoA synthetase (ACS) family is a subfamily of adenylate-forming enzymes, which has a close evolutionary relationship with the 4-coumarate:CoA ligase (4CL) family. In this study, two ACS genes were cloned from Populus trichocarpa and were named PtrACS1 and PtrACS2. Bioinformatics characterization of PtrACS1 and PtrACS2 showed that they contained the key ACS residues and a putative peroxisome targeting sequence 1 (PTS1) at the end of the C-terminal sequence. Real-time PCR results showed that PtrACS1 and PtrACS2 were expressed in the phloem, xylem, leaves, and roots of one-year-old P. trichocarpa, but were expressed primarily in the leaves. The ACS enzyme activity was higher in leaves than other tissues in P. trichocarpa. Two overexpressed recombinant proteins showed no catalytic activity toward the substrates of 4CL, but did have notable catalytic activity toward sodium acetate and substrates of ACS. The relative activities of PtrACS1 and PtrACS2 were 194.16 ± 11.23 and 422.25 ± 21.69 μM min(-1) mg(-1), respectively. The K m and V max of PtrACS1 were 0.25 mM and 698.85 μM min(-1) mg(-1), while those for PtrACS2 were 0.72 mM and 245.96 μM min(-1) mg(-1), respectively. Our results revealed that both proteins belong to the ACS family, and provide a theoretical foundation for the identification and functional analysis of members of the adenylate-forming enzyme superfamily.

Characterization and expression of AMP-forming Acetyl-CoA Synthetase from Dunaliella tertiolecta and its response to nitrogen starvation stress

AMP-forming acetyl-CoA synthetase (ACS) catalyzes the formation of acetyl-CoA. Here, a cDNA of ACS from Dunaliella tertiolecta (DtACS) was isolated using RACEs. The full-length DtACS cDNA (GenBank: KT692941) is 2,464 bp with a putative ORF of 2,184 bp, which encodes 727 amino acids with a predicted molecular weight of 79.72 kDa. DtACS has a close relationship with Chlamydomonas reinhardtii and Volvox carteri f. nagariensis. ACSs existing in Bacteria, Archaea and Eukaryota share ten conserved motifs (A1–A10) and three signature motifs (I–III) of the acyl-adenylate/thioester forming enzyme superfamily. DtACS was expressed in E. coli BL21 as Trx-His-tagged fusion protein (~100 kDa) and the enzymatic activity was detected. The recombinant DtACS was purified by HisTrap^TM HP affinity chromatography to obtain a specific activity of 52.873 U/mg with a yield of 56.26%, which approached the specific activity of ACS isolated from other eukaryotes. Kinetic analysis indicated that the Km of DtACS was 3.59 mM for potassium acetate, and the purified DtACS exhibited a temperature optimum of 37 °C and a pH optimum of 8.0. In addition, the expression levels of DtACS were increased after nitrogen starvation cultivation, indicating that ACS activity may be related to the lipid accumulation under nitrogen deficient condition.

AMP-forming acetyl coenzyme A synthetase in the outermost membrane of the hyperthermophilic crenarchaeon Ignicoccus hospitalis

Ignicoccus hospitalis, a hyperthermophilic, chemolithoautotrophic crenarchaeon was found to possess a new CO(2) fixation pathway, the dicarboxylate/4-hydroxybutyrate cycle. The primary acceptor molecule for this pathway is acetyl coenzyme A (acetyl-CoA), which is regenerated in the cycle via the characteristic intermediate 4-hydroxybutyrate. In the presence of acetate, acetyl-CoA can alternatively be formed in a one-step mechanism via an AMP-forming acetyl-CoA synthetase (ACS). This enzyme was identified after membrane preparation by two-dimensional native PAGE/SDS-PAGE, followed by matrix-assisted laser desorption ionization-time of flight tandem mass spectrometry and N-terminal sequencing. The ACS of I. hospitalis exhibits a molecular mass of ∼690 kDa with a monomeric molecular mass of 77 kDa. Activity tests on isolated membranes and bioinformatic analyses indicated that the ACS is a constitutive membrane-associated (but not an integral) protein complex. Unexpectedly, immunolabeling on cells of I. hospitalis and other described Ignicoccus species revealed that the ACS is localized at the outermost membrane. This perfectly coincides with recent results that the ATP synthase and the H(2):sulfur oxidoreductase complexes are also located in the outermost membrane of I. hospitalis. These results imply that the intermembrane compartment of I. hospitalis is not only the site of ATP synthesis but may also be involved in the primary steps of CO(2) fixation.

AMP-forming acetyl-CoA synthetase from the extremely halophilic archaeon Haloarcula marismortui: purification, identification and expression of the encoding gene, and phylogenetic affiliation

Halophilic archaea activate acetate via an (acetate)-inducible AMP-forming acetyl-CoA synthetase (ACS), (Acetate+ATP+CoA --> Acetyl-CoA+AMP+PP(i)). The enzyme from Haloarcula marismortui was purified to homogeneity. It constitutes a 72-kDa monomer and exhibited a temperature optimum of 41 degrees C and a pH optimum of 7.5. For optimal activity, concentrations between 1 M and 1.5 M KCl were required, whereas NaCl had no effect. The enzyme was specific for acetate (100%) additionally accepting only propionate (30%) as substrate. The kinetic constants were determined in both directions of the reaction at 37 degrees C. Using the N-terminal amino acid sequence an open reading frame - coding for a 74 kDa protein - was identified in the partially sequenced genome of H. marismortui. The function of the ORF as acs gene was proven by functional overexpression in Escherichia coli. The recombinant enzyme was reactivated from inclusion bodies, following solubilization in urea and refolding in the presence of salts, reduced and oxidized glutathione and substrates. Refolding was dependent on salt concentrations of at least 2 M KCl. The recombinant enzyme showed almost identical molecular and catalytic properties as the native enzyme. Sequence comparison of the Haloarcula ACS indicate high similarity to characterized ACSs from bacteria and eukarya and the archaeon Methanosaeta. Phylogenetic analysis of ACS sequences from all three domains revealed a distinct archaeal cluster suggesting monophyletic origin of archaeal ACS.

A novel octameric AMP-forming acetyl-CoA synthetase from the hyperthermophilic crenarchaeon Pyrobaculum aerophilum

AMP-forming acetyl-CoA synthetases (ACSs) are ubiquitous in all three domains of life. Here, we report the first characterization of an ACS from a hyperthermophilic organism, from the archaeon Pyrobaculum aerophilum. The recombinant ACS, the gene product of ORF PAE2867, showed extremely high thermostability and thermoactivity at temperatures around 100 degrees C. In contrast to known monomeric or homodimeric mesophilic ACSs, the P. aerophilum ACS was a 610 kDa homooctameric protein, with a significant lower content of thermolabile (Cys, Asn, and Gln) and higher content of charged (Glu, Lys, and Arg) amino acids. Kinetic analyses revealed an unusual broad substrate spectrum for organic acids and an extremely high affinity for acetate (K(m) 3 microM).

Expression of a yeast acetyl CoA hydrolase in the mitochondrion of tobacco plants inhibits growth and restricts photosynthesis

Acetyl Coenzyme A (acetyl CoA) is required in the mitochondria to fuel the operation of the Krebs cycle and within the cytosolic, peroxisomal and plastidial compartments wherein it acts as the immediate precursor for a wide range of anabolic functions. Since this metabolite is impermeable to membranes it follows that discrete pathways both for its synthesis and for its utilization must be present in each of these organelles and that the size of the various compartmented pools are independently regulated. To determine the specific role of acetyl CoA in the mitochondria we exploited a transgenic approach to introduce a yeast acetyl CoA hydrolase (EC 3.1.2.1.) into this compartment in tobacco plants. Despite the facts that the introduced enzyme was correctly targeted and that there were marked reductions in the levels of citrate and malate and an increase in the acetate content of the transformants, the transgenic plants surprisingly exhibited increased acetyl CoA levels. The lines were further characterised by a severe growth retardation, abnormal leaf colouration and a dramatic reduction in photosynthetic activity correlated with a marked reduction in the levels of transcripts of photosynthesis and in the content of photosynthetic pigments. The altered rate of photosynthesis in the transgenics was also reflected by a modified carbon partitioning in leaves of these lines, however, further studies revealed that this was most likely caused by a decreased source to sink transport of carbohydrate. In summary these results suggest that the content of acetyl CoA is under tight control and that alterations in the level of this central metabolite have severe metabolic and developmental consequences in tobacco.

Role of acetyl-coenzyme A synthetase in leaves of Arabidopsis thaliana

Acetyl-coenzyme A synthetase (ACS) is a plastidic enzyme that forms acetyl-coenzyme A (acetyl-CoA) from acetate and coenzyme A using the energy from ATP. Traditionally it has been thought to be the major source for the production of acetyl-CoA destined for fatty acid formation. Recent work suggested that the accumulation of lipids in developing Arabidopsis seeds was more closely correlated with the expression of pyruvate dehydrogenase complex than with the expression of ACS, suggesting that most of the carbon for fatty acid formation in the plastids of seeds comes from pyruvate rather than from acetate. To explore the role of this enzyme, Arabidopsis plants with altered amounts of ACS were generated by overexpressing its cDNA in either the sense or the antisense configuration. The resulting plants had in vitro enzyme activities that ranged from about 5% to over 400% of wild-type levels. The rate of [1-14C]acetate conversion into fatty acids was closely related to the in vitro ACS activity, showing that the amount of enzyme clearly limited the capacity of leaves to convert exogenous acetate to fatty acids. There was, however, no relationship between the ACS level and the capacity of the plants to incorporate 14CO2 into 14C-labeled fatty acids. These data strongly support the idea that, although plants can convert acetate into fatty acids, relatively little carbon moves through this pathway under normal conditions.

Carbon flux and fatty acid synthesis in plants

The de novo synthesis of fatty acids in plants occurs in the plastids through the activity of fatty acid synthetase. The synthesis of the malonyl-coenzyme A that is required for acyl-chain elongation requires the import of metabolites from the cytosol and their subsequent metabolism. Early studies had implicated acetate as the carbon source for plastidial fatty acid synthesis but more recent experiments have provided data that argue against this. A range of cytosolic metabolites including glucose 6-phosphate, malate, phosphoenolpyruvate and pyruvate support high rates of fatty acid synthesis by isolated plastids, the relative utilisation of which depends upon the plant species and the organ from which the plastids are isolated. The import of these metabolites occurs via specific transporters on the plastid envelope and recent advances in the understanding of the role of these transporters are discussed. Chloroplasts are able to generate the reducing power and ATP required for fatty acid synthesis by capture of light energy in the reactions of photosynthetic electron transport. Regulation of chloroplast fatty acid synthesis is mediated by the response of acetyl-CoA carboxylase to the redox state of the plastid, which ensures that the carbon metabolism is linked to the energy status. The regulation of fatty acid synthesis in plastids of heterotrophic cells is much less well understood and is of particular interest in the tissues that accumulate large amounts of the storage oil, triacylglycerol. In these heterotrophic cells the plastids import ATP and oxidise imported carbon sources to produce the required reducing power. The sequencing of the genome of Arabidopsis thaliana has now enabled a number of aspects of plant fatty acid synthesis to be re-addressed, particularly those areas in which in vitro biochemical analysis had provided equivocal answers. Examples of such aspects and future opportunities for our understanding of plant fatty acid synthesis are presented and discussed.

Stromal concentrations of coenzyme A and its esters are insufficient to account for rates of chloroplast fatty acid synthesis: evidence for substrate channelling within the chloroplast fatty acid synthase

Concentrations of total CoAs in chloroplasts freshly isolated from spinach and peas were 10-20 microM, assuming a stromal volume of 66 microl per mg of chlorophyll. Acetyl-CoA and CoASH constituted at least 90% of the total CoA in freshly isolated chloroplasts. For a given chloroplast preparation, the concentration of endogenous acetyl-CoA was the same when extractions were performed using HClO4, trichloroacetic acid, propan-2-ol or chloroform/methanol, and the extracts analysed by quantitative HPLC after minimal processing. During fatty acid synthesis from acetate, concentrations of CoASH within spinach and pea chloroplasts varied from less than 0.1 to 5.0 microM. Malonyl-CoA concentrations were also very low (<0.1-3.0 microM) during fatty acid synthesis but could be calculated from radioactivity incorporated from [1-14C]acetate. Concentrations of CoASH in chloroplasts synthesizing fatty acids could be doubled in the presence of Triton X-100, suggesting that the detergent stimulates fatty acid synthesis by increasing the turnover rate of acyl-CoA. However, although taken up, exogenous CoASH (1 microM) did not stimulate fatty acid synthesis by permeabilized spinach chloroplasts. Calculated rates for acetyl-CoA synthetase, acetyl-CoA carboxylase and malonyl-CoA-acyl-carrier protein transacylase reactions at the concentrations of metabolites measured here are < 0.1-4% of the observed rates of fatty acid synthesis from acetate by isolated chloroplasts. The results suggest that CoA and its esters are probably confined within, and channelled through, the initial stages of a fatty acid synthase multienzyme complex.

Purification and properties of acetyl-CoA synthetase (ADP-forming), an archaeal enzyme of acetate formation and ATP synthesis, from the hyperthermophile Pyrococcus furiosus

Acetyl-CoA synthetase (ADP-forming) is an enzyme in Archaea that catalyzes the formation of acetate from acetyl-CoA and couples this reaction with the synthesis of ATP from ADP and Pi (acetyl-CoA + ADP + Pi --> acetate + ATP + CoA) [Schifer, T., Selig, M. & Schonheit, P. (1993) Arch. Microbiol. 159, 72-83]. The enzyme from the anaerobic hyperthermophile Pyrococcus furiosus was purified 96-fold with a yield of 20% to apparent electrophoretic homogeneity. The oxygen-stable enzyme had an apparent molecular mass of 145 kDa and was composed of two subunits with apparent molecular masses of 47 kDa and 25 kDa, indicating an alpha2beta2 structure. The N-terminal amino acid sequences of both subunits were determined; they do not show significant identity to other proteins in databases. The purified enzyme catalyzed the reversible conversion of acetyl-CoA, ADP and Pi to acetate, ATP and CoA. The apparent Vmax value in the direction of acetate formation was 18 U/mg (55 degrees C), the apparent Km values for acetyl-CoA, ADP and Pi were 17 microM, 60 microM and 200 microM, respectively. ADP and Pi could not be replaced by AMP and PPi, defining the enzyme as an ADP-forming rather than an AMP-forming acetyl-CoA synthetase. The apparent Vmax value in the direction of acetyl-CoA formation was about 40 U/mg (55 degrees C), and the apparent Km values for acetate, ATP and CoA were 660 microM, 80 microM and 30 microM, respectively. The purified enzyme was not specific for acetyl-CoA or acetate, in addition to acetyl-CoA (100%), the enzyme accepts propionyl-CoA (110%) and butyryl-CoA (92%), and in addition to acetate (100%), the enzyme accepts propionate (100%), butyrate (92%), isobutyrate (79%), valerate (36%) and isovalerate (34%), indicating that the enzyme functions as an acyl-CoA synthetase (ADP-forming) with a broad substrate spectrum. Succinate, phenylacetate and indoleacetate did not serve as substrates for the enzyme (<3%). In addition to ADP (100%), GDP (220%) and IDP (250%) were used, and in addition to ATP (100%), GTP (210%) and ITP (320%) were used. Pyrimidine nucleotides were not accepted. The enzyme was dependent on Mg2+, which could be partly substituted by Mn2+ and Co2+. The pH optimum was pH 7. The enzyme has a temperature optimum at 90 degrees C, which is in accordance with its physiological function under hyperthermophilic conditions. The enzyme was stabilized against heat inactivation by salts. In the presence of KCI (1 M), which was most effective, the enzyme did not loose activity after 2 h incubation at 100 degrees C.

A Developmental Analysis of the Enolase Isozymes from Ricinus communis

Enolase activity was measured in clarified homogenates of various tissues during the life cycle of the castor oil plant (Ricinus communis L. cv Baker 296). The proportions of total activity due to the plastid and cytosolic isozymes were determined after separation by ion-exchange chromatography. The contribution of the plastid isozyme varied from more than 30% of the total at the midpoint of endosperm development to less than 1% in mature leaves and roots. During endosperm development, enolase activity increased to a peak coincident with the maximum rate of storage lipid accumulation, then decreased to nearly undetectable levels in the mature seed. Plastid enolase protein, measured using an enzyme-linked immunosorbent assay, increased in parallel with the increase in activity but decreased less rapidly and was still easily detectable in mature seeds.