Acetyl-coenzyme A carboxylase in maize leaves

Chloroplast acetyl-CoA carboxylase activity is 2-oxoglutarate-regulated by interaction of PII with the biotin carboxyl carrier subunit

The PII protein is a signal integrator involved in the regulation of nitrogen metabolism in bacteria and plants. Upon sensing of cellular carbon and energy availability, PII conveys the signal by interacting with target proteins, thereby modulating their biological activity. Plant PII is located to plastids; therefore, to identify new PII target proteins, PII-affinity chromatography of soluble extracts from Arabidopsis leaf chloroplasts was performed. Several proteins were retained only when Mg-ATP was present in the binding medium and they were specifically released from the resin by application of a 2-oxoglutarate-containing elution buffer. Mass spectroscopy of SDS/PAGE-resolved protein bands identified the biotin carboxyl carrier protein subunits of the plastidial acetyl-CoA carboxylase (ACCase) and three other proteins containing a similar biotin/lipoyl-binding motif as putative PII targets. ACCase is a key enzyme initiating the synthesis of fatty acids in plastids. In in vitro reconstituted assays supplemented with exogenous ATP, recombinant Arabidopsis PII inhibited chloroplastic ACCase activity, and this was completely reversed in the presence of 2-oxoglutarate, pyruvate, or oxaloacetate. The inhibitory effect was PII-dose-dependent and appeared to be PII-specific because ACCase activity was not altered in the presence of other tested proteins. PII decreased the V(max) of the ACCase reaction without altering the K(m) for acetyl-CoA. These data show that PII function has evolved between bacterial and plant systems to control the carbon metabolism pathway of fatty acid synthesis in plastids.

Plant biotin-containing carboxylases

Biotin-containing proteins are found in all forms of life, and they catalyze carboxylation, decarboxylation, or transcarboxylation reactions that are central to metabolism. In plants, five biotin-containing proteins have been characterized. Of these, four are catalysts, namely the two structurally distinct acetyl-CoA carboxylases (heteromeric and homomeric), 3-methylcrotonyl-CoA carboxylase and geranoyl-CoA carboxylase. In addition, plants contain a noncatalytic biotin protein that accumulates in seeds and is thought to play a role in storing biotin. Acetyl-CoA carboxylases generate two pools of malonyl-CoA, one in plastids that is the precursor for de novo fatty acid biosynthesis and the other in the cytosol that is the precursor for fatty acid elongation and a large number of secondary metabolites. 3-Methylcrotonyl-CoA carboxylase catalyzes a reaction in the mitochondrial pathway for leucine catabolism. The exact metabolic function of geranoyl-CoA carboxylase is as yet unknown, but it may be involved in isoprenoid metabolism. This minireview summarizes the recent developments in our understanding of the structure, regulation, and metabolic functions of these proteins in plants.

Induced beta-carotene synthesis driven by triacylglycerol deposition in the unicellular alga dunaliella bardawil

Under stress conditions such as high light intensity or nutrient starvation, cells of the unicellular alga Dunaliella bardawil overproduce beta-carotene, which is accumulated in the plastids in newly formed triacylglycerol droplets. We report here that the formation of these sequestering structures and beta-carotene are interdependent. When the synthesis of triacylglycerol is blocked, the overproduction of beta-carotene is also inhibited. During overproduction of beta-carotene no up-regulation of phytoene synthase or phytoene desaturase is observed on the transcriptional or translational level, whereas at the same time acetyl-CoA carboxylase, the key regulatory enzyme of acyl lipid biosynthesis, is increased, at least in its enzymatic activity. We conclude that under normal conditions the carotenogenic pathway is not maximally active and may be appreciably stimulated in the presence of sequestering structures, creating a plastid-localized sink for the end product of the carotenoid biosynthetic pathway.

Propionyl-CoA carboxylase from Streptomyces coelicolor A3(2): cloning of the gene encoding the biotin-containing subunit

In Streptomyces coelicolor A3(2), polyketides are made from malonyl-CoA, which is presumed to be derived from acetyl-CoA by the action of acetyl-CoA carboxylase (ACC). No ACC activity was found in cell-free extracts of S. coelicolor. However, propionyl-CoA carboxylase (PCC) activity was detected at substantial levels. Fixation of CO2 by ACC and PCC occurs by covalent bonding of CO2 to a biotin-containing protein. Most bacteria have a single small biotinylated protein of approximately 22 kDa, but S. coelicolor contains three larger biotin-containing proteins (approximately 145, 88 and 70 kDa). To determine which biotinylated protein was associated with PCC activity, the enzyme was purified and shown to comprise an alpha subunit (biotin-containing) of 88 kDa and a beta subunit of 66 kDa. The N-terminal sequences of these proteins were determined and, using an oligonucleotide probe, the gene for the alpha subunit (pccA) was cloned.

Molecular cloning of two different cDNAs for maize acetyl CoA carboxylase

Acetyl CoA carboxylase (EC 6.4.1.2) in plants is a chloroplast-localized, biotin-containing enzyme that catalyses the carboxylation of acetyl CoA to malonyl CoA, the first committed step of the fatty acid biosynthesis pathway. Acetyl CoA carboxylase is the target site for the monocotyledon-specific aryloxyphenoxypropionate and cyclohexanedione groups of herbicides. We have purified a herbicide-sensitive acetyl CoA carboxylase from maize leaves to homogeneity (specific activity 7 mumol min-1 mg-1), separating it during the purification from a minor herbicide-resistant acetyl CoA carboxylase. The purified enzyme is a dimer of 230 kDa subunits. Antibodies raised to the purified acetyl CoA carboxylase detected three cross-reacting clones in a maize leaf cDNA expression library, each having an insert of 4-4.5 kb. Restriction analysis and sequencing showed that the cDNAs were derived from two different transcripts. Comparison of the deduced amino acid sequences with those of chicken and yeast acetyl CoA carboxylases confirmed that both types encoded acetyl CoA carboxylase, corresponding to the C-terminal half of the enzyme. The overall identity of the maize and chicken sequences was 37% (58% similarity) but for some shorter regions was much higher. Analysis of six other acetyl CoA carboxylase clones recovered from the maize cDNA library showed four belonged to one type and two to the other. The nucleotide sequence similarity between the two types of cDNA was approximately 95% in the coding region but considerably less in the 3'-untranslated region. Northern blot analysis of maize RNA showed a single band of 8.2-8.5 kb for acetyl CoA carboxylase mRNA. Southern blot hybridisations indicated that there are probably no more than two genes in maize for acetyl CoA carboxylase. The possible significance of two different cDNAs for acetyl CoA carboxylase is discussed.

Plants contain multiple biotin enzymes: discovery of 3-methylcrotonyl-CoA carboxylase, propionyl-CoA carboxylase and pyruvate carboxylase in the plant kingdom

Acetyl-CoA carboxylase is the sole biotin enzyme previously reported in plants. Western analysis with 125I-streptavidin of proteins extracted from carrot somatic embryos visualized six biotin-containing polypeptides, the relative molecular masses of which are 210,000, 140,000, 73,000, 50,000, 39,000, and 34,000. This multiplicity of the biotin-containing polypeptides can be partly explained by the discovery of 3-methylcrotonyl-CoA carboxylase, propionyl-CoA carboxylase, and pyruvate carboxylase in extracts of somatic carrot embryos, biotin enzymes previously unknown in the plant kingdom. These biotin enzymes seem to be widely distributed in the plant kingdom.

Changes in the activities of various lipid and carbohydrate biosynthetic enzymes in the diatom Cyclotella cryptica in response to silicon deficiency

The effects of silicon deficiency on the activities of several enzymes involved in lipid and storage carbohydrate synthesis in the diatom Cyclotella cryptica were determined. The activity of UDPglucose pyrophosphorylase was not affected after 4 h of silicon-deficient growth, but the activity of UDPglucose: beta-(1----3)-glucan-beta-3-glucosyltransferase (chrysolaminarin synthase) was reduced by 31% during this period. Acetyl-CoA synthetase, acetyl-CoA hydrolase, and citrate synthase activities were present in cell-free extracts of C. cryptica, but did not change in response to 4 h of silicon deficiency. However, the activity of acetyl-CoA carboxylase increased approximately two- and fourfold after 4 and 15 h of silicon-deficient growth, respectively. This induction could be blocked by cycloheximide (20 micrograms/ml) and actinomycin D (10 micrograms/ml), suggesting that silicon deficiency may induce an increase in the rate of acetyl-CoA carboxylase synthesis. These changes in enzymatic activity may be partially responsible for the accumulation of lipids that has been observed in C. cryptica and other diatoms in response to silicon deficiency.

Inhibition of plant acetyl-coenzyme A carboxylase by the herbicides sethoxydim and haloxyfop

Incorporation of [14C]acetate or [14C]pyruvate into fatty acids in isolated corn seedling chloroplasts was inhibited 90% or greater by 10 microM sethoxydim or 1 microM haloxyfop. At these concentrations, neither sethoxydim nor haloxyfop inhibited [14C]acetate incorporation into fatty acids in isolated pea chloroplasts. Sethoxydim (10 microM) and haloxyfop (1 microM) did not inhibit incorporation of [14C]malonyl-CoA into fatty acids in cell free extracts from corn tissue cultures. Acetyl coenzyme A carboxylase (EC 6.4.1.2) from corn seedling chloroplasts was inhibited by both sethoxydim and haloxyfop, with I50 values of 2.9 and 0.5 microM, respectively. This enzyme in pea was not inhibited by 10 microM sethoxydim or 1 microM haloxyfop.

Acetyl-CoA-carboxylase activity in normally developing wheat leaves

In order to investigate the role of acetyl CoA carboxylase (ACC) in the regulation of fatty-acid biosynthesis in chloroplasts, the activities and relative amounts of the enzyme have been measured in the tissue of wheat (Triticum aestivum L.) leaves undergoing development and cellular differentiation. The total activity in the first leaves of 5- to 7-d-old plants was similar but decreased to less than half in 9-d-old plants. The activity of ACC in the cells of the first leaf of 7-d-old plants doubled when cell age increased from 24 to 48 h, remained relatively constant for a further 24 h and then declined. The amount of ACC in cells increased 15-fold during the first 36 h of cell enlargement. Cells more than 36 h old contained about two-thirds the maximum amount of ACC found in younger cells. The most rapid phase of fatty-acyl accumulation in lipids was in cells aged between 60 and 84 h. Tenfold changes in the activity of ACC were observed when the assay conditions with respect to ATP, ADP, Mg(2+) and pH were changed to correspond to the physiological conditions in chloroplasts during light/dark transitions. This observation and the magnitude of the changes in the optimum activity and amount of ACC in leaf cells undergoing development are consistent with a role for ACC in the regulation of the flow of carbon from acetyl CoA to fatty acids in chloroplasts.

Use of streptavidin to detect biotin-containing proteins in plants

A procedure to detect biotinyl proteins after fractionation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was developed. Proteins were immobilized on nitrocellulose and biotin-containing proteins were detected by probing with 125I-streptavidin. Using this procedure a small survey of biotinyl protein in plants was undertaken. In total four biotin-containing proteins were detected in higher plants of molecular weights 62,000, 50,000, 34,000, and 31,000. These biotinyl proteins were not ubiquitous in the plants surveyed. In the cyanobacterium Anabeana variabilis, a single biotin-containing protein of 21,000 Da was detected. In isolated spinach chloroplasts, the two biotinyl proteins detected were soluble. The results are discussed in relation to acetyl-CoA carboxylase.

Localization of chloroplastic fatty acid synthesis de novo in the stroma

The synthesis of fatty acids de novo from [2-14C]malonyl-CoA was studied in fractions from lettuce (Lactuca sativa) and pea (Pisum sativum) chloroplasts. When lettuce chloroplasts were subjected to osmotic lysis, disintegration through a Yeda press and high-speed centrifugation, essentially all of the fatty-acid-synthetic activity was found to be soluble. The distribution of the activity in various chloroplast fractions was similar to that of soluble marker enzymes such as ribulose-1,5-bisphosphate carboxylase and NADP+-linked glyceraldehyde-3-phosphate dehydrogenase. Marked differences were apparent in the quality of products from fatty acid synthesis de novo in the various fractions of chloroplasts. Thus soluble fractions produced predominantly stearate, whereas those containing membranes produced a greater proportion of palmitate. In pea chloroplasts, osmotic lysis released almost all of the fatty acid synthetase into the stromal fraction. In this instance, no major alterations in the products of fatty acid synthesis were observed. The fatty-acid-synthetic activity of the stromal fraction was still soluble after prolonged ultracentrifugation. The results show clearly the soluble nature of fatty acid synthesis de novo in lettuce and pea chloroplasts. Thus fatty acid synthesis measured in microsomal fractions from such plant tissues is not due to the presence of chloroplastic membranes.

Subcellular distribution of acetyl-coenzyme A carboxylase in mesophyll cells of barley and sorghum leaves

The subcellular distribution of acetyl-CoA carboxylase [acetyl-CoA-carbon dioxide ligase (ADP-forming), EC 6.4.1.2] was determined in mesophyll protoplasts isolation from barley, a C3 plant, and sorghum, a C4 plant. In both species, all of the mesophyll acetyl-CoA carboxylase was demonstrated to be chloroplastic. In barley leaves and mesophyll protoplasts, a single biotinyl protein of 60,000 Da was identified by a modified Western-blotting procedure. The subcellular distribution of this biotinyl protein was identical to that found for acetyl-CoA carboxylase. These results are discussed in relation to the compartmentation of reactions requiring malonyl-CoA as a substrate.

The effect of hypolipidemic drugs on plant lipid metabolism

The effect of hypolipidemic drugs, WY14643 and DH990, on plant lipid metabolism has been studied. The total incorporation of [14C]acetate into lipids was inhibited by addition of both drugs to aged potato (Solanum tuberosum) tuber discs, spinach (Spinacia oleracea) leaves, and spinach chloroplasts, while the incorporation in Chlorella vulgaris cells was affected only by DH990. Moreover, DH990 inhibited the incorporation of 14C-labeled fatty acids into phosphatidylcholine and phosphatidylethanolamine of potato discs, and decreased the incorporation into phosphatidylglycerol of Chlorella cells. DH990 inhibited the formation of polyunsaturated fatty acids in potato discs, Chlorella cells, and spinach leaves, whereas WY14643 had no effect on the formation of these fatty acids. Stearoyl-ACP desaturase from safflower (Carthamus tinctorius) seeds was very sensitive to both drugs, especially DH990, which completely blocked the activity at 2 mM levels. When safflower lysophospholipid acyltransferases were solubilized by detergent treatment, only DH990 inhibited the incorporation of [14C]oleoyl-CoA into lysophosphatidylcholine or lysophosphatidylethanolamine. Both drugs inhibited fatty acid synthesis from [14C]malonyl-CoA in the microsomal fraction from safflower seeds, but only DH990 inhibited FAS activity in the soluble fraction; both drugs inhibited severely the formation of stearic acid. Both acetyl-CoA carboxylase and acetyl-CoA synthetase were sensitive to both drugs.

Purification and characterization of maize leaf acetyl-coenzyme A carboxylase

Maize leaf acetyl-CoA carboxylase was purified from whole tissue homogenates by precipitation with polyethylene glycol and ammonium sulfate, and gel filtration. Recoveries were approximately 5% with 100-fold increases in specific activity. The molecular weight of the native enzyme is estimated at 500,000 from the elution volume of a calibrated Ultrogel AcA 22 column. Electrophoresis in polyacrylamide gel containing 1% sodium dodecyl sulfate revealed a single subunit of Mr 60,000-61,000. Investigation of the kinetic properties of the purified enzyme indicates that Mg X ATP is the active substrate, with free ATP inhibiting and Mg2+ activating the enzyme. Km's for acetyl-CoA and HCO3- are about 0.1 and 2 mM, respectively. ADP inhibition is competitive with respect to ATP, but uncompetitive with respect to acetyl-CoA. The observed responses of purified acetyl-CoA carboxylase to changes in pH, and in concentrations of Mg2+, ATP, and ADP, and the reported changes in the chloroplastic concentrations of these effectors during light-dark transitions of chloroplasts are consistent with increased acetyl-CoA carboxylase activity upon illumination of chloroplasts.

Acetyl-coenzyme A carboxylase from the developing endosperm of Ricinus communis. I. Isolation and characterization

Acetyl-coenzyme A carboxylase has been purified from the plastids of developing castor oil seeds. High concentrations of the enzyme are required for stability as well as the presence of dithiothreitol, glycerol, bicarbonate, Triton X-100, and polyvinyl-pyrrolidone. It has a molecular weight of approximately 528,000 and appears to be membrane associated. Acetyl-CoA carboxylase is active over a wide pH range with an optimum at 8.0. Arrhenius plots are biphasic. The enzyme displays normal Michaelis-Menten kinetics with limiting Michaelis constants of KATP, 0.1 mM; KHCO-3, 3.0 mM; and Kacetyl-CoA, 0.05 mM. Monovalent cations, such as K+ and Cs+, exert a small activating effect on the enzyme while a divalent cation, Mn2+ or Mg2+, is essential for activity. The enzyme does not appear to be highly regulated by cellular metabolites.

Changes in the activity of acetyl-CoA carboxylase during rape-seed formation

During the formation of rape-seeds, lipid accumulated in the cotyledons from 16 days after pollination, rising to a plateau after 28 days. The accumulation of lipid was preceded by a marked rise in acetyl-CoA carboxylase activity, which declined rapidly, correlating with the decline in rate of lipid formation. Incubation of rape-seed extracts with avidin-agarose resulted in a decrease in acetyl-CoA carboxylase activity in the extract. Polyacrylamide-gel electrophoresis of polypeptides bound to avidin-agarose showed the presence of a polypeptide of Mr 225 000. The intensity of this band increased during the period of increase of acetyl-CoA carboxylase activity in the seeds.

The use of acetyl-CoA carboxylase activity and changes in wall composition as measures of embryogenesis in tissue cultures of oil palm (Elaeis guineensis)

With some lines of oil-palm tissue cultures embryogenesis occurs spontaneously within the callus grown on a medium containing 2.5 mg of 3-naphthylacetic acid/litre. One of the initial biochemical events that occurs just before the embryoid can be seen is the accumulation of fat droplets within the cells. This accumulation of lipid is correlated with an increase in acetyl-CoA carboxylase activity. The carboxylase is thus probably a rate-limiting step in fatty acid synthesis in these cells and can be used as a quantitative marker of somatic embryogenesis within the tissue. During the development of the embryoid tissue there is an increase in cell division and the differentiation of vascular cells with secondary thickened walls. These stages of the differentiation may be monitored by measuring the ratio of pectin synthesis (polygalacturonic acid formation) to hemicellulose synthesis (xylan formation).