Purification, and phosphorylation in vivo and in vitro, of phosphoenolpyruvate carboxykinase from cucumber cotyledons

Defining Mechanisms of C3 to CAM Photosynthesis Transition toward Enhancing Crop Stress Resilience

Global climate change and population growth are persistently posing threats to natural resources (e.g., freshwater) and agricultural production. Crassulacean acid metabolism (CAM) evolved from C3 photosynthesis as an adaptive form of photosynthesis in hot and arid regions. It features the nocturnal opening of stomata for CO2 assimilation, diurnal closure of stomata for water conservation, and high water-use efficiency. To cope with global climate challenges, the CAM mechanism has attracted renewed attention. Facultative CAM is a specialized form of CAM that normally employs C3 or C4 photosynthesis but can shift to CAM under stress conditions. It not only serves as a model for studying the molecular mechanisms underlying the CAM evolution, but also provides a plausible solution for creating stress-resilient crops with facultative CAM traits. This review mainly discusses the recent research effort in defining the C3 to CAM transition of facultative CAM plants, and highlights challenges and future directions in this important research area with great application potential.

Protein-Protein Interactions and Quantitative Phosphoproteomic Analysis Reveal Potential Mitochondrial Substrates of Protein Phosphatase 2A-B'ζ Holoenzyme

Protein phosphatase 2A (PP2A) is a heterotrimeric conserved serine/threonine phosphatase complex that includes catalytic, scaffolding, and regulatory subunits. The 3 A subunits, 17 B subunits, and 5 C subunits that are encoded by the Arabidopsis genome allow 255 possible PP2A holoenzyme combinations. The regulatory subunits are crucial for substrate specificity and PP2A complex localization and are classified into the B, B', and B" non-related families in land plants. In Arabidopsis, the close homologs B'η, B'θ, B'γ, and B'ζ are further classified into a subfamily of B' called B'η. Previous studies have suggested that mitochondrial targeted PP2A subunits (B'ζ) play a role in energy metabolism and plant innate immunity. Potentially, the PP2A-B'ζ holoenzyme is involved in the regulation of the mitochondrial succinate/fumarate translocator, and it may affect the enzymes involved in energy metabolism. To investigate this hypothesis, the interactions between PP2A-B'ζ and the enzymes involved in the mitochondrial energy flow were investigated using bimolecular fluorescence complementation in tobacco and onion cells. Interactions were confirmed between the B'ζ subunit and the Krebs cycle proteins succinate/fumarate translocator (mSFC1), malate dehydrogenase (mMDH2), and aconitase (ACO3). Additional putative interacting candidates were deduced by comparing the enriched phosphoproteomes of wild type and B'ζ mutants: the mitochondrial regulator Arabidopsis pentatricopeptide repeat 6 (PPR6) and the two metabolic enzymes phosphoenolpyruvate carboxylase (PPC3) and phosphoenolpyruvate carboxykinase (PCK1). Overall, this study identifies potential PP2A substrates and highlights the role of PP2A in regulating energy metabolism in mitochondria.

Integrating multiple regulations on enzyme activity: the case of phosphoenolpyruvate carboxykinases

Data on protein post-translational modifications (PTMs) increased exponentially in the last years due to the refinement of mass spectrometry techniques and the development of databases to store and share datasets. Nevertheless, these data per se do not create comprehensive biochemical knowledge. Complementary studies on protein biochemistry are necessary to fully understand the function of these PTMs at the molecular level and beyond, for example, designing rational metabolic engineering strategies to improve crops. Phosphoenolpyruvate carboxykinases (PEPCKs) are critical enzymes for plant metabolism with diverse roles in plant development and growth. Multiple lines of evidence showed the complex regulation of PEPCKs, including PTMs. Herein, we present PEPCKs as an example of the integration of combined mechanisms modulating enzyme activity and metabolic pathways. PEPCK studies strongly advanced after the production of the recombinant enzyme and the establishment of standardized biochemical assays. Finally, we discuss emerging open questions for future research and the challenges in integrating all available data into functional biochemical models.

Two phosphoenolpyruvate carboxykinases with differing biochemical properties in Chlamydomonas reinhardtii

Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the reversible reaction of decarboxylation and phosphorylation of oxaloacetate (OAA) to generate phosphoenolpyruvate (PEP) and CO₂ playing mainly a gluconeogenic role in green algae. We found two PEPCK isoforms in Chlamydomonas reinhardtii and we cloned, purified and characterised both enzymes. ChlrePEPCK1 is more active as decarboxylase than ChlrePEPCK2. ChlrePEPCK1 is hexameric and its activity is affected by citrate, phenylalanine and malate, while ChlrePEPCK2 is monomeric and it is regulated by citrate, phenylalanine and glutamine. We postulate that the two PEPCK isoforms found originate from alternative splicing of the gene or regulated proteolysis of the enzyme. The presence of these two isoforms would be part of a mechanism to finely regulate the biological activity of PEPCKs.

Gluconeogenesis in Plants: A Key Interface between Organic Acid/Amino Acid/Lipid and Sugar Metabolism

Gluconeogenesis is a key interface between organic acid/amino acid/lipid and sugar metabolism. The aims of this article are four-fold. First, to provide a concise overview of plant gluconeogenesis. Second, to emphasise the widespread occurrence of gluconeogenesis and its utilisation in diverse processes. Third, to stress the importance of the vacuolar storage and release of Krebs cycle acids/nitrogenous compounds, and of the role of gluconeogenesis and malic enzyme in this process. Fourth, to outline the contribution of fine control of enzyme activity to the coordinate-regulation of gluconeogenesis and malate metabolism, and the importance of cytosolic pH in this.

Tyr-Asp inhibition of glyceraldehyde 3-phosphate dehydrogenase affects plant redox metabolism

How organisms integrate metabolism with the external environment is a central question in biology. Here, we describe a novel regulatory small molecule, a proteogenic dipeptide Tyr-Asp, which improves plant tolerance to oxidative stress by directly interfering with glucose metabolism. Specifically, Tyr-Asp inhibits the activity of a key glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase (GAPC), and redirects glucose toward pentose phosphate pathway (PPP) and NADPH production. In line with the metabolic data, Tyr-Asp supplementation improved the growth performance of both Arabidopsis and tobacco seedlings subjected to oxidative stress conditions. Moreover, inhibition of Arabidopsis phosphoenolpyruvate carboxykinase (PEPCK) activity by a group of branched-chain amino acid-containing dipeptides, but not by Tyr-Asp, points to a multisite regulation of glycolytic/gluconeogenic pathway by dipeptides. In summary, our results open the intriguing possibility that proteogenic dipeptides act as evolutionarily conserved small-molecule regulators at the nexus of stress, protein degradation, and metabolism.

Engineering of Crassulacean Acid Metabolism

Crassulacean acid metabolism (CAM) has evolved from a C₃ ground state to increase water use efficiency of photosynthesis. During CAM evolution, selective pressures altered the abundance and expression patterns of C₃ genes and their regulators to enable the trait. The circadian pattern of CO₂ fixation and the stomatal opening pattern observed in CAM can be explained largely with a regulatory architecture already present in C₃ plants. The metabolic CAM cycle relies on enzymes and transporters that exist in C₃ plants and requires tight regulatory control to avoid futile cycles between carboxylation and decarboxylation. Ecological observations and modeling point to mesophyll conductance as a major factor during CAM evolution. The present state of knowledge enables suggestions for genes for a minimal CAM cycle for proof-of-concept engineering, assuming altered regulation of starch synthesis and degradation are not critical elements of CAM photosynthesis and sufficient malic acid export from the vacuole is possible.

Proteolytic cleavage of Arabidopsis thaliana phosphoenolpyruvate carboxykinase-1 modifies its allosteric regulation

Phosphoenolpyruvate carboxykinase (PEPCK) plays a crucial role in gluconeogenesis. In this work, we analyze the proteolysis of Arabidopsis thaliana PEPCK1 (AthPEPCK1) in germinating seedlings. We found that the amount of AthPEPCK1 protein peaks at 24-48 h post-imbibition. Concomitantly, we observed shorter versions of AthPEPCK1, putatively generated by metacaspase-9 (AthMC9). To study the impact of AthMC9 cleavage on the kinetic and regulatory properties of AthPEPCK1, we produced truncated mutants based on the reported AthMC9 cleavage sites. The Δ19 and Δ101 truncated mutants of AthPEPCK1 showed similar kinetic parameters and the same quaternary structure as the wild type. However, activation by malate and inhibition by glucose 6-phosphate were abolished in the Δ101 mutant. We propose that proteolysis of AthPEPCK1 in germinating seedlings operates as a mechanism to adapt the sensitivity to allosteric regulation during the sink-to-source transition.

Biochemical characterization of phosphoenolpyruvate carboxykinases from Arabidopsis thaliana

ATP-dependent phosphoenolpyruvate carboxykinases (PEPCKs, EC 4.1.1.49) from C4 and CAM plants have been widely studied due to their crucial role in photosynthetic CO2 fixation. However, our knowledge on the structural, kinetic and regulatory properties of the enzymes from C3 species is still limited. In this work, we report the recombinant production and biochemical characterization of two PEPCKs identified in Arabidopsis thaliana: AthPEPCK1 and AthPEPCK2. We found that both enzymes exhibited high affinity for oxaloacetate and ATP, reinforcing their role as decarboxylases. We employed a high-throughput screening for putative allosteric regulators using differential scanning fluorometry and confirmed their effect on enzyme activity by performing enzyme kinetics. AthPEPCK1 and AthPEPCK2 are allosterically modulated by key intermediates of plant metabolism, namely succinate, fumarate, citrate and α-ketoglutarate. Interestingly, malate activated and glucose 6-phosphate inhibited AthPEPCK1 but had no effect on AthPEPCK2. Overall, our results demonstrate that the enzymes involved in the critical metabolic node constituted by phosphoenolpyruvate are targets of fine allosteric regulation.

Phylogenic and phosphorylation regulation difference of phosphoenolpyruvate carboxykinase of C3 and C4 plants

In C4 plants, phosphoenolpyruvate carboxykinase (PEPCK) plays a key role in the C4 cycle. PEPCK is also involved in gluconeogenesis and is conserved in both lower and higher organisms, including in animals and plants. A phylogenic tree constructed from PEPCK sequences from bacteria to higher plants indicates that the C4 Poaceae PEPCKs are conserved and have diverged from the PEPCKs of C3 plants. The maximum enzymatic activities of wild-type and phosphorylation mimic PEPCK proteins indicate that there is a significant difference between C3 and C4 plant PEPCKs. The conserved PEPCK phosphorylation sites are regulated differently in C3 and C4 plants. These results suggest that the functions of PEPCK have been conserved, but that sequences have diverged and regulation of PEPCK is important in C4 plants, but not in herbaceous and, in particular, woody C3 plants.

Phosphorylation of phosphoenolpyruvate carboxykinase (PEPCK) and phosphoenolpyruvate carboxylase (PEPC) in the flesh of fruits

This study determined whether phosphoenolpyruvate carboxykinase (PEPCK) and phosphoenolpyruvate carboxylase (PEPC) are phosphorylated in the flesh of a range of fruits. This was done by incubating fruit flesh with ³²P[P] (where ³²P[P] = ³²PO₄^3-), then PEPCK and PEPC were immunoprecipitated from extracts using specific antisera. The incorporation of ³²P[P] into these enzymes was then determined by autoradiography of SDS-PAGE gels. Both enzymes were subject to phosphorylation in vivo in the flesh of grape, tomato, cherry and plum. PEPCK was also subject to phosphorylation in vivo in developing grape seeds. Proteolytic cleavage of PEPCK showed that it was phosphorylated at a site(s) located on its N-terminal extension. Potentially phosphorylation of these enzymes could contribute to the coordinate regulation of their activities in the flesh of fruits and in developing seeds.

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency affects the germination, growth and fruit sugar content in tomato (Solanum lycopersicum L.)

Phosphoenolpyruvate carboxykinase (PEPCK) is a key regulatory enzyme and is utilized in the gluconeogenesis pathway in plants. Although, its catalytic and regulatory properties are quite well understood, there are uncertainties regarding its physiological role in many plants tissues such as the flesh of developing fruits. To further understand the function of PEPCK in fruits and other tissues, RNAi transgenic tomato plants in which SlPEPCK transcription was down-regulated by either CaMV 35S constitutive promoter or the fruit-specific E8 promoter were generated and characterized on the basis of their phenotypic and metabolic aspects. In the PEPCK-deficient lines, prominent growth suppression of germinated seedlings was observed and other vegetative suppression appeared during the early stage of plant growth in the 35S promoter-driven lines. In particular, root elongation was most obviously suppressed in the germinated seedlings, indicating that the gluconeogenesis pathway is involved in the root growth of seedlings. Regarding the primary metabolism in fruit, the soluble sugar content tended to decrease, whereas the malate content tended to increase in ripening fruits of the RNAi lines compared with the wild type. These results indicate that activation of the gluconeogenesis pathway from organic acids to sugars occurs during ripening but is suppressed by the knocking down of the PEPCK gene, suggesting that PEPCK participates in determining the sugar/acid ratio in ripening fruit.

The pepper phosphoenolpyruvate carboxykinase CaPEPCK1 is involved in plant immunity against bacterial and oomycete pathogens

Phosphoenolpyruvate carboxykinase, a member of the lyase family, is involved in the metabolic pathway of gluconeogenesis in organisms. Although the major function of PEPCK in gluconeogenesis is well established, it is unclear whether this enzyme is involved in plant immunity. Here, we isolated and identified the pepper (Capsicum annuum) PEPCK (CaPEPCK1) gene from pepper leaves infected with Xanthomonas campestris pv. vesicatoria (Xcv). CaPEPCK1 was strongly expressed in pepper leaves during the incompatible interaction with avirulent Xcv and in response to environmental stresses, especially salicylic acid (SA) treatment. PEPCK activity was low in healthy leaves but dramatically increased in avirulent Xcv-infected leaves. Knock-down expression of CaPEPCK1 by virus-induced gene silencing resulted in high levels of susceptibility to both virulent and avirulent Xcv infection. CaPEPCK1 silencing in pepper compromised induction of the basal defense-marker genes CaPR1 (pathogenesis-related 1 protein), CaPR10 (pathogenesis-related 10 protein) and CaDEF1 (defensin) during Xcv infection. SA accumulation was also significantly suppressed in the CaPEPCK1-silenced pepper leaves infected with Xcv. CaPEPCK1 in an Arabidopsis overexpression (OX) line inhibited the proliferation of Pseudomonas syringae pv. tomato (Pst) and Hyaloperonospora arabidopsidis (Hpa). CaPEPCK1-OX plants developed more rapidly, with enlarged leaves, compared to wild-type plants. The T-DNA insertion Arabidopsis orthologous mutants pck1-3 and pck1-4 were more susceptible to the bacterial Pst and oomycete Hpa pathogens than the wild type. Taken together, these results suggest that CaPEPCK positively contributes to plant innate immunity against hemibiotrophic bacterial and obligate biotrophic oomycete pathogens.

Arabidopsis uses two gluconeogenic gateways for organic acids to fuel seedling establishment

Gluconeogenesis is a fundamental metabolic process that allows organisms to make sugars from non-carbohydrate stores such as lipids and protein. In eukaryotes only one gluconeogenic route has been described from organic acid intermediates and this relies on the enzyme phosphoenolpyruvate carboxykinase (PCK). Here we show that two routes exist in Arabidopsis, and that the second uses pyruvate, orthophosphate dikinase (PPDK). Gluconeogenesis is critical to fuel the transition from seed to seedling. Arabidopsis pck1 and ppdk mutants are compromised in seed-storage reserve mobilization and seedling establishment. Radiolabelling studies show that PCK predominantly allows sugars to be made from dicarboxylic acids, which are products of lipid breakdown. However, PPDK also allows sugars to be made from pyruvate, which is a major product of protein breakdown. We propose that both routes have been evolutionarily conserved in plants because, while PCK expends less energy, PPDK is twice as efficient at recovering carbon from pyruvate.

During seed germination plants use gluconeogenesis to mobilize noncarbohydrate energy reserves. Here Eastmond et al. show that plants, unlike other eukaryotes, do not solely rely on a gluconeogenic pathway via the enzyme PCK but also use a second pathway relying on PPDK.

Light quality modulates metabolic synchronization over the diel phases of crassulacean acid metabolism

Temporal compartmentation of carboxylation processes is a defining feature of crassulacean acid metabolism and involves circadian control of key metabolic and transport steps that regulate the supply and demand for carbon over a 24h cycle. Recent insights on the molecular workings of the circadian clock and its connection with environmental inputs raise new questions on the importance of light quality and, by analogy, certain photoreceptors for synchronizing the metabolic components of CAM. The present work tested the hypothesis that optimal coupling of stomatal conductance, net CO2 uptake, and the reciprocal turnover of carbohydrates and organic acids over the diel CAM cycle requires both blue and red light input signals. Contrasting monochromatic wavelengths of blue, green, and red light (i.e. 475, 530, 630nm) with low fluence rates (10 μmol m(-2) s(-1)) were administered for 16 hours each diel cycle for a total treatment time of 48 hours to the obligate CAM bromeliad, Aechmea 'Maya'. Of the light treatments imposed, low-fluence blue light was a key determinant in regulating stomatal responses, organic acid mobilization from the vacuole, and daytime decarboxylation. However, the reciprocal relationship between starch and organic acid turnover that is typical for CAM was uncoupled under low-fluence blue light. Under low-fluence red or green light, the diel turnover of storage carbohydrates was orchestrated in line with the requirements of CAM, but a consistent delay in acid consumption at dawn compared with plants under white or low-fluence blue light was noted. Consistent with the acknowledged influences of both red and blue light as input signals for the circadian clock, the data stress the importance of both red and blue-light signalling pathways for synchronizing the metabolic and physiological components of CAM over the day/night cycle.

Light-regulated phosphorylation of maize phosphoenolpyruvate carboxykinase plays a vital role in its activity

Phosphoenolpyruvate carboxykinase (PEPCK)-the major decarboxylase in PEPCK-type C4 plants-is also present in appreciable amounts in the bundle sheath cells of NADP-malic enzyme-type C4 plants, such as maize (Zea mays), where it plays an apparent crucial role during photosynthesis (Wingler et al., in Plant Physiol 120(2):539-546, 1999; Furumoto et al., in Plant Mol Biol 41(3):301-311, 1999). Herein, we describe the use of mass spectrometry to demonstrate phosphorylation of maize PEPCK residues Ser55, Thr58, Thr59, and Thr120. Western blotting indicated that the extent of Ser55 phosphorylation dramatically increases in the leaves of maize seedlings when the seedlings are transferred from darkness to light, and decreases in the leaves of seedlings transferred from light to darkness. The effect of light on phosphorylation of this residue is opposite that of the effect of light on PEPCK activity, with the decarboxylase activity of PEPCK being less in illuminated leaves than in leaves left in the dark. This inverse relationship between PEPCK activity and the extent of phosphorylation suggests that the suppressive effect of light on PEPCK decarboxylation activity might be mediated by reversible phosphorylation of Ser55.

The Arabidopsis metacaspase9 degradome

Metacaspases are distant relatives of the metazoan caspases, found in plants, fungi, and protists. However, in contrast with caspases, information about the physiological substrates of metacaspases is still scarce. By means of N-terminal combined fractional diagonal chromatography, the physiological substrates of metacaspase9 (MC9; AT5G04200) were identified in young seedlings of Arabidopsis thaliana on the proteome-wide level, providing additional insight into MC9 cleavage specificity and revealing a previously unknown preference for acidic residues at the substrate prime site position P1'. The functionalities of the identified MC9 substrates hinted at metacaspase functions other than those related to cell death. These results allowed us to resolve the substrate specificity of MC9 in more detail and indicated that the activity of phosphoenolpyruvate carboxykinase 1 (AT4G37870), a key enzyme in gluconeogenesis, is enhanced upon MC9-dependent proteolysis.

Two phosphoenolpyruvate carboxykinases coexist in the Crassulacean Acid Metabolism plant Ananas comosus. Isolation and characterization of the smaller 65 kDa form

Two phosphoenolpyruvate carboxykinase (PEPCK, EC 4.1.1.49) isoforms of 74 and 65 kDa were found to coexist in vivo in pineapple leaves, a constitutive Crassulacean Acid Metabolism plant. The 65 kDa form was not the result of proteolytic cleavage of the larger form since extraction methods reported to prevent PEPCK proteolysis in other plant tissues failed to yield a single immunoreactive PEPCK polypeptide in leaf extracts. In this work, the smaller form of 65 kDa was purified to homogeneity and physically and kinetically characterized and showed parameters compatible with a fully active enzyme. The specific activity was nearly twice higher for decarboxylation of oxaloacetate when compared to carboxylation of phosphoenolpyruvate. Kinetic parameters fell within the range of those estimated for other plant PEPCKs. Its activity was affected by several metabolites, as shown by inhibition by 3-phosphoglycerate, citrate, malate, fructose-1,6-bisphosphate, l-asparagine and activation of the decarboxylating activity by succinate. A break in the Arrhenius plot at about 30°C indicates that PEPCK structure is responsive to changes in temperature. The results indicate that pineapple leaves contain two PEPCK forms. The biochemical characterization of the smaller isoform performed in this work suggests that it could participate in both carbon and nitrogen metabolism in vivo by acting as a decarboxylase.

Regulation of malate metabolism in grape berry and other developing fruits

Organic acids are present in all plants, supporting numerous and varied facets of cellular metabolism. The type of organic acid found, and the levels to which they accumulate are extremely variable between species, developmental stages and tissue types. Acidity plays important roles in the organoleptic properties of plant tissues, where examples of both enhanced and reduced palatability can be ascribed to the presence of specific organic acids. In fruits, sourness is generally attributed to proton release from acids such as citric, malic, oxalic, quinic, succinic and tartaric, while the anion forms each contribute a distinct taste. Acidity imposes a strong influence on crop quality, and is an important factor in deciding the harvest date, particularly for fruits where acidity is important for further processing, as in wine grapes. In the grape, as for many other fruits, malate is one of the most prevalent acids, and is an important participant in numerous cellular functions. The accumulation of malate is thought to be due in large part to de novo synthesis in fruits such as the grape, through metabolism of assimilates translocated from leaf tissues, as well as photosynthetic activity within the fruit itself. During ripening, the processes through which malate is catabolised are of interest for advancing metabolic understanding, as well as for potential crop enhancement through agricultural or molecular practices. A body of literature describes research that has begun to unravel the regulatory mechanisms of enzymes involved in malate metabolism during fruit development, through exploration of protein and gene transcript levels. Datasets derived from a series of recent microarray experiments comparing transcript levels at several stages of grape berry development have been revisited, and are presented here with a focus on transcripts associated with malate metabolism. Developmental transcript patterns for enzymes potentially involved in grape malate metabolism have shown that some flux may occur through pathways that are less commonly regarded in ripening fruit, such as aerobic ethanol production. The data also suggest pyruvate as an important intermediate during malate catabolism in fruit. This review will combine an analysis of microarray data with information available on protein and enzyme activity patterns in grapes and other fruits, to explore pathways through which malate is conditionally metabolised, and how these may be controlled in response to developmental and climatic changes. Currently, an insufficient understanding of the complex pathways through which malate is degraded, and how these are regulated, prevents targeted genetic manipulation aimed at modifying fruit malate metabolism in response to environmental conditions.

Stereochemistry of the carboxylation reaction catalyzed by the ATP-dependent phosphoenolpyruvate carboxykinases from Saccharomyces cerevisiae and Anaerobiospirillum succiniciproducens

The stereochemistry of CO(2) addition to phosphoenolpyruvate (PEP) to yield oxaloacetate catalyzed by ATP-dependent Saccharomyces cerevisiae and Anaerobiospirillum succiniciproducens PEP carboxykinases was determined using (Z)-3-fluorophosphoenolpyruvate ((Z)-F-PEP) as a substrate analog. A. succiniciproducens and S. cerevisiae PEP carboxykinases utilized (Z)-F-PEP with 1/14 and 1/47 the respective K(m) values for PEP. On the other hand, in the bacterial and yeast enzymes k(cat) was reduced to 1/67 and 1/48 the value with PEP, respectively. The binding affinity of pyridoxylphosphate-labeled S. cerevisiae and A. succiniciproducens PEP carboxykinases for PEP and (Z)-F-PEP was checked and found to be of similar magnitude for both substrates, suggesting that the lowered K(m) values for the fluorine-containing PEP analog are due to kinetic effects. The lowered k(cat) values when using (Z)-F-PEP as substrate suggest that the electron withdrawing effect of fluorine affects the nucleophilic attack of the double bond of (Z)-F-PEP to CO(2). For the stereochemical analyses, the carboxylation of (Z)-F-PEP was coupled to malate dehydrogenase to yield 3-fluoromalate, which was analyzed by (19)F NMR. The fluoromalate obtained was identified as (2R, 3R)-3-fluoromalate for both the A. succiniciproducens and S. cerevisiae PEP carboxykinases, thus indicating that CO(2) addition to (Z)-F-PEP, and hence PEP, takes place through the 2-si face of the double bond. These results, together with previously published data [Rose, I.A. et al. J. Biol. Chem. 244 (1969) 6130-6133; Hwang, S.H. and Nowak, T. Biochemistry 25 (1986) 5590-5595] indicate that PEP carboxykinases, no matter their nucleotide specificity, catalyze the carboxylation of PEP from the 2-si face of the double bond.

An Arabidopsis mutant disrupted in valine catabolism is also compromised in peroxisomal fatty acid beta-oxidation

Characterisation of the Arabidopsis dbr5 mutant, which was isolated on the basis of 2,4-dichlorophenoxybutyric acid (2,4-DB) resistance, revealed that it is disrupted in the CHY1 gene. CHY1 encodes a peroxisomal protein that is 43% identical to the mammalian beta-hydroxyisobutryl-CoA hydrolase of valine catabolism. We show that 2,4-DB resistance and the associated sucrose dependent seedling growth are due to a large activity decrease of 3-ketoacyl-CoA thiolase, which is involved in peroxisomal fatty acid beta-oxidation. (14)C-feeding studies demonstrate that dbr5 and chy1 seedlings are reduced in valine catabolism. These data support the hypothesis that CHY1 plays a key role in peroxisomal valine catabolism and that disruption of this enzyme results in accumulation of a toxic intermediate, methacrylyl-CoA, that inhibits 3-ketoacyl-CoA thiolase activity and thus blocks peroxisomal beta-oxidation. We also show that CHY1 is repressed in seedlings grown on sugars, which suggests that branched chain amino acid catabolism is transcriptionally regulated by nutritional status.

Regulation and roles of phosphoenolpyruvate carboxykinase in plants

Phosphoenolpyruvate carboxykinase (PCK) is probably ubiquitous in flowering plants, but is confined to certain cells or tissues. It is regulated by phosphorylation, which renders it less active by altering both its substrate affinities and its sensitivity to regulation by adenylates. In the leaves of some C4 plants, such as Panicum maximum, dephosphorylation increases its activity in the light. In other tissues such regulation probably avoids futile cycling between phosphoenolpyruvate and oxaloacetate. Although PCK generally acts as a decarboxylase in plants, its affinity for CO2 measured at physiological concentrations of metal ions is high and would allow it to be freely reversible in vivo. While its function in gluconeogenesis in seeds postgermination and in leaves of C4 and crassulacean acid metabolism plants is clearly established, the possible functions of PCK in other plant cells are discussed, drawing parallels with those in animals, including its integrated function in cataplerosis, nitrogen metabolism, pH regulation, and gluconeogenesis.

Effects of phosphorylation on phosphoenolpyruvate carboxykinase from the C4 plant Guinea grass

In the C4 plant Guinea grass (Panicum maximum), phosphoenolpyruvate carboxykinase (PEPCK) is phosphorylated in darkened leaves and dephosphorylated in illuminated leaves. To determine whether the properties of phosphorylated and non-phosphorylated PEPCK were different, PEPCK was purified to homogeneity from both illuminated and darkened leaves. The final step of the purification procedure, gel filtration chromatography, further separated phosphorylated and non-phosphorylated forms. In the presence of a high ratio of ATP to ADP, the non-phosphorylated enzyme had a higher affinity for its substrates, oxaloacetate and phosphoenolpyruvate. The activity of the non-phosphorylated form was up to 6-fold higher when measured at low substrate concentrations. Comparison of proteoloytically cleaved PEPCK from Guinea grass, which lacked its N-terminal extension, from yeast (Saccharomyces cerevisiae), which does not possess an N-terminal extension, and from the C4 plant Urochloa panicoides, which possesses an N-terminal extension but is not subject to phosphorylation, revealed similar properties to the non-phosphorylated full-length form from Guinea grass. Assay of PEPCK activity in crude extracts of Guinea grass leaves, showed a large difference between illuminated and darkened leaves when measured in a selective assay (a low concentration of phosphoenolpyruvate and a high ratio of ATP to ADP), but there was no difference under assay conditions used to estimate maximum activity. Immunoblots of sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels showed no difference in the abundance of PEPCK protein in illuminated and darkened leaves. There were no light/dark differences in activity detected in maize (Zea mays) leaves, in which PEPCK is not subject to phosphorylation.

Using immunohistochemistry to study plant metabolism: the examples of its use in the localization of amino acids in plant tissues, and of phosphoenolpyruvate carboxykinase and its possible role in pH regulation

To understand many aspects of the metabolism of complex plant structures such as leaves, fruit and roots it is important to understand how metabolic processes are compartmentalized between tissues. The aim of this article is to show how immunohistochemistry, in conjunction with biochemical and physiological studies, is useful in understanding both the function of an enzyme in a tissue and metabolic processes occurring in plant tissues. This is illustrated by two examples. Firstly, the use of immunohistochemisty in the localization of amino acids in plant tissues is described. Secondly, the use of immunohistochemistry in understanding the function of an enzyme in a tissue and the metabolic processes occurring within the tissue is described. To illustrate this the example of phosophoenolpyruvate carboxykinase (PEPCK), an enzyme which is present in many plant tissues in which its function is unknown, is used. Evidence is provided that PEPCK may play a role in pH regulation in tissues active in the metabolism of nitrogen.

Changes in photosynthetic carbon flow in transgenic rice plants that express C4-type phosphoenolpyruvate carboxykinase from Urochloa panicoides

A cDNA encoding phosphoenolpyruvate carboxykinase (PCK) of Urochloa panicoides (a PCK-type C4 plant) was expressed in rice (Oryza sativa cv Tsukinohikari) plants under the control of the promoter of a maize (Zea mays) gene for phosphoenolpyruvate carboxylase or pyruvate, orthophosphate dikinase with the transit peptide of the small subunit of Rubisco. Crude extracts prepared from the green leaves of transgenic plants had high PCK activity and the newly expressed PCK was localized in chloroplasts. In labeling experiments with (14)CO(2) up to 20% of the radioactivity was incorporated into 4C compounds (malate, oxaloacetate, and aspartate) in excised leaves of transgenic plants, as compared with about 1% in excised leaves of control plants. There was a positive correlation between PCK activity and the extent of labeling of 4C compounds. When L-[4-(14)C]malate was fed to excised leaves the extent of incorporation of radioactivity into sucrose was 3-fold greater in transgenic plants than in control plants and the level of radiolabeled aspartate was significantly lower in transgenic plants. These results indicate that the ectopic expression of PCK in rice chloroplasts was able partially to change the carbon flow in mesophyll cells into a C4-like photosynthetic pathway. Such a strategy appears to provide a possible method for enhancing the photosynthetic capacity of C3 plants.

Phosphoenolpyruvate carboxykinase in the C(4) monocot Urochloa panicoides is encoded by four differentially expressed genes

Previous screening of a cDNA library of leaf poly(A(+)) RNA from Urochloa panicoides, a phosphoenolpyruvate carboxykinase (PCK)-type C(4) monocot, led to the characterization of cDNAs encoding the U. panicoides PCK subunit PCK1. A second PCK sequence, designated PCK2, has now been found by rescreening the library. The deduced PCK2 polypeptide is 626 residues in length, has a predicted molecular mass of 68,686 D, and is 96% identical to the deduced PCK1 sequence. Isolation and characterization of genomic DNA fragments revealed that the PCK1 and PCK2 genes are each closely linked to another PCK gene. These additional genes have been designated PCK3 and PCK4, respectively. In each case, the second gene is located upstream and in the same transcriptional orientation as the gene characterized through cDNA analysis. A reverse transcription-polymerase chain reaction assay was used to demonstrate that PCK1 and PCK2 transcripts predominate in leaves, whereas PCK3 and PCK4 transcripts predominate in roots. Moreover, accumulation of PCK1 and PCK2 transcripts is light dependent. Direct N-terminal sequencing of PCK polypeptides purified from leaves demonstrated that PCK2 is produced. These results strongly suggest that PCK1 and PCK2 are involved in the photosynthetic CO(2)-concentrating mechanism active in U. panicoides.

Phosphoenolpyruvate carboxykinase is involved in the decarboxylation of aspartate in the bundle sheath of maize

We recently showed that maize (Zea mays L.) leaves contain appreciable amounts of phosphoenolpyruvate carboxykinase (PEPCK; R.P. Walker, R.M. Acheson, L.I. Tecsi, R.C. Leegood [1997] Aust J Plant Physiol 24: 459-468). In the present study, we investigated the role of PEPCK in C4 photosynthesis in maize. PEPCK activity and protein were enriched in extracts from bundle-sheath (BS) strands compared with whole-leaf extracts. Decarboxylation of [4-14C]aspartate (Asp) by BS strands was dependent on the presence of 2-oxoglutarate and Mn2+, was stimulated by ATP, was inhibited by the PEPCK-specific inhibitor 3-mercaptopicolinic acid, and was independent of illumination. The principal product of Asp metabolism was phosphoenolpyruvate, whereas pyruvate was a minor product. Decarboxylation of [4-14C]malate was stimulated severalfold by Asp and 3-phosphoglycerate, was only slightly reduced in the absence of Mn2+ or in the presence of 3-mercaptopicolinic acid, and was light dependent. Our data show that decarboxylation of Asp and malate in BS cells of maize occurs via two different pathways: Whereas malate is mainly decarboxylated by NADP-malic enzyme, decarboxylation of Asp is dependent on the activity of PEPCK.

Phosphorylation of phosphoenolpyruvate carboxykinase in plants. Studies in plants with C4 photosynthesis and Crassulacean acid metabolism and in germinating seeds

We have previously shown that phosphoenolpyruvate carboxykinase (PEPCK) is phosphorylated in vivo in the cotyledons of darkened cucumber seedlings and that phosphorylation is reversed by light [Walker and Leegood (1995) FEBS Lett. 362, 70-74]. In this study the molecular mass of PEPCK was estimated in a range of gluconeogenic seedlings and in leaves of C4 plants and plants with Crassulacean acid metabolism (CAM). Phosphorylation of PEPCK was studied in these plants by feeding tissues with [32P]Pi and assessing phosphorylation by SDS/PAGE and autoradiography of either total proteins or of immunoprecipitated protein. In gluconeogenic seedlings and most CAM plants PEPCK had a molecular mass of 74 kDa, whereas in C4 grasses the molecular mass of PEPCK was always smaller and varied from 67-71 kDa. In all gluconeogenic seedlings and leaves of CAM plants PEPCK was phosphorylated, but it was not phosphorylated in all species of C4 grasses studied. In CAM plants, phosphorylation of PEPCK occurred at night and dephosphorylation occurred during the day. In C4 grasses phosphorylation occurred when leaves were darkened and the enzyme was dephosphorylated following illumination, but it was only phosphorylated in those plants with larger (71 kDa) molecular mass forms of PEPCK.

THE ORGANIZATION AND REGULATION OF PLANT GLYCOLYSIS

This review discusses the organization and regulation of the glycolytic pathway in plants and compares and contrasts plant and nonplant glycolysis. Plant glycolysis exists both in the cytosol and plastid, and the parallel reactions are catalyzed by distinct nuclear-encoded isozymes. Cytosolic glycolysis is a complex network containing alternative enzymatic reactions. Two alternate cytosolic reactions enhance the pathway's ATP yield through the use of pyrophosphate in place of ATP. The cytosolic glycolytic network may provide an essential metabolic flexibility that facilitates plant development and acclimation to environmental stress. The regulation of plant glycolytic flux is assessed, with a focus on the fine control of enzymes involved in the metabolism of fructose-6-phosphate and phosphoenolpyruvate. Plant and nonplant glycolysis are regulated from the "bottom up" and "top down," respectively. Research on tissue- and developmental-specific isozymes of plant glycolytic enzymes is summarized. Potential pitfalls associated with studies of glycolytic enzymes are considered. Some glycolytic enzymes may be multifunctional proteins involved in processes other than carbohydrate metabolism.

PLANT PROTEIN PHOSPHATASES

Posttranslational modification of proteins by phosphorylation is a universal mechanism for regulating diverse biological functions. Recognition that many cellular proteins are reversibly phosphorylated in response to external stimuli or intracellular signals has generated an ongoing interest in identifying and characterizing plant protein kinases and protein phosphatases that modulate the phosphorylation status of proteins. This review discusses recent advances in our understanding of the structure, regulation, and function of plant protein phosphatases. Three major classes of enzymes have been reported in plants that are homologues of the mammalian type-1, -2A, and -2C protein serine/threonine phosphatases. Molecular genetic and biochemical studies reveal a role for some of these enzymes in signal transduction, cell cycle progression, and hormonal regulation. Studies also point to the presence of additional phosphatases in plants that are unrelated to these major classes.