Inhibition of phosphoenolpyruvate carboxylase by malate

Loss of LaMATE impairs isoflavonoid release from cluster roots of phosphorus-deficient white lupin

White lupin (Lupinus albus L.) forms brush-like root structures called cluster roots under phosphorus-deficient conditions. Clusters secrete citrate and other organic compounds to mobilize sparingly soluble soil phosphates. In the context of aluminum toxicity tolerance mechanisms in other species, citrate is released via a subgroup of MATE/DTX proteins (multidrug and toxic compound extrusion/detoxification). White lupin contains 56 MATE/DTX genes. Many of these are closely related to gene orthologs with known substrates in other species. LaMATE is a marker gene for functional, mature clusters and is, together with its close homolog LaMATE3, a candidate for the citrate release. Both were highest expressed in mature clusters and when expressed in oocytes, induced inward-rectifying currents that were likely carried by endogenous channels. No citrate efflux was associated with LaMATE and LaMATE3 expression in oocytes. Furthermore, citrate secretion was largely unaffected in P-deficient composite mutant plants with genome-edited or RNAi-silenced LaMATE in roots. Moderately lower concentrations of citrate and malate in the root tissue and consequently less organic acid anion secretion and lower malate in the xylem sap were identified. Interestingly, however, less genistein was consistently found in mutant exudates, opening the possibility that LaMATE is involved in isoflavonoid release.

The coordination of major events in C4 photosynthesis evolution in the genus Flaveria

C4 photosynthesis is a remarkable complex trait, elucidations of the evolutionary trajectory of C4 photosynthesis from its ancestral C3 pathway can help us better understand the generic principles of the evolution of complex traits and guide the engineering of C3 crops for higher yields. Here, we used the genus Flaveria that contains C3, C3-C4, C4-like and C4 species as a system to study the evolution of C4 photosynthesis. We first mapped transcript abundance, protein sequence and morphological features onto the phylogenetic tree of the genus Flaveria, and calculated the evolutionary correlation of different features; we then predicted the relative changes of ancestral nodes of those features to illustrate the major events during the evolution of C4 photosynthesis. We found that gene expression and protein sequence showed consistent modification patterns in the phylogenetic tree. High correlation coefficients ranging from 0.46 to 0.9 among gene expression, protein sequence and morphology were observed. The greatest modification of those different features consistently occurred at the transition between C3-C4 species and C4-like species. Our results show highly coordinated changes in gene expression, protein sequence and morphological features, which support evolutionary major events during the evolution of C4 metabolism.

Metabolic flux from the chloroplast provides signals controlling photosynthetic acclimation to cold in Arabidopsis thaliana

Photosynthesis is especially sensitive to environmental conditions, and the composition of the photosynthetic apparatus can be modulated in response to environmental change, a process termed photosynthetic acclimation. Previously, we identified a role for a cytosolic fumarase, FUM2 in acclimation to low temperature in Arabidopsis thaliana. Mutant lines lacking FUM2 were unable to acclimate their photosynthetic apparatus to cold. Here, using gas exchange measurements and metabolite assays of acclimating and non-acclimating plants, we show that acclimation to low temperature results in a change in the distribution of photosynthetically fixed carbon to different storage pools during the day. Proteomic analysis of wild-type Col-0 Arabidopsis and of a fum2 mutant, which was unable to acclimate to cold, indicates that extensive changes occurring in response to cold are affected in the mutant. Metabolic and proteomic data were used to parameterize metabolic models. Using an approach called flux sampling, we show how the relative export of triose phosphate and 3-phosphoglycerate provides a signal of the chloroplast redox state that could underlie photosynthetic acclimation to cold.

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.

A single serine to alanine substitution decreases bicarbonate affinity of phosphoenolpyruvate carboxylase in C4Flaveria trinervia

Phosphoenolpyruvate (PEP) carboxylase (PEPc) catalyzes the first committed step of C4 photosynthesis generating oxaloacetate from bicarbonate (HCO3-) and PEP. It is hypothesized that PEPc affinity for HCO3- has undergone selective pressure for a lower KHCO3 (Km for HCO3-) to increase the carbon flux entering the C4 cycle, particularly during conditions that limit CO2 availability. However, the decrease in KHCO3 has been hypothesized to cause an unavoidable increase in KPEP (Km for PEP). Therefore, the amino acid residue S774 in the C4 enzyme, which has been shown to increase KPEP, should lead to a decrease in KHCO3. Several studies reported the effect S774 has on KPEP; however, the influence of this amino acid substitution on KHCO3 has not been tested. To test these hypotheses, membrane-inlet mass spectrometry (MIMS) was used to measure the KHCO3 of the photosynthetic PEPc from the C4Flaveria trinervia and the non-photosynthetic PEPc from the C3F. pringlei. The cDNAs for these enzymes were overexpressed and purified from the PEPc-less PCR1 Escherichia coli strain. Our work in comparison with previous reports suggests that KHCO3 and KPEP are linked by specific amino acids, such as S774; however, these kinetic parameters respond differently to the tested allosteric regulators, malate and glucose-6-phosphate.

Heterologous Expression of Key C and N Metabolic Enzymes Improves Re-assimilation of Photorespired CO2 and NH3, and Growth

We investigated the effect of the heterologous expression of phosphoenolpyruvate carboxylase (ZmPepcase), aspartate aminotransferase (GmAspAT), and glutamine synthetase (NtGS) on carbon (C) and nitrogen (N) metabolism in Arabidopsis (Arabidopsis thaliana). These transgenes were expressed either separately or in different combinations. The highest gains in shoot dry weight were observed in transgenic lines coexpressing all three genes. Tracer experiments using NaH14CO3 suggested that the coexpression of ZmPepcase, GmAspAT, and NtGS resulted in a higher flux of assimilated CO2 toward sugars and amino acids. Upon feeding the leaf discs with glycine-1-14C, transgenic lines evolved significantly lower 14CO2 levels than the wild type, suggesting that a higher reassimilation of CO2 evolved during photorespiration. Leaves of transgenic plants accumulated significantly lower ammonium without any significant difference in the levels of photorespiratory ammonium relative to the wild type, suggesting a higher reassimilation of photorespired NH3 Transgenic lines also showed improved photosynthetic rates, higher shoot biomass accumulation, and improved seed yield in comparison with wild-type plants under both optimum and limiting N conditions. This work demonstrates that the heterologous coexpression of ZmPepcase, GmAspAT, and NtGS reduced the photorespiratory loss of C and N with concomitant enhancements in shoot biomass and seed yield.

Evidence That Isoprene Emission Is Not Limited by Cytosolic Metabolites. Exogenous Malate Does Not Invert the Reverse Sensitivity of Isoprene Emission to High [CO2]

Isoprene is synthesized via the chloroplastic 2-C-methyl-d-erythritol 4-phosphate/1-deoxy-d-xylulose 5-phosphate pathway (MEP/DOXP), and its synthesis is directly related to photosynthesis, except under high CO₂ concentration, when the rate of photosynthesis increases but isoprene emission decreases. Suppression of MEP/DOXP pathway activity by high CO₂ has been explained either by limited supply of the cytosolic substrate precursor, phosphoenolpyruvate (PEP), into chloroplast as the result of enhanced activity of cytosolic PEP carboxylase or by limited supply of energetic and reductive equivalents. We tested the PEP-limitation hypotheses by feeding leaves with the PEP carboxylase competitive inhibitors malate and diethyl oxalacetate (DOA) in the strong isoprene emitter hybrid aspen (Populus tremula × Populus tremuloides). Malate feeding resulted in the inhibition of net assimilation, photosynthetic electron transport, and isoprene emission rates, but DOA feeding did not affect any of these processes except at very high application concentrations. Both malate and DOA did not alter the sensitivity of isoprene emission to high CO₂ concentration. Malate inhibition of isoprene emission was associated with enhanced chloroplastic reductive status that suppressed light reactions of photosynthesis, ultimately leading to reduced isoprene substrate dimethylallyl diphosphate pool size. Additional experiments with altered oxygen concentrations in conditions of feedback-limited and non-feedback-limited photosynthesis further indicated that changes in isoprene emission rate in control and malate-inhibited leaves were associated with changes in the share of ATP and reductive equivalent supply for isoprene synthesis. The results of this study collectively indicate that malate importantly controls the chloroplast reductive status and, thereby, affects isoprene emission, but they do not support the hypothesis that cytosolic metabolite availability alters the response of isoprene emission to changes in atmospheric composition.

A cysteine in the C-terminal region of alanyl-tRNA synthetase is important for aminoacylation activity

Alanyl-tRNA synthetase (AlaRS) from Escherichia coli is a multimeric enzyme that catalyzes the esterification of alanine to tRNA(Ala) in the ATP-dependent aminoacylation reaction. The functional binding of all three substrates follows Michaelis-Menten kinetics. The role of cysteines in this enzyme has been evaluated via modification of these residues with p-(hydroxymercuri)phenylsulfonic acid, monobromobimane, and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). The former two reagents induce nearly complete inactivation of AlaRS aminoacylation activity and the release of all tightly bound zinc. In the case of mild DTNB treatment, only two of the six cysteines in AlaRS are modified, with release of all zinc and partial loss of aminoacylation activity. These experiments indicate the importance of one or more cysteines, other than those thought to be coordinated with zinc, in the aminoacylation reaction. Substitution of each of the cysteine residues outside the zinc-binding motif with serine does not disrupt zinc binding. However, the cysteine most removed in primary sequence from the active site (Cys665) is identified as important in the aminoacylation step. Mutation of Cys665 to serine induces a 120-fold decrease in the catalytic efficiency of this enzyme, primarily through a kcat effect, and introduces sigmoidal kinetics (nH = 1.8) with respect to the RNA substrate. The results demonstrate that a simple manipulation in the C-terminal region can introduce positive cooperativity in this otherwise noncooperative enzyme.

Oligomerization and the Affinity of Maize Phosphoenolpyruvate Carboxylase for Its Substrate

When two different forms of phosphoenolpyruvate carboxylase (PEPC) from maize (Zea mays L.) leaves are present in an assay it is possible to estimate the ratio of Vmax to Km (V/K) for the two forms separately. This measure of the binding of the substrate by the enzyme permits evaluation of the effects of various treatments on the relative substrate-binding velocity of the enzyme. PEPC diluted 1/20 is present in a mixture of a tetrameric form with a high affinity for phosphoenolpyruvate and a dimeric form with a low affinity (M.-X. Wu, C.R. Meyer, K.O. Willeford, R.T. Wedding [1990] Arch Biochem Biophys 281: 324-329). Malate at 5 mM reduced (V/K)1,[mdash]the V/K of the probable tetrameric form[mdash]almost to zero, but reduced (V/K)2[mdash]the V/K of the probable dimer[mdash]by only about 80%. Glucose-6-phosphate (Glc-6-P) at 5 mM increased (V/K)1 to 155% of the control but had no effect on (V/K)2. Glycerol (20%) alone increased both V/Ks, and its effects are additive to the Glc-6-P effects, implying different mechanisms for activation by Glc-6-P and glycerol.

Regulatory protein phosphorylation of phosphoenolpyruvate carboxylase in the facultative crassulacean-acid-metabolism plant Mesembryanthemum crystallinum L

Phosphoenolpyruvate PyrP carboxylase (PyrPC) and PyrPC kinase were copurified from dark-adapted leaves of the common ice plant Mesembryanthemum crystallinum L. with crassulacean-acid metabolism (CAM). Purification by (NH4)2SO4 fractionation, chromatography on Fractogel-DEAE and hydroxylapatite resulted in a PyrPC preparation with a specific activity of 23-25 U/mg protein and a protein kinase activity of 255 mumol Pi.mol-1 PyrPC.s-1. After in vitro phosphorylation, the most prominently phosphorylated polypeptide was identified as PyrPC by immunoblotting and sequencing. Phosphorylation of PyrPC in vitro by incubation with 400 microM MgATP decreased its sensitivity towards malate. When purified in the absence of the protease inhibitor chymostatin, PyrPC lost an N-terminal sequence of 128 amino acids. Although the carboxylation reaction was unaffected, the truncated PyrPC could neither be phosphorylated in vitro nor inhibited by malate. This result and data obtained by limited proteolysis concur with the hypothesis [Jiao, J.A. & Chollet, R. (1989) Arch. Biochem. Biophys. 283, 300-305] that Ser11 is the phosphorylation site of the CAM PyrPC of M. crystallinum. At pH 7.0, the Km for ATP of the protein kinase was 25 microM; phosphorylation of PyrPC was maximal after 30 min at pH 7.0. The kinase showed also activity with histone III-S but not with dephosphorylated casein. It was inhibited by malate. The results show, that reversible protein phosphorylation is an important factor in the regulation of PyrPC in the facultative CAM plant M. crystallinum, similar to C4 and constitutive CAM plants.

Inactivation of maize leaf phosphoenolpyruvate carboxylase by the binding to chloroplast membranes

Phosphoenolpyruvate carboxylase (PEPC) purified from maize (Zea mays L.) leaves associates with maize leaf chloroplast membrane in vitro. The binding of PEPC to the membrane results in enzyme inactivation. A protein isolated from a maize leaf chloroplast membrane preparation inactivated PEPC. Treatment with membrane preparation or with partially purified inactivating protein accelerates PEPC inactivation at low temperature (4 degrees C). Interaction of PEPC with chloroplast membrane or inactivating protein may inactivate the enzyme by influencing dissociation of the enzyme active tetramer.

Oligomerization and regulation of higher plant phosphoenolpyruvate carboxylase

The specific activity of phosphoenolpyruvate (PEP) measured at a saturating level of substrate diminishes as the enzyme is diluted at about the same rate that specific light scattering by the diluted enzyme decreases. The presence of PEP in the assay causes an increase in activity with increasing dilution. This is accompanied by an increase in light scattering of the diluted enzyme. The reverse situation obtains with the addition of malate to assays: the activity decreases with increasing dilution but light scattering is not substantially changed, indicating that the enzyme is already brought to a smaller aggregate by the dilution itself. In this case, the inhibition by malate in the assay probably is the noncompetitive type not involved in regulatory control by malate. Glucose-6-phosphate in the range from 1 to 6 millimolar causes an increase in activity of the enzyme run at a substrate level less than K(m), and an associated increase in light scattering is found, indicating an increase in the mean size of the enzyme. When PEP is added to a 1/80 diluted enzyme, light scattering increases and is associated with a more rapid activity of the enzyme. When malate is added to the same cuvette, the activity decreases and the light scattering diminishes, thus showing that the ligand response is immediately reversible. When malate is added first, followed by PEP, the reverse sequence of activity and light scattering change is observed.

Quantitation of Rates of Transport, Metabolic Fluxes, and Cytoplasmic Levels of Inorganic Carbon in Maize Root Tips during K Ion Uptake

Our aim was to determine whether fixation of inorganic carbon (C(i)), due to phosphoenolpyruvate carboxylase activity, is limited by the availability of C(i) in the cytoplasm of maize (Zea mays L.) root tips. Rates of C(i) uptake and metabolism were measured during K(2)SO(4) treatment, which stimulates dark C(i) fixation. (13)C(i) uptake was followed by (13)C-nuclear magnetic resonance (NMR); 5 millimolar K(2)SO(4) had no significant effect on (13)C(i) influx. The contribution of respiratory CO(2) production to cytoplasmic HCO(3) (-) was measured using in vivo(13)C-NMR and (1)H-NMR of cell extracts; K(2)SO(4) treatment had no effect on respiratory CO(2) production. The concentration of cytoplasmic HCO(3) (-) was estimated to be approximately 11 millimolar, again with K(2)SO(4) having no significant effect. These experiments allowed us to determine the extent to which extracellularly supplied (14)C(i) was diluted in the cytoplasm by respiratory CO(2) and thereby measure phosphoenolpyruvate (PEP) carboxylase activity in vivo using (14)C(i). PEP carboxylase activity in root tips was enhanced approximately 70% over controls within 12 minutes of the addition of 5 millimolar K(2)SO(4). The activity of carbonic anhydrase, which provides PEP carboxylase with C(i), was determined by saturation transfer (13)C-NMR to be more than 200 times that of PEP carboxylase in vivo. The regulation of PEP carboxylase in K(2)SO(4)-treated roots is discussed.

Regulation of Crassula argentea phosphoenolpyruvate carboxylase in relation to temperature

The effect of temperature on the kinetic parameters of phosphoenolpyruvate carboxylase purified from Crassula argentea was such that both the Vmax and Km(MgPEP) values tended upward over the range from 11 to 35 degrees C. The increased rate at low temperatures due to the low Km is at least partially offset by the increased Vmax at higher temperatures, potentially leading to a broad plateau of enzyme activity and a relatively small effect of temperature on the enzyme. The cooperativity was negative at 11 degrees C, but above 15 degrees C it became positive. The presence of 5 mM glucose-6-phosphate has relatively little effect on Vmax but it clearly reduces Km and overcomes any effect of temperature on this parameter in the range studied. Positive cooperativity is observed only at temperatures above 25 degrees C. The size of the native enzyme, as determined by dynamic light scattering, was strongly toward the tetrameric form. At a temperature of 40 degrees C and above, a considerable oligomerization takes place. No loss of activity can be observed in this range of temperature. In the presence of either glucose-6-phosphate or magnesium phosphoenolpyruvate, at temperatures under 25 degrees C, the equilibrium is displaced toward higher levels of aggregation. Maximal accumulation of lead malate occurred at 10 to 12 degrees C in vivo with reduction to about 25% at 35 degrees C. Glucose-6-phosphate followed a similar curve in response to temperature, but the overall difference was about 50%. The sum of phosphoenolpyruvate plus pyruvate is level at night temperatures below 25 degrees C, doubling at 35 degrees C. Calculated concentrations of malate, glucose-6-phosphate, and phosphoenolpyruvate plus pyruvate indicate that the concentrations present are equal to or greater than Ki, Ka, and Km values for these metabolites, respectively.

Role of cysteine in activation and allosteric regulation of maize phosphoenolpyruvate carboxylase

The effect of 5-5'-dithiobis-2-nitrobenzoate (DTNB) on the kinetic parameters and structure of phosphoenolpyruvate carboxylase purified from maize (Zea mays L.) has been studied. The V(max) is found to be independent of the presence of this thiol reagent. The K(m) is increased upon oxidation of cysteines by DTNB. At a substrate concentration higher than K(m) (3.1 millimolar Mgphosphoenolpyruvate), a significant reversible decrease of the activity is observed. Malate has little effect in preventing the modification of these cysteines. The V type inhibition by malate was also studied at a saturating phosphoenolpyruvate level (9.3 millimolar Mgphosphoenolpyruvate). In the presence of 50 micromolar DTNB, up to 60% inhibition is caused by 15 millimolar malate; however, in the presence of both 50 micromolar DTNB and 50 millimolar dithiothreitol (DTT) this inhibition is reduced to 20%. The presence of DTT alone increases the size of the phosphoenolpyruvate carboxylase molecule as determined by light scattering. The activity at nonsaturating substrate concentration is increased by 36% in the presence of DTT. The oligomerization equilibrium between the dimer and the tetrameric form of the enzyme is affected by cysteine. The K(m) for the substrate, the sensitivity toward malate, and the size of the enzyme are found to be modified upon incubation in the presence of DTT.

Fluorescence Study of Chemical Modification of Phosphoenolpyruvate Carboxylase from Crassula argentea

The chemical modification of phosphoenolpyruvate carboxylase purified from Crassula argentea leaves was studied using the fluorescence of the extrinsic probe 8-anilino-1-naphalenesulfonate. The effects of ligands on kinetic parameters of phosphoenolpyruvate carboxylase activity, and its response to pH and metal cations, were associated with the binding of the ligands to the enzyme as measured by fluorescence. Binding of the ligands phosphoenolpyruvate, malate, and glucose-6-phosphate revealed by fluorescence measurements corresponds to competitive phenomena observed in kinetic studies. The fluorescence measurements also suggest the involvement of specific amino acids in the binding of a given ligand. Arginyl residues modified by 2,3-butanedione appear to be directly involved in the binding of phosphoenolpyruvate and malate to the active and the inhibition sites, respectively. A histidyl residue was involved in the binding of malate, accounting for the lack of inhibition by malate in kinetic studies of the enzyme treated with diethylpyrocarbonate. Although activity was lost, there was no decrease in the ability of the treated enzyme to bind phosphoenolpyruvate, suggesting that additional histidyl residues are essential for activity although not directly involved in the binding of phosphoenolpyruvate. The lysine reagent trinitrobenzenesulfonate caused a loss of activity and a reduction in malate inhibition and glucose-6-phosphate activation, but these modifications were not related to changes in the ability of the enzyme to bind any of the three ligands. This suggests that lysine residues were not directly involved in the binding of these ligands.

Regulation of Phosphoenolpyruvate carboxylase from Crassula argentea: effect of incubation with ligands and dilution on oligomeric state, activity, and allosteric properties

The relationship between the aggregation state and allosteric properties of purified phosphoenolpyruvate carboxylase from Crassula argentea was examined using both kinetic and physical techniques. Analysis by native polyacrylamide gel electrophoresis showed that dilution induced a dissociation of the active tetramer to a less active dimer. Kinetic assays showed that inhibition of phosphoenolpyruvate carboxylase by 5 mM malate measured at a saturating phosphoenolpyruvate concentration rose to nearly 80% with increasing preassay dilution while the activity in the absence of malate remained constant. Kinetic bursts were observed when enzyme-initiated assays were measured at a subsaturating phosphoenolpyruvate concentration. At saturating phosphoenolpyruvate concentrations, however, increasing lags developed in response to increasing the preassay dilution of the enzyme. Further, dynamic laser-light scattering measurements showed that preincubation of the dilute enzyme with phosphoenolpyruvate stabilized the tetramer while the presence of malate induced dimer formation. These observations confirm and extend earlier work with the extracted active malate insensitive night and less active, malate-sensitive day forms of the enzyme (Wu and Wedding [1985] Plant Physiol. 77, 667-675). Activity measured at subsaturating phosphoenolpyruvate concentrations dropped with increasing preassay dilution of enzyme, while activation by 3.2 mM glucose 6-phosphate, assayed at a low phosphoenolpyruvate concentration (0.044 mM), increased with dilution to nearly 400%. In this case activation results from a decrease in the control rate as the activity measured in the presence of glucose 6-phosphate was nearly constant, similar in effect to saturating phosphoenolpyruvate in the assay. Glucose 6-phosphate induced tetramer formation of the dilute enzyme as measured by light-scattering similar to the effects induced by PEP. In addition, when diluted (dimeric) PEPC was preincubated with PEP or glucose 6-phosphate the enzyme became less sensitive to malate inhibition, while the active-site directed ligand 2-phosphoglycolate had no effect on malate inhibition. These results indicate that both the substrate PEP and the activator glucose 6-phosphate stabilize the active tetramer via binding and interaction at an activator site separate from the active site.

Posttranslational regulation of phosphoenolpyruvate carboxylase in c(4) and crassulacean Acid metabolism plants

Control of C(4) photosynthesis and Crassulacean acid metabolism (CAM) is, in part, mediated by the diel regulation of phosphoenolpyruvate carboxylase (PEPC) activity. The nature of this regulation of PEPC in the leaf cell cytoplasm of C(4) and CAM plants is both metabolite-related and posttranslational. Specificially, the regulatory properties of the enzyme vary in accord with the physiological activity of C(4) photosynthesis and CAM: PEPC is less sensitive to feedback inhibition by l-malate under light (C(4) plants) or at night (CAM plants) than in darkness (C(4)) or during the day (CAM). While the view that a light-induced change in the aggregation state of the holoenzyme is a general mechanism for the diel regulation of PEPC activity in CAM plants is currently in dispute, there is no supportive in vivo evidence for such a tetramer/dimer interconversion in C(4) plants. In contrast, a wealth of in vitro and in vivo data has accumulated in support of the view that the reversible phosphorylation of a specific, N-terminal regulatory serine residue in PEPC (e.g. Ser-15 or Ser-8 in the maize or sorghum enzymes, respectively) plays a key, if not cardinal, role in the posttranslational regulation of the carboxylase by light/dark or day/night transitions in both C(4) and CAM plants, respectively.

Cytoplasmic malate levels in maize root tips during K+ ion uptake determined by 13C-NMR spectroscopy

13C-NMR spectroscopy was used to determine the level of cytoplasmic malate in maize root tips that exhibited different rates of malate synthesis. Intracellular malate was 13C-labeled at carbons 1 and 4 by perfusing root tips with 5 nM H13CO3-. This labeling reflects the activities of phosphoenolpyruvate carboxylase and malate dehydrogenase (production of [4-13C]malate), and fumarase (scrambling of 13C-label between C1 and C4 of malate). In vivo 13C-NMR spectra contained a clearly resolved resonance from cytoplasmic [4-13C]malate, while the resonance from cytoplasmic [1-13C]malate overlapped with others. After 90 min of H13CO3- treatment, 13C-labeling of organic acid pools had reached steady-state. Thereafter, the ratios [13C]malate/[12C + 13C]malate and [1-13C]malate/[4-13C]malate in tissue extracts remained constant; evidence is presented that these ratios were the same for both cytoplasmic and total cellular malate. Hence, the intensity of the cytoplasmic [4-13C]malate signal was proportional to the amount of cytoplasmic malate in root tips. Potassium sulfate stimulate malate synthesis in maize root tips, relative to root tips perfused with HCO3- alone; total cellular malate doubled after approx. 1 h of 5 mM K2SO4-treatment. Cytoplasmic malate increased from approx. 3.5 mM to approx. 7.5 mM within 45 min of the onset of K2SO4-treatment, declining slightly thereafter. The possible effects of these changing cytoplasmic malate concentration on the enzymes involved in malate metabolism are discussed.

Effects of pH on inactivation of maize phosphoenolpyruvate carboxylase

Maize leaf phosphoenolpyruvate carboxylase (PEPC) is inactivated by incubation at pH's above neutrality. Both the amount and the rapidity of inactivation increase as the pH rises. The presence of phosphoenolpyruvate (PEP), malate, glucose 6-phosphate and dithiothreitol in the incubation medium give protection to the enzyme. While the presence of PEP during incubation at pH 8 prevents inactivation, the level of PEP in the assay after incubation has no effect on the relative inactivation. When the enzyme is incubated at pH 7 with 5 mM malate (a treatment known to cause dimerization) subsequent assay at saturating levels of MgPEP completely restores activity while assay at less than Km MgPEP produces greater than 99% inhibition of the same sample, showing that high PEP concentration has reconverted the PEPC to the malate-resistant tetramer. Thus the protective effect of PEP against inactivation at high pH probably is not related to its effect on the aggregation state of the enzyme but rather is due to the presence of PEP at the active site. Protection of PEPC at pH 8 by EDTA and its inactivation by low concentrations of Cu2- indicates that the loss of activity at high pH probably is in a sense an artifact resulting from the binding to a deprotinated cysteine of heavy metal ions contaminating the enzyme preparation or present in reagents. This suggests that caution should be used in the interpretation of experiments involving PEPC activity at alkaline pH's.

Metabolite regulation of partially purified soybean nodule phosphoenolpyruvate carboxylase

Phosphoenolpyruvate carboxylase (PEPC) was purified 40-fold from soybean (Glycine max L. Merr.) nodules to a specific activity of 5.2 units per milligram per protein and an estimated purity of 28%. Native and subunit molecular masses were determined to be 440 and 100 kilodaltons, respectively, indicating that the enzyme is a homotetramer. The response of enzyme activity to phosphoenolpyruvate (PEP) concentration and to various effectors was influenced by assay pH and glycerol addition to the assay. At pH 7 in the absence of glycerol, the K(m) (PEP) was about twofold greater than at pH 7 in the presence of glycerol or at pH 8. At pH 7 or pH 8 the K(m) (MgPEP) was found to be significantly lower than the respective K(m) (PEP) values. Glucose-6-phosphate, fructose-6-phosphate, glucose-1-phosphate, and dihydroxyacetone phosphate activated PEPC at pH 7 in the absence of glycerol, but had no effect under the other assay conditions. Malate, aspartate, glutamate, citrate, and 2-oxoglutarate were potent inhibitors of PEPC at pH 7 in the absence of glycerol, but their effectiveness was decreased by raising the pH to 8 and/or by adding glycerol. In contrast, 3-phosphoglycerate and 2-phosphoglycerate were less effective inhibitors at pH 7 in the absence of glycerol than under the other assay conditions. Inorganic phosphate (up to 20 millimolar) was an activator at pH 7 in the absence of glycerol but an inhibitor under the other assay conditions. The possible significance of metabolite regulation of PEPC is discussed in relation to the proposed functions of this enzyme in legume nodule metabolism.