Properties Of Phosphoenolpyruvate Carboxylase Research Articles

Regulatory Properties of Phosphoenolpyruvate Carboxylase in Crassulacean Acid Metabolism Plants: Diurnal Changes in Phosphorylation State and Regulation of Gene Expression

Regulatory properties of Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in three CAM species, Kalanchoë pinnata, K. daigremontiana and Ananas comosus (pineapple) were examined. PEPC activity in the leaves of the three CAM species exhibited diurnal changes peaking during the first 2-h of darkness in Kalanchoë species and at midnight in pineapple, and then decreasing drastically until dawn. The oscillations of PEPC activity were far greater in Kalanchoë species than in pineapple. In the presence of 2 mM malate, the activity of PEPC decreased in all three CAM species, but the sensitivity of PEPC to malate was markedly different between pineapple and the Kalanchoë species. The malate sensitivity was 2- to 3-times higher in pineapple than in the Kalanchoë species during the dark period, but it was almost the same during the light period. PEPC in the three CAM species was phosphorylated only during the dark period. PEPC proteins were highly phosphorylated during the first 2-h of darkness in Kalanchoë species and at midnight in pineapple, and then they decreased drastically during the latter part of darkness. CAM-specific isoforms of PEPC in the leaves of the three CAM species contained a highly conserved phosphorylation site of Ser-11 at the N-terminus. These PEPC isoforms displayed diurnal changes in transcript abundance, with the peak of transcripts occurring during the dark period. The day/night changes in PEPC transcript abundance were mirrored by changes in the PEPC protein and corresponding enzyme activity over the diurnal cycle. These findings suggest that the diurnal regulation in PEPC activity is determined by the amount of PEPC protein as well as the posttranslational control in these CAM species.

Modulation of phosphoenolpyruvate carboxylase in vivo by Ca 2+ in Amaranthus hypochondriacus, a NAD-ME type C 4 plant: Possible involvement of Ca 2+ in up-regulation of PEPC-protein kinase in vivo

The properties of phosphoenolpyruvate carboxylase (PEPC) were studied, with respect to calcium (Ca2+), in leaves of Amaranthus hypochondriacus, a C4 plant. Experiments were conducted in vitro (by adding Ca2+ during enzyme assay) or in vivo (by feeding Ca2+ to intact leaves through petiole). Inclusion of 10 microM Ca2+ during assay marginally increased (<30%) malate sensitivity of PEPC in extracts from dark-adapted leaves. The effect of Ca2+ was marginal on PEPC in extracts from illuminated leaves. Upon applying a low concentration of Ca2+ to leaves, the PEPC activity in leaves increased by 1.5-fold, while inhibition by malate decreased markedly. The light activation of PEPC in Ca2+-fed leaves was slightly higher than in the absence of Ca2+-ethyleneglycol-bis-(beta-aminoethyl ether) N,N,N',N'-tetra acetic acid (EGTA). To assess further the role of Ca2+, 5 mM EGTA (Ca2+ chelator) was either added during the enzyme assay or fed to leaves through petiole. EGTA had no effect on PEPC, when added during enzyme assay. Upon feeding EGTA, the PEPC activity in the dark-adapted leaf extracts increased by 30%, and the effect on malate sensitivity was marginal. However, there was a decrease in PEPC activity in illuminated extracts, resulting in a marked decrease in the extent of light activation of PEPC. The extent of phosphorylation of PEPC was much higher in Ca2+ or Ca2+-EGTA-fed leaves than in the control, but EGTA decreased the light-induced phosphorylation. Our results suggest that optimal alone concentration of Ca2+ is essential for PEPC in leaves of A. hypochondriacus, particularly in vivo. We suggest that Ca2+ regulates PEPC, at an upstream level, such as transcription, by modulating PEPC-protein kinase, thus facilitating the light activation of PEPC.

Modulation in vivo by Nitrate Salts of the Activity and Properties of Phosphoenolpyruvate Carboxylase in Leaves of Alternanthera pungens (C&lt;sub&gt;4&lt;/sub&gt; plant) and A. sessilis (C&lt;sub&gt;3&lt;/sub&gt; species)

Feeding K+ or Na+ nitrate salts in vivo enhanced the activity of phosphoenolpyruvate carboxylase (PEPC) in the leaf extracts of Alternanthera pungens (C4 plant) and A. sessilis (C3 species). The increase was more pronounced in A. pungens than in A. sessilis. Chloride salts increased the PEPC activity only marginally. However, the sulfate salts were either not effective or inhibitory. Feeding nitrate modulated the regulatory properties of PEPC in A. pungens, resulting in increased KI (malate) and decreased KA (glucose-6-P). The sensitivity of PEPC to malate, which gives a measure of phosphorylation status of the enzyme, indicated that feeding leaves with NO3- enhanced the phosphorylation status of the enzyme. The reduction in PEPC activity due to cycloheximide treatment suggested that increased synthesis of PEPC protein kinase may be one of the reasons for the enhancement in PEPC activity, after the nitrate feeding. We suggest that nitrate salts could be used as a tool to modulate and analyze the properties of PEPC in C3 and C4 plants.

Marked modulation by phosphate of phosphoenolpyruvate carboxylase in leaves of Amaranthus hypochondriacus, a NAD-ME type C4 plant: decrease in malate sensitivity but no change in the phosphorylation status.

The effect of Pi on the properties of phosphoenolpyruvate carboxylase (PEPC) from Amaranthus hypochondriacus, a NAD-ME type C4 plant, was studied in leaf extracts as well as with purified protein. Efforts were also made to modulate the Pi status of the leaf by feeding leaves with either Pi or mannose. Inclusion of 30 mM Pi during the assay enhanced the enzyme activity in leaf extracts or of purified protein by >2-fold. The effect of Pi on the enzyme purified from dark-adapted leaves was more pronounced than that from light-adapted ones. The Ki for malate increased >2.3-fold and >1.9-fold by Pi in the enzyme purified from dark-adapted leaves and light-adapted leaves, respectively. Pi also induced an almost 50-60% increase in Km for PEP or Ka for glucose-6-phosphate. Feeding the leaves with Pi also increased the activity of PEPC in leaf extracts, while decreasing the malate sensitivity of the enzyme. On the other hand, Pi sequestering by mannose marginally decreased the activity, while markedly suppressing the light activation, of PEPC. There was no change in phosphorylation of PEPC in leaves of A. hypochondriacus due to the feeding of 30 mM Pi. However, feeding with mannose decreased the light-enhanced phosphorylation of PEPC. The marked decrease in malate sensitivity of PEPC with no change in phosphorylation state indicates that the changes induced by Pi are independent of the phosphorylation of PEPC. It is suggested here that Pi is an important factor in regulating PEPC in vivo and could also be used as a tool to analyse the properties of PEPC.

Induction of a Crassulacean acid like metabolism in the C(4) succulent plant, Portulaca oleracea L.: physiological and morphological changes are accompanied by specific modifications in phosphoenolpyruvate carboxylase.

The induction of a Crassulacean acid like metabolism (CAM) was evidenced after 21-23 days of drought stress in the C(4) succulent plant Portulaca oleracea L. by changes in the CO(2) exchange pattern, in malic acid content and in titratable acidity during the day-night cycle. Light microscopy studies also revealed differences in the leaf structure after the drought treatment. Following the induction of the CAM-like metabolism, the regulatory properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), the enzyme responsible for the diurnal fixation of CO(2) in C(4) plants but nocturnal in CAM plants, were studied. The enzyme from stressed plants showed different kinetic properties with respect to controls, notably its lack of cooperativity, higher sensitivity to L-malate inhibition, higher PEP affinity and lower enzyme content on a protein basis. In both conditions, PEPC's subunit mass was 110 kDa, although changes in the isoelectric point and electrophoretic mobility of the native enzyme were observed. In vivo phosphorylation and native isoelectrofocusing studies indicated variations in the phosphorylation status of the enzyme of samples collected during the night and day, which was clearly different for the control and stressed groups of plants. The results presented suggest that PEPC activity and regulation are modified upon drought stress treatment in a way that allows P. oleracea to perform a CAM-like metabolism.

Molecular forms and kinetic properties of phosphoenolpyruvate carboxylase from barnyard grass (Echinochloa crus-galli(L.) Beauv.: Poaceae)

The thermal, kinetic, and electrophoretic properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were analyzed in plants from two ecotypes of barnyard grass (Echinochloa crus-galli (L.) Beauv.: Poaceae) originated from sites of contrasting climates in Quebec (QUE) and Mississippi (MISS). The thermostability, cold inactivation, the apparent energy of activation (Ea), the Michaelis-Menten constant (Km), and Vmax/Kmratios for phosphoenolpyruvate (PEP) and Mg2+were analyzed with desalted Sephadex G-25 crude PEPC extracts, with partially purified PEPC from the polyethylene glycol (PEG) 13% fraction and with purified PEPC obtained after elution from DEAE-Sepharose affinity chromatography. PEPC from illuminated leaves from both ecotypes consisted of one isomorph with the same electrophoretic mobility in polyacrylamide gels, similar molecular masses for the native enzyme (400 kDa) and for each subunit of the tetramer (100 kDa), and a same isoelectric point (pI) of 4.95. The only kinetic property for PEPC for which differences of physiological importance among ecotypes were observed at the three levels of purification was Kmfor PEP for which values for QUE plants were significantly lower at low assay temperatures. Differences among ecotypes for thermostability were only observed in assays with crude and partially purified PEPC extracts, while no differences were found for cold inactivation rates, Km(Mg2+) estimates at any level of purification or for Vmax/Kmratios (PEP or Mg2+) from purified PEPC. Significant differences among the two ecotypes were found for catalytic constant (Kcat) estimates obtained with purified PEPC. However, results show higher catalytic efficiency for PEPC from MISS plants at high assay temperatures but no indication of an improved catalytic efficiency for PEPC from QUE plants at low assay temperatures. The lack of ecotypic differences for most thermal and kinetic properties observed with purified PEPC casts doubts about the evolutionary interpretations of results obtained in previous kinetic comparative analyses, which were based on crude or partially purified enzymatic preparations of PEPC extracted from E. crus-galli plants.Key words: phosphoenolpyruvate carboxylase, enzyme kinetics, thermal adaptation, barnyard grass, electrophoresis, Echinochloa crus-galli.

Changes in Properties of Phosphoenolpyruvate Carboxylase with Induction of Crassulacean Acid Metabolism (CAM) in the C&lt;sub&gt;4&lt;/sub&gt; Plant Portulaca Oleracea

Aiming at understanding the odd case of CAM expression by a C4 plant, some properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31, orthophosphate: oxaloacetate carboxylyase, phosphorylating) were comparatively studied in leaves of CAM-expressing and non-expressing Portulaca oleracea L. plants. CAM expression was induced by growing plants under an 8-h photoperiod and under water-stress. CAM induction in leaves of these plants (designated as CAM) is indicated by the nocturnal acidification and by the clear diurnal oscillation pattern and amplitude of acidity, malic acid, and PEPC activity characteristic of CAM plants. Treatment of the other plant group (designated as C4) by growth under a 16-h photoperiod and well-watered conditions did not induce expression of the tested criteria of CAM in plants. In these C4 plants, the mentioned CAM criteria were undetectable. PEPC from CAM and C4Portulaca responded differently to any of the studied assay conditions or effectors. For example, extent and timing of sensitivity of PEPC to pH change, inhibition by malate, activation by glucose-6-phosphate or inorganic phosphate, and the enzyme affinity to the substrate PEP were reversed with induction of CAM from the C4-P. oleracea. These contrasting responses indicate distinct kinetic and regulatory properties of PEPC of the two modes. Thus by shifting to CAM in the C4Portulaca a new PEPC isoform may be synthesised to meet CAM requirements. Simultaneous occurrence of both C4 and CAM is suggested in P. oleracea when challenged with growth under stress.

Phosphoenolpyruvate Carboxylase Purified from Leaves of C&lt;sub&gt;3&lt;/sub&gt;, C&lt;sub&gt;4&lt;/sub&gt;, and C&lt;sub&gt;3&lt;/sub&gt;-C&lt;sub&gt;4&lt;/sub&gt; intermediate species of Alternanthera: Properties at Limiting and Saturating Bicarbonate

Phosphoenolpyruvate carboxylase (PEPC) was purified from leaves of four species of Alternanthera differing in their photosynthetic carbon metabolism: Alternanthera sessilis (C3), A. pungens (C4), A. ficoides and A. tenella (C3-C4 intermediates or C3-C4). The activity and properties of PEPC were examined at limiting (0.05 mM) or saturating (10 mM) bicarbonate concentrations. The Vmax as well as Km values (for Mg2+ or PEP) of PEPC from A. ficoides and A. tenella (C3-C4 intermediates) were in between those of C3 (A. sessilis) and C4 species (A. pungens). Similarly, the sensitivity of PEPC to malate (an inhibitor) or G-6-P (an activator) of A. ficoides and A. tenella (C3-C4) was also of intermediate status between those of C3 and C4 species of A. sessilis and A. pungens, respectively. In all the four species, the maximal activity (Vmax), affinity for PEP (Km), and the sensitivity to malate (KI) or G-6-P (KA) of PEPC were higher at 10 mM bicarbonate than at 0.05 mM bicarbonate. Again, the sensitivity to bicarbonate of PEPC from C3-C4 intermediates was in between those of C3- and C4-species. Thus the characteristics of PEPC of C3-C4 intermediate species of Alternanthera are intermediate between C3- and C4-type, in both their kinetic and regulatory properties. Bicarbonate could be an important modulator of PEPC, particularly in C4 plants.

Molecular forms and kinetic properties of phosphoenolpyruvate carboxylase from barnyard grass (&lt;i&gt;Echinochloa crus-galli&lt;/i&gt; (L.) Beauv.: Poaceae)

The thermal, kinetic, and electrophoretic properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were analyzed in plants from two ecotypes of barnyard grass (Echinochloa crus-galli (L.) Beauv.: Poaceae) originated from sites of contrasting climates in Quebec (QUE) and Mississippi (MISS). The thermostability, cold inactivation, the apparent energy of activation (Ea), the Michaelis-Menten constant (Km), and Vmax/Km ratios for phosphoenolpyruvate (PEP) and Mg2+ were analyzed with desalted Sephadex G-25 crude PEPC extracts, with partially purified PEPC from the polyethylene glycol (PEG) 13% fraction and with purified PEPC obtained after elution from DEAE-Sepharose affinity chromatography. PEPC from illuminated leaves from both ecotypes consisted of one isomorph with the same electrophoretic mobility in polyacrylamide gels, similar molecular masses for the native enzyme (400 kDa) and for each subunit of the tetramer (100 kDa), and a same isoelectric point (pI) of 4.95. The only kinetic property for ...

Phosphoenolpyruvate carboxylase of an ammonia-oxidizing chemoautotrophic bacterium, Nitrosomonas europaea ATCC 25978

The properties of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) of an obligate chemoautotrophic bacterium, Nitrosomonas europaea ATCC 25978 were studied. The pH and temperature optima of the PEPC activity were about 8.0–8.5 and 45–50°C, respectively. The PEPC was stable in the pH range 6.0–10.0 and up to 35°C. It required Mg 2+ (20 mM) or Mn 2+ (10 mM), and was activated by acetyl-CoA (0.16 mM). SDS (1 mM), pHMB (1 mM) and l-malate (30 mM) were found to be inhibitory against the PEPC activity, while l-aspartate, FBP and various nucleotides had no effect.

Re-Evaluation of the Effects of Glucose-6-Phosphate and Malate on the Catalytic Properties of Phosphoenolpyruvate Carboxylase From Cynodon dactylon Under Physiological Assay Conditions

Some catalytic and regulatory properties of the day- and night-form of phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Cynodon dactylon (L.) Pers. leaves were re-evaluated in the presence of high concentrations of glycerol in the assay medium. This cosolute, known to be preferentially excluded from the hydration sphere of some proteins, apparently promotes aggregation, through an increase of the enzyme concentration in a fraction of the total volume. In the presence of this cosolute in the assay medium: (a) glucose-6-phosphate does not activate the enzyme, (b) inhibition of the Vmax by malate is increased, particularly with the night-form of phosphoenolpyruvate carboxylase, whereas the day-form is more responsive at the Km levels, (c) the light activation, detected as an increase in the apparent affinity of the substrate phosphoenolpyruvate is maintained under the new assay conditions.

Analysis of the control of photosynthesis in C 4 plants by changes in light and carbon dioxide

Parallel measurements of contents of photosynthetic intermediates, activities of enzymes of photosynthetic carbon assimilation, gas-exchange rates and components of chlorophyll-fluorescence quenching in leaves of C 4 plants are considered in relation to changes in photon flux density (PFD) and CO 2 . The influence of varying light and CO 2 concentration upon changes in the amounts of phosphoenolpyruvate (PEP) in leaves of C 4 plants during steady-state photosynthesis are interpreted in terms of the regulatory properties of PEP carboxylase and in terms of feedback interactions between the Calvin cycle and the C 4 cycle. Relations between electron transport and carbon assimilation are discussed in terms of the regulation of the supply of ATP and NADPH and the demands of carbon assimilation. In low light these relations differ in C 3 and C 4 plants. The lag in photosynthetic carbon assimilation in maize that follows a decrease in PFD has been analysed. The changes that occur in enzyme activities, metabolites and components of chlorophyll-fluorescence quenching following the transition from high to low light indicate that diminished production of ATP and NADPH is responsible for the lag in photosynthetic carbon assimilation and may reflect a stimulation of cyclic electron flow to make up a deficit in ATP.

Regulation of Phosphoenolpyruvate Carboxylase Activity in Maize Leaves

The aim of this work was to investigate how light regulates the activity of phosphoenolpyruvate carboxylase in vivo in C(4) plants. The properties of phosphoenolpyruvate carboxylase were investigated in extracts which were rapidly prepared (in less than 30 seconds) from darkened and illuminated leaves of Zea mays. Illumination resulted in a significant decrease in the S(0.5)(phosphoenolpyruvate) but there was no change in V(max). The form of the enzyme from illuminated leaves was less sensitive to malate inhibition than was the form from darkened leaves. At low concentrations of phosphoenolpyruvate, the activity of the enzyme was strongly stimulated by glucose-6-phosphate, fructose-6-phosphate, triose-phosphate, alanine, serine, and glycine and was inhibited by organic acids. The enzyme was assayed in mixtures of metabolites at concentrations believed to be present in the mesophyll cytosol in the light and in the dark. It displayed low activity in a simulated ;dark' cytosol and high activity in a simulated ;light' cytosol, but activities were different for the enzyme from darkened compared to illuminated leaves.

Purification and Properties of Phosphoenolpyruvate Carboxylase from Immature Pods of Chickpea (Cicer arietinum L.)

Phosphoenolpyruvate carboxylase (EC 4.1.1.31) was purified to homogeneity with about 29% recovery from immature pods of chickpea using ammonium sulfate fractionation, DEAE-cellulose chromatography, and gel filtration through Sephadex G-200. The purified enzyme with molecular weight of about 200,000 daltons was a tetramer of four identical subunits and exhibited maximum activity at pH 8.1. Mg(2+) ions were specifically required for the enzyme activity. The enzyme showed typical hyperbolic kinetics with phosphoenolpyruvate with a K(m) of 0.74 millimolar, whereas sigmoidal response was observed with increasing concentrations of HCO(3) (-) with S(0.5) value as 7.6 millimolar. The enzyme was activated by inorganic phosphate and phosphate esters like glucose-6-phosphate, alpha-glycerophosphate, 3-phosphoglyceric acid, and fructose-1,6-bisphosphate, and inhibited by nucleotide triphosphates, organic acids, and divalent cations Ca(2+) and Mn(2+). Oxaloacetate and malate inhibited the enzyme noncompetitively. Glucose-6-phosphate reversed the inhibitory effects of oxaloacetate and malate.

Adaptive changes in the properties of phosphoenolpyruvate carboxylase in water-stressed plants

Adaptive changes in the properties of phosphoenolpyruvate carboxylase in water-stressed plants

Effects of NaCl on the properties of phosphoenolpyruvate carboxylase from Suaeda monoica and Chloris gayana

The effects of NaCl on the kinetic properties of desalted phosphoenolpyruvate carboxylase (PEP carboxylase, EC 4.1.1.31) from two halophytes, Suaeda monoica Forssk. ex. J.F. Gmel and Chloris gayana Kunth. were investigated. The tolerance of PEP carboxylase to NaCl in the reaction medium depends on the enzyme pre‐conditioning as well as on the concentration of its substrate PEP in the assay medium. Addition of PEP to the extraction and the storage medium, stabilizes the enzyme. Such a pre‐treated enzyme is inhibited by NaCl in the presence of low concentrations of PEP in the assay medium but is activated by NaCl in the presence of PEP at concentrations above 1.0 mM. NaCl modifies the nH value, K’ and Vmax, and seems to act as an allosteric effector.

Purification and some properties of phosphoenolpyruvate carboxylase from Brevibacterium flavum and its aspartate-overproducing mutant.

Phosphoenolpyruvate (PEP) carboxylases (PC) were purified from a wild strain and an aspartate-producing mutant of Brevibacterium flavum to electrophoretic homogeneity. The molecular weights of the enzymes were determined to be 4.1 X 10(5) by the gel-filtration technique. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme gave only one protein band with a molecular weight of 1.07 X 10(5). The enzyme was labile and stabilized by substrate PEP, activators, metallic cofactors, an allosteric inhibitor and ammonium sulfate. The mechanism for the PC reaction was rapid equilibrium random Bi Bi with a dead end complex, enzyme-bicarbonate-Pi. The KmS for PEP and bicarbonate were 2.5 and 0.63 mM, respectively, and the apparent KmS were not affected by the secondary substrate concentrations. Dissociation constants for Pi of enzyme-Pi and the dead end complex were 5.0 and 16 mM, respectively. Aspartate inhibition was completely competitive with both the substrates, PEP and bicarbonate, with an inhibitor constant of 0.044 mM. An activator, acetyl-CoA, did not alter the apparent Km for bicarbonate but decreased that for PEP. The activator constants for the enzyme-PEP complex and free enzyme were 6.3 and 40 microM, respectively. Double reciprocal plots of reaction rate against PEP concentration were not linear at lower PEP concentrations. Hill coefficients for PEP were 1.6 in the absence of any effectors, 1.0 in the presence of acetyl-CoA, and 2.3 in the presence of aspartate. As to the mutant enzyme, only the inhibitor constant for aspartate was increased, being 0.18 mM, but other constants, coefficients, as described above, and specific activity were almost the same as those of the wild-type enzyme.

A comparison of the kinetic properties of phosphoenolpyruvate carboxylase from guard-cell and mesophyll-cell protoplasts of Commelina communis

Some kinetic properties of partially purified phosphoenolpyruvate carboxylase (PEPCase) from guard-cell and mesophyll-cell protoplasts of Commelina communis are described. The PEPCase activity inherent to each cell type was determined and the apparent K m (phosphoenolpyruvate) and K i (malate) were compared. Malate sensitivity was much higher (K i malate 0.4 mol m(-3)) in the extract of guard-cell protoplasts than in that of mesophyllcell protoplasts (K i malate 4.2 mol m(-3)). The stimulation of activity by glucose-6-phosphate in the presence of malate ('deinhibition') was also investigated in extracts from both cell types and was found to be similar to previously reported results with epidermal tissue. The effect of contamination of an extract of guard-cell protoplasts with mesophyll-cell protoplasts was measured in the presence and absence of malate. It was found that a small amount to mesophyll-cell contaminant appears to desensitize the malate inhibition of PEPCase from guard-cell protoplasts. It is concluded that experiments which use epidermal tissue to study guardcell PEPCase may give misleading information as a consequence of mesophyll contamination.

Diurnal changes in the properties of phosphoenolpyruvate carboxylase in Bryophyllum leaves: a possible co valent modification

In plants that show Crassulacean acid metabolism, phosphoenolpyruvate carboxylase catalyses the key step of CO 2 fixation at night. We show here that the properties of this enzyme from Bryophyllum fedtschenkoi undergo marked changes between night and day; the night form is much less sensitive to feedback inhibition by malate than is the day form. Incubation of leaves with 32P i followed by extraction and immunoprecipitation of phosphoenolpyruvate carboxylase showed that only the night form contained 32P. This suggests that the activity of the enzyme is controlled by a covalent modification mechanism.

Phosphoenolpyruvate carboxylase from pennywort (Umbilicus rupestris). Changes in properties after exposure to water stress

Umbilicus rupestris (pennywort) switches from C3 photosynthesis to an incomplete form of crassulacean acid metabolism (referred to as 'CAM-idling') when exposed to water stress (drought). This switch is accompanied by an increase in the activity of phosphoenolpyruvate carboxylase. This enzyme also shows several changes in properties, including a marked decrease in sensitivity to acid pH, a lower Km for phosphoenolpyruvate, very much decreased sensitivity to the allosteric inhibitor malate, and increased responsiveness to the allosteric effector glucose 6-phosphate. The Mr of the enzyme remains unchanged, at approx. 185 000. These changes in properties of phosphoenolpyruvate carboxylase are discussed in relation to the roles of the enzyme in C3 and in CAM plants.