Phosphoenolpyruvate carboxylase isoenzymes: separation and properties of three forms from cotton leaf tissue

Evolution of C4 phosphoenolpyruvate carboxylase

C4 plants are known to be of polyphyletic origin and to have evolved independently several times during the evolution of angiosperms. This implies that the C4 isoform of phosphoenolpyruvate carboxylase (PEPC) originated from a nonphotosynthetic PEPC gene that was already present in the C3 ancestral species. To meet the special requirements of the C4 photosynthetic pathway the expression program of the C4 PEPC gene had to be changed to achieve a strong and selective expression in leaf mesophyll cells. In addition, the altered metabolite concentrations around C4 PEPC in the mesophyll cytoplasm necessitated changes in the enzyme's kinetic and regulatory properties. To obtain insight into the evolutionary steps involved in these altered enzyme characteristics, and even the order of these steps, the dicot genus Flaveria (Asteraceae) appears to be the experimental system of choice. Flaveria contains closely related C3, C3-C4, and C4 species that can be ordered by their gradual increase in C4 photosynthetic traits. The C4 PEPC of F. trinervia, which is encoded by the ppcA gene class, possesses typical kinetic and regulatory features of a C4-type PEPC. Its nearest neighbor is the orthologous ppcA gene of the C3 species F. pringlei. This latter gene encodes a typical nonphotosynthetic C3-type PEPC which is believed to be similar to the C3 ancestral PEPC. This pair of orthologous PEPCs has been used to map C4-specific molecular determinants for the kinetic and regulatory characteristics of C4 PEPCs. The most notable finding from these investigations was the identification of a C4 PEPC invariant site-specific mutation from alanine (C3) to serine (C4) at position 774 that was a necessary and late step in the evolution of C3 to C4 PEPC. The C3-C4 intermediate ppcA PEPCs are used to identify the sequence of events leading from a C3- to a C4-type PEPC.

Regulation of the supply of cytosolic oxaloacetate for mitochondrial metabolism via phosphoenolpyruvate carboxylase in barley leaf protoplasts. I. The effect of covalent modification on PEPC activity, pH response, and kinetic properties

The regulation of the supply of oxaloacetate (OAA) for mitochondrial metabolism via phosphoenolpyruvate carboxylase (PEPC) by covalent modification is studied in barley (Hordeum vulgare L.) leaf protoplasts in light or darkness as well as under photorespiratory or non-photorespiratory conditions. Extracts for studies on in vivo PEPC phosphorylation were prepared from barley leaf protoplasts by rapid filtration, fractionating the cell within less than 1 s. Measurements of in vitro PEPC activity were performed on samples quickly frozen in liquid nitrogen to break the cell and stop metabolism and thus preserve the in vivo activation state. The relative PEPC phosphorylation state increased upon illumination and decreased upon redarkening under photorespiratory and non-photorespiratory conditions. PEPC activity measured in the presence of malate (3 mM) under photorespiratory conditions showed the same response indicating that a light-induced increase in PEPC activity and decrease in malate sensitivity is caused by an increased phosphorylation level of the PEPC protein. PEPC activity was pH dependent. At the physiological cytosolic pH, activity was suboptimal, but most sensitive towards malate inhibition and glucose 6-phosphate stimulation. The presence of malate increased the sensitivity of PEPC activity towards pH changes. The response of PEPC activity to changing pH was not affected by changes in the activation state of the enzyme. The Km (phosphoenolpyruvate, PEP) is about 1 mM. Upon illumination the Km (PEP) decrease significantly. Vmax was unaffected by the light treatment. The presence of physiological concentrations of glucose 6-phosphate decreased Km (PEP) 5- to 10-fold and increased Vmax by about 35%. The effect of glucose 6-phosphate was strongest (up to 7-fold) at subsaturating PEP concentrations stimulating PEPC activity to nearly maximal rates. The results show that an increase in PEPC phosphorylation state causes an increase in PEPC activity as well as in substrate affinity leading to an increased production of OAA in the light.

Regulation of the supply of oxaloacetate for mitochondrial metabolism via phosphoenolpyruvate carboxylase in barley leaf protoplasts. II. Effects of metabolites on PEPC activity at different activation states of the protein

The regulation of the supply of oxaloacetate (OAA) for mitochondrial metabolism via phosphoenolpyruvate carboxylase (PEPC) by metabolites is studied in barley (Hordeum vulgare L.) leaf protoplasts in light or darkness as well as under photorespiratory or non-photorespiratory conditions. Measurements on PEPC activity were performed on samples quickly frozen in liquid nitrogen to break the cell and stop metabolism and thus preserve the in vivo activation state. Glycine, serine, pyruvate, acetyl-CoA, glycolate, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate and ADP had no significant effect on PEPC activity. Malate, aspartate and glutamate were strong inhibitors of PEPC activity decreasing the activity more in light versus darkness. However, at the physiological cytosolic concentration of these metabolites under the respective conditions, inhibition of PEPC activity was about the same with the exception of aspartate which inhibits more under non-photorespiratory than under photorespiratory conditions. 2-Oxoglutarate and glyoxylate decreased PEPC activity by 20 to 40% in the range of its physiological cytosolic concentration. Inhibition by physiological cytosolic concentrations of glutamine was limited. Glucose 6-phosphate, fructose 6-phosphate, 3-phosphoglycerate, dihydroxyacetonphosphate and P(i) stimulated PEPC activity significantly in their physiological cytosolic concentration range. Physiological cytosolic concentrations of glucose 6-phosphate and fructose 6-phosphate activated PEPC activity to about the same extent under all conditions applied, while 3-phosphoglycerate and dihydroxyacetonphosphate stimulating stronger under non-photorespiratory versus photorespiratory conditions. Moreover, dihydroxyacetonphosphate stimulated PEPC activity more in light versus darkness under non-photorespiratory conditions. P(i) activation of PEPC activity decreases in light versus darkness under non-photorespiratory conditions. Stimulation of PEPC activity by citrate in its physiological concentration range is limited. Glucose 1-phosphate and AMP activated PEPC activity only at concentrations higher than their physiological levels in the cytosol. Determinations of PEPC activity in the presence of different malate/glucose 6-phosphate ratios revealed that glucose 6-phosphate totally relieved the inhibitory effect of malate. The regulatory properties of PEPC activity will be discussed in relation to its functions in C3 plants.

Kinetic evidence of the existence of a regulatory phosphoenolpyruvate binding site in maize leaf phosphoenolpyruvate carboxylase

Phenylphosphate, a structural analog of phosphoenolpyruvate (PEP), was found to be an activator of phosphoenolpyruvate carboxylase (PEP carboxylase) purified from maize leaves. This finding suggested the presence in the enzyme of a regulatory site, to which PEP could bind. We carried out kinetic studies on this enzyme using controlled concentrations of free PEP and of Mg-PEP complex and developed a theoretical kinetic model of the reaction. In summary, the main conclusions drawn from our results, and taken as assumptions of the model, were the following: (i) The affinity of the active site for the complex Mg-PEP is much higher than that for free PEP and Mg2+ ions, and therefore it can be considered that the preferential substrate of the PEP-catalyzed reaction is Mg-PEP. (ii) The enzyme has a regulatory site specific for free PEP, to which Mg2+ ions can not bind. (iii) The binding of free PEP, or an analog molecule, to this regulatory site yields a modified enzyme that has much lower apparent Km values and apparent Vmax values than the unmodified enzyme. So, free PEP behaves as an excellent activator of the reaction at subsaturating substrate concentrations, and as an inhibitor at saturating substrate concentrations. These findings may have important physiological implications on the regulation of the PEP carboxylase in vivo activity and, consequently, of the C4 pathway, since increased reaction rates would be obtained when the concentration of PEP rises, even at limiting Mg2+ concentrations.

Phosphoenolpyruvate carboxylase in Hydrilla plants with varying CO2 compensation points

Incubation of the submersed aquatic macrophyte, Hydrilla verticillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO2/1) and low (6 to 8 μl CO2/l) CO2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C3 and C4 species, respectively.Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO3 (-)) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg(2+)) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively.Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C4 acid inhibition and protection were not reliable methods to distinguish C3 and C4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from hight Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C3 and C4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C4 type.

Phosphoenolpyruvate carboxylase in Hydrilla plants with varying CO2 compensation points

Incubation of the submersed aquatic macrophyte, Hydrilla vertieillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO2/l) and low (6 to 8 μl CO2/l) CO2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C3 and C4 species, respectively.Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO3 (-)) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg(2+)) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively.Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C4 acid inhibition and protection were not reliable methods to distinguish C3 and C4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from high Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C3 and C4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C4 type.

Cell organelles from crassulacean acid metabolism (CAM) plants : II. Compartmentation of enzymes of the crassulacean acid metabolism

The intracellular distribution of enzymes involved in the Crassulacean acid metabolism (CAM) has been studied in Bryophyllum calycinum Salisb. and Crassula lycopodioides Lam. After separation of cell organelles by isopycnic centrifugation, enzymes of the Crassulacean acid metabolism were found in the following cell fractions: Phosphoenolpyruvate carboxylase in the chloroplasts; NAD-dependent malate dehydrogenase in the mitochondria and in the supernatant; NADP-dependent malate dehydrogenase and phosphoenolpyruvate carboxykinase in the chloroplasts; NADP-dependent malic enzyme in the supernatant and to a minor extent in the chloroplasts; NAD-dependent malic enzyme in the supernatant and to some degree in the mitochondria; and pyruvate; orthophosphate dikinase in the chloroplasts. The activity of the NAD-dependent malate dehydrogenase was due to three isoenzymes separated by (NH4)2SO4 gradient solubilization. These isoenzymes represented 17, 78, and 5% of the activity recovered, respectively, in the order of elution. The isoenzyme eluting first was associated with the mitochondria and the second isoenzyme was of cytosolic origin, while the intracellular location of the third isoenzyme was probably the peroxisome. Based on these findings, the metabolic path of Crassulacean acid metabolism within cells of CAM plants is discussed.

Phosphoenolpyruvate carboxylase from soybean nodule cytosol. Evidence for isoenzymes and kinetics of the most active component

Phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxylase (phosphorylating), EC 4.1.1.31) from plant cells of soybean nodules was studied to assess its role in providing carbon skeletons for aspartate and asparagine synthesis. The enzyme was purified 119-fold by (NH4)2SO4 fractionation and DEAE-cellulose, BioGel A-1.5m, and hydroxyapatite chromatography. Five activity bands were resolved with discontinuous polyacrylamide gel electrophoresis. A small quantity of enzyme from the most active band was separated from the others by preparative electrophoresis. The apparent Michaelis constants of this enzyme for phosphoenolpyruvate and HCO3- were 9.4.10(-2) and 4.1.10(-1) mM, respectively. A series of metabolite tested at 1 mM had no significant effect on enzyme activity. These experiments indicate that the major factors directly controlling phosphoenolpyruvate carboxylase activity in vivo are phosphoenolpypyruvate and HCO3- concentrations.

Changes in the isozymic pattern of phosphoenolpyruvate : An early step in photoperiodic control of crassulacean acid metabolism level

Two major isofunctional forms of phosphoenolpyruvate carboxylase (EC 4.1.1.31) have been separated from the leaves of Kalanchoe blossfeldiana Poelln. Tom Thumb by acrylamide gel electrophoresis and diethylaminoethyl cellulose techniques: one of the forms prevails under long-day treatment (low crassulacean acid metabolism level), the other develops under short-day treatment (high Crassulacean acid metabolism level). Molecular weights are significantly different: 175·10(3) and 186·10(3), respectively. These results indicate that two populations of phosphoenolyruvate carboxylase are present in the plant, one of which is responsible for Crassulacean acid metabolism activity under the control of photoperiod.The Crassulacean acid metabolism appears to depend on the same endogenous clock that governs other photoperiodically controlled events (e.g. flowering). The metabolic and energetic significance of this feature is discussed. It is suggested that modification in isozymic composition could be an early step in the response to photoperiodism at the metabolic level.

Correlations between photosynthetic enzymes, CO2-fixation and plastid structure in an albino mutant of Zea mays L

An albino seedling of Zea mays L. was investigated for its potential for CO2-assimilation. In the mesophyll the number, dimensions and fine structure of chloroplasts are drastically reduced but to a lesser extent in the bundle sheath. Chlorophyll concentration is zero and carotenoid concentration almost zero. Albinism also exerts a strong influence on the stroma of bundle sheath chloroplasts; ribulose-1.5-biphosphate carboxylase (EC 4.1.1.39) activity and glyceraldehyde-3-phosphate dehydrogenase (NADP) (EC 1.2.1.13) activity is not detectable. The C4-enzymes phosphoenolpyruvate carboxylase (EC 4.1.1.31) and malate dehydrogenase (decarboxylating) (EC 1.1.1.40) and the non-photosynthetic linked enzymes malate dehydrogenase (NAD) (EC 1.1.1.37), aspartate-2-oxoglutarate aminotransferase (EC 1.1.1.37), aspartate-2-oxoglutarate aminotransferase (EC 2.6.1.1.) and glyceraldehyde-3-phosphate dehydrogenase (NAD) (EC 1.2.1.1.) are present in the albino seedling with activities comparable to those in etiolated maize seedlings. The potential for CO2 fixation of the albino seedlings exceeds that of comparable dark seedlings considerably. The results are discussed with regard to enzyme localization of the C4 pathway of photosynthesis.

The C-4 pathway in Pennisetum purpureum : I. The allosteric nature of PEP carboxylase

Phosphoenol pyruvate (PEP) carboxylase has been partially purified from leaves of the C-4 tropical grass Pennisetum purpureum and shown to have allosteric properties. When initial velocities of incorporation of (14)C from NaH(14)CO3 into oxaloacetate were determined as a function of concentration of either HCO3-or Mg(2+) typical Michaelis-Menten kinetics were observed. Both Lineweaver-Burk and Hill plots were linear with values of n (interaction coefficients) of about one. Sigmoid Michaelis-Menten plots were obtained with PEP as the variable substrate. Following (NH4)2SO4 fractionation and DEAE-cellulose chromatography Lineweaver-Burk plots were concave upwards and Hill plots gave n values of two. With enzyme purified further by Sephadex G-200 chromatography Lineweaver-Burk plots were concave downwards and Hill plots gave values of n of 0.5 at low concentrations of PEP increasing to about 4 at high concentrations of PEP. Enzyme activity was modified by inclusion of glucose-6-phosphate (G6P) in the assay mixtures. When the eoncentration of G6P exceeded that of PEP, the initial velocity tended towards zero. When the concentration of G6P equalled that of PEP activity was increased. When the concentration of PEP exceeded that of G6P, the velocity approached that recorded in control samples at saturating concentrations of PEP. The rate of reaction was also increased on addition of NADH, and decreased by oxaloacetate and malate.