Cytokinin BAP promotes the accumulation of hexoses and increases the activity of phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase during the delay of leaf senescence in wheat. Agrociencia [online]. ISSN Cytokinins reduce their concentration during leaf senescence; but spraying of the cytokinin bencylaminopurine BAP delays this process; this may have applications in the control of the senescence pasture, ornamental foliage and green vegetables.
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Received February 23, Accepted June 8, Key words: Zea mays L. Palabras clave: Zea mays L. In leaves of C4 plants the initial reaction in the assimilation pathway of atmospheric CO 2 is the essentially irreversible carboxylation of phospho eno lpyruvate PEP by phospho eno lpyruvate carboxylase orthophosphate: oxaloacetate carboxylyase phosphorylating , PEPC, EC 4.
But they are by no means redundant. The concentration of phosphorylated sugars increases when the Calvin cycle is active. Neutral amino acids concentrations, particularly that of Gly, increase under photorespiration conditions . Therefore, the two kinds of activators act as metabolic signals that indicate the necessity of increasing the flux through the C4 cycle, in order to keep pace with the flux rate of the Calvin cycle in the case of Glc6P, or to increase the supply of CO 2 to the bundle sheath cells to prevent photorespiration, in the case of Gly.
While Glc6P is unable to revert the inhibition caused by a physiological concentration of malate, Gly can produce an enzyme almost as active than that in the absence of the inhibitor . The kinetic differences between the allosteric activators acquire special relevance under conditions close to those prevailing under illumination, i. Activation by Glc6P could be important during the night or at the onset of illumination before the buildup of malate that takes place during the first hour after illumination .
Once the levels of malate are high, saturation of the Glc6P allosteric site would give only a marginal advantage. Photorespiration surely follows the buildup of malate during the day because of a decrease of the C4 cycle flux, a decrease due to both PEPC inhibition by the increased malate concentration and depletion of the available CO 2 by a very active Calvin cycle.
Activation by Gly helps in increasing the flux through the C4 pathway by effectively counteracting the inhibitory effects of malate, and, therefore, it helps in increasing the concentrations of CO2 in the bundle sheet cells thus overcoming photorespiration.
The bicarbonate concentration in an assay medium in contact with air at pH 7. Also, because regulation of PEPC activity by metabolite effectors is mostly exerted at subsaturating concentrations of substrate , in the studies with the allosteric effectors we used a fixed total PEP tPEP concentration of only 0. The results of these kinetic experiments are shown in Figure 1 and summarized in dable 1.
These two limiting concentrations of tPEP are close to those existing in the cytosol of the mesophyll cells during the dark and light periods, respectively [22, 23]. Glc6P binds cooperatively to both enzymes, with h values close to 2. The activation by Glc6P may, however, play an important role increasing the flux of the C4 cycle at the onset of the light conditions, as mentioned above. Table 1. The differences between the two enzymes in the degree of cooperativity in the binding of PEP in the presence of a high malate concentration are in full agreement with their differences in malate affinity.
Although the S 0. When near physiological concentrations were used, Glc6P was very ineffective in overcoming malate inhibition . Gly has been found to be much more effective than Glc6P in this respect under conditions close to those existing in vivo during the light period .
We tested now the relative contribution of the two kinds of activators in relieving malate inhibition of the two C4 isoenzymes at the tPEP concentration existing during the night, 0. These results indicate that the binding of malate and that of Glc6P to the amaranth enzyme are competitive. These findings suggest that the binding of Glc6P is not affected by the binding of the inhibitor to this enzyme. This is consistent with a lack of effect of malate on the binding of Glc6P and, reciprocally, a lack of effect of Glc6P on the binding of malate.
Therefore, under our experimental conditions Glc6P is no more effective in counteracting malate inhibition of the amaranth than of the maize enzyme Fig. This is consistent with competition between inhibitor and activator for their binding to the enzyme.
These results show that Gly is not an activator of the dicot enzyme either in the absence or in the presence of the inhibitor malate. Six of these sequences are from monocot plants and the other seven from dicot plants. The overall identity among monocot isoenzymes ranged from 80 to In this loop there are several amino acid residues that are conserved, or with conservative substitutions, within each group of monocots or dicots enzymes, but that differ from one group to the other marked with an asterisk in Figure 3.
The neutral amino acid binding site is not yet known because no structure with this kind of ligand has been determined so far. In the homology models of both enzymes, these parts are forming loops, as expected. Of particular interest to us is the loop analyzed in the sequence alignments of Figure 3.
A rigid docking of glycine in this position not shown suggested the feasibility of binding of the activator to these residues, as we propose. As a consequence of this, D and K in the maize enzyme model are not as well positioned to bind the activator molecule as they are in the amaranth enzyme model, as indicated by a rigid docking of the Gly molecule in this site not shown , which is consistent with the A 0.
The allosteric transition would not occur in the amaranth enzyme, thus accounting for the huge differences between the amaranth and the maize enzymes in their degree of activation achieved at saturation by Gly. Both types of isoenzymes also differ in their affinity for the substrate PEP, the activator Glc6P and the inhibitor malate. The lack of activation by Gly of the dicot isoenzymes is mainly compensated by their higher affinity for the substrate PEP and their lower affinity for the inhibitor malate than those exhibited by the monocot isoenzymes.
EDTA ethylenediaminetetraacetic acid disodium salt was from Merck. All other chemicals of analytical grade were from standard suppliers. Plants of maize Zea mays L. Fully expanded leaves were used for the experiments.
Amaranth Amaranthus hypochondriacus L. Phosphoenolpyruvate carboxylase extraction, purification and assay. Plants were kept in darkness for at least 6 h prior to extraction. Protein was measured by the method of Bradford , using bovine serum albumin as the standard. Phosphoenolpyruvate carboxylase assay and kinetic studies. The standard assay medium, final volume of 0. The reaction was started by addition of the enzyme preparation. Rates in the absence of PEP were negligible.
Each determination was performed at least in duplicate. One unit of PEPC is defined as the amount of enzyme needed to catalyze the formation of 1 umol of oxalacetate per min under our experimental conditions. No exogenous bicarbonate was added to the assay media, so that the concentration of bicarbonate was 0. We display the results of the kinetics of saturation of the enzyme by its substrate PEP by considering tPEP as the variable substrate, instead of MgPEP, to facilitate the evaluation of the data in the physiological range of concentration of this metabolite.
Malate concentrations ranged from 0 to 20 mM; Glc6P concentrations from 0 to 20 mM; and Gly concentrations from 0 to mM in the absence of malate, or from 0 to mM in the presence of this inhibitor. Kinetic data were analyzed by nonlinear regression calculations using a commercial computing program formulated with the algorithm of Marquardt . Initial velocity data depending upon varied concentration of substrate were fitted to a Hill equation equation 1 :.
In the experiments in which the concentration of the activator was varied at constant concentration of substrates, equation 2 was used:. When the concentration of inhibitor was varied at constant concentration of substrates, the experimental data were fitted to equation The points in the figures are the experimentally determined values, whereas the curves are calculated from fits of these data to the appropriate equation.
The best fits were determined by the relative fit error, error of the constants and absence of significant correlation between the residuals, and other relevant variables like observed velocities, substrate concentration and data number. Sequence alignments and homology model building. Progressive multiple sequence alignment was carried out with the ClustalX package , using penalties based on secondary structure.
The same solution was always obtained after repeated submissions of the data to this server. The models were validated using ProCheck . The figure was created with PyMOL . Hatch, M. Plant Cell Physiol. Coombs, J.
Wong, K. Bandarian, V. Plant Physiol. Doncaster, H. Nishikido, T; Takanashi, H. Huber, S. Jiao, J. Duff, S. Leegood, R. Matsumura, H. Yazaki, Y. Jenkins, C. Rajagopalan, A. Photosynth Res. Stitt, M. Wedding, R. Yuan, J. Endo, T. Gonzalez, L. Kai, Y. FEBS Lett. Laemmli, U. Nature , , Bradford, M.
La fosfoenolpiruvato carboxilasa (PEPC): enzima clave de los metabolismos fotosintéticos C4 y CAM
Citrate release and activity of phosphoenolpyruvate carboxylase in roots of white lupin in response to varying phosphorus supply. ISSN It has been propoosed that one of the functions of the enzyme phosphoenolpyruvate carboxylase PEPCase in the roots of white lupines consists in providing the carbon required to support the significant quantity of citrate that is excreted by P-starved plants. To demonstrate that citrate excretion by roots is an event more sensitive to P concentration than PEPCase, the activity of the enzyme extracted from roots of white lupines growing in soil as well as its activity and citrate release in plants growing in a nutrient solution were measured. PEPCase activity of plants growing in soil at five P treatments with P added to obtain shoot P ranging from deficient to adequate varied from 0. In a broad range of P concentrations in nutrient solutions with P added to obtain shoot P ranging from deficient to near toxicity , the enzyme activity and citrate release were reduced to almost undetectable levels when shoot P was increased to 0. The results indicate that in vitro PEPCase activity does not significantly change with the range of shoot P from deficient to adequate, and suggest that the mechanism associated with citrate excretion might be impaired at P concentrations lower than those required to inhibit PEPCase activity.
Received February 23, Accepted June 8, Key words: Zea mays L. Palabras clave: Zea mays L. In leaves of C4 plants the initial reaction in the assimilation pathway of atmospheric CO 2 is the essentially irreversible carboxylation of phospho eno lpyruvate PEP by phospho eno lpyruvate carboxylase orthophosphate: oxaloacetate carboxylyase phosphorylating , PEPC, EC 4. But they are by no means redundant. The concentration of phosphorylated sugars increases when the Calvin cycle is active.
Araceli; Herrera-Cabrera, B. Edgar The objective of the present study was to investigate the effect of BAP application on primary metabolism during the leaf senescence of Triticum aestivum L. Temporalera studying the photosynthetic pigments content, soluble pr mas oteins, Rubisco, carbohydrates, enzyme activity of phosphoenolpyruvate carboxylase PEPC and phosphoenolpyruvate carboxykinase PEPCK , major components of plant primary metabolism. Seedlings 21 d after sowing DAS were sprayed with 0. The BAP promoted the accumulation of hexoses. It is discussed the function of these enzymes in the regulation of pH and the translocation of nitrogen compounds during the delay of leaf senescence.