Corn leaf phosphoenolpyruvate carboxylases. Purification and properties of two isoenzymes

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.

Structure and expression of the maize gene encoding the phosphoenolpyruvate carboxylase isozyme involved in C4 photosynthesis

We have determined the structure of the maize (Zea mays L. subsp.mays line B73) nuclear gene encoding the phosphoenolpyruvate (PEP) carboxylase isozyme involved in C4 photosynthesis. The gene is 5.3 kb long and has ten exons that range in size from 85 to 999 bp. The nine introns vary from 97 to 872 bp. The sequence of 663 bp of 5'-flanking and 205 bp of 3'-flanking DNA is reported along with the entire gene sequence. Several short repetitive sequences were found in the 5'-flanking DNA that have characteristics similar to elements important in the light regulation of pea genes encoding the small subunit of ribulose 1,5-bisphosphate carboxylase. In addition, some 5'-flanking sequence similarities were found in a comparison with other light-regulated genes from maize and wheat.The level of DNA sequence variation among different PEP carboxylase alleles is similar to the allelic variation observed for several other maize nuclear genes. The data suggest modern maize variaties have retained much of the genetic variation present in their ancestral forms.Finally, accumulation of transcripts encoding the PEP carboxylase isozyme involved in C4 photosynthesis is quite high in several structures besides leaves, including inner leaf sheaths, tassels and husks. This indicates that expression of this gene is not leaf-specific and may not necessarily be coupled to the development of Kranz anatomy.

Artifacts in the assay of maize leaf phosphoenolpyruvate carboxylase activity due to its instability

When the assay of maize leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity is started with phosphoenolpyruvate, much lower reaction rates are obtained as compared to the enzyme-initiated reaction. The difference is due to the lability of the dilute enzyme in the absence of its substrate and is increased with incubation time in the absence of substrate or stabilizers. The activation of the enzyme by glucose-6-phosphate is overestimated with the substrate-initiated assay since a part of the apparent activation is due to stabilization of the enzymic activity by this effector during the minus-substrate preincubation. In contrast, the inhibitory effect of malate is underestimated when the reaction is started with the substrate. The enzyme-initiated assay is recommended provided that the necessary corrections for apparent activity in the absence of substrate and for inactivation during the assay at low substrate levels are made.

Modification of an essential amino group of phosphoenolpyruvate carboxylase from maize leaves by pyridoxal phosphate and by pyridoxal phosphate-sensitized photooxidation

Phosphoenolpyruvate carboxylase from maize leaves was inactivated by pyridoxal 5'-phosphate in the dark and in the light. A two-step reversible mechanism is proposed for inactivation in the dark, which involves the formation of a noncovalent complex prior to a Schiff base with amino groups of the enzyme. Spectral analysis of pyridoxal 5'-phosphate-modified phosphoenolpyruvate carboxylase showed absorption maxima at 432 and 327 nm, before and after reduction with NaBH4, respectively, suggesting that epsilon-amino groups of lysine residues are the reactive groups in the enzyme. A correlation between spectral data and the maximal inactivation obtained with several concentrations of inhibitor allowed us to establish that the incorporation of 4 mol of pyridoxal 5'-phosphate per mole of holoenzyme accounts for total inactivation. The absence of modifier bound to phosphoenolpyruvate carboxylase when the modification was carried out in the presence of phosphoenolpyruvate and MgCl2 suggests the existence of an essential lysine residue at the catalytic site of the enzyme. Modification of phosphoenolpyruvate carboxylase in the light under an oxygen atmosphere resulted in an irreversible inactivation, which was completely protected by phosphoenolpyruvate and MgCl2. Spectral analysis of the photomodified enzyme showed an absorption peak of 320 nm, suggesting light-mediated addition of a nucleophilic residue (probably an imidazole group) to the pyridoxal 5'-phosphate-lysine azomethine bond.

Involvement of thiol groups in the activity of phosphoenolpyruvate carboxylase from maize leaves

Purified maize leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) was completely inactivated by several thiol-modifying reagents, including, CuCl2, CdCl2 and N-ethylmaleimide. The inactivation by CuCl2 could be reversed by dithiothreitol, suggesting the involvement of vicinal dithiols in the inactivation process.Complete inactivation of phosphoenolpyruvate carboxylase was correlated with the incorporation of two mol ((3)H)N-ethylmaleimide per 100-kilodalton subunit. The total protection of the enzyme against N-ethylmaleimide inactivation afforded by the substrate, phosphoenolpyruvate, was correlated with the protection of one mol ((3)H)N-ethylmaleimide reactive residue per mol subunit.The complete inactivation of phosphoenolpyruvate carboxylase by N-ethylmaleimide and the protection afforded by phosphoenolpyruvate against modification suggest the presence of an essential cysteine residue in the catalytic site of the C4 leaf enzyme.

Changes in properties of phosphoenolpyruvate carboxylase from the CAM plant Sedum praealtum D.C. upon dark/light transition and their stabilization by glycerol

A prenounced decrease in phosphoenolpyruvate earboxylase (PEPC) activity is observed upon dark/light transition in Sedum praealtum D.C., only when glycerol is included in the extraction medium. If glycerol is omitted, the activity extracted in light is initially low, but soon reaches night levels. The stabilization of the light-induced form of the enzyme by glycerol, in crude or desalted extracts, made it possible to study its kinetic properties in comparison to those of the dark form. The behaviour towards substrate (PEP) changes from hyperbolic (dark) to sigmoid (light), S0.5 is increased and the enzymic activity becomes more sensitive to malate inhibition. Quite different activity/pH profiles are also obtained for the two forms of PEPC.It is inferred that the in vivo regulation of PEPC in CAM is effected by a concerted action of light, malate and pH shifting.

A study of the in-vitro regulation of phosphoenolpyruvate carboxylase from the epidermis of Commelina communis by malate and glucose-6-phosphate

Phosphoenolpyruvate (PEP) carboxylase activity in epidermal extracts of Commelina communis has been compared in the presence of malate and glucose-6-phosphate. The activity of PEP carboxylase was inhibited by increasing malate concentrations at several substrate (PEP) concentrations and changes in both the apparent K m (PEP) and V max values in the presence of malate suggested the occurence of mixed-type inhibiton. In the presence of glucose-6-phosphate no increase in enzyme activity was observed, although there was a slight decrease in the K m (PEP). However, glucose-6-phosphate appeared to alleviate the inhibition caused by malate. The possible implications of these properties in the control of malate production in guard cells is discussed.

Kinetic and isotope effect studies of maize phosphoenolpyruvate carboxylase

Carbon isotope effects for the carbon atom arising from bicarbonate have been measured for the phosphoenolpyruvate carboxylase from maize. At pH 7.5, 25 degrees C, the isotope effect is K12/k13 = 1.0029 +/- 0.0005 in the presence of Mg2+. The isotope effect decreases with increasing pH, reaching a value of 0.9973 at pH 10.0. All these isotope effects are relative to HCO3(-) taken as the starting state. If CO2 is considered the starting state, the isotope effects are all inverse. These values suggest that the carboxylation of phosphoenolpyruvate occurs by way of a stepwise mechanism involving an enzyme-bound carboxyphosphate intermediate, with formation of the intermediate being the primary rate-determining step. Steady-state kinetics reveal that Vmax is independent of pH over the range pH 7.5-10.0 Vmax/Km (phosphoenolpyruvate) is bell shaped in the same interval. Two pKa values near 7 are observed; the first is attributed to ionization of the phosphate group of phosphoenolpyruvate and the second to an unidentified group on the enzyme. Activity of the enzyme also depends on protonation of a group on the enzyme with a pKa near 10. Several metal ions were tested as activators of phosphoenolpyruvate carboxylase. Under saturating conditions, Mg2+ and Mn2+ show equal activity but different carbon isotope effects. Co2+ has about half the activity of Mg2+ and shows an inverse carbon isotope effect.

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.