Kinetic analysis of effectors of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi

Effect of carbon sources differing in oxidation state and transport route on succinate production in metabolically engineered Escherichia coli

In mixed-acid fermentation, succinate synthesis requires one mole of phosphoenolpyruvate (PEP), one mole of CO2, and two moles of NADH for every mole of succinate to be formed. Different carbon sources with different properties were used to address these requirements. Sorbitol generates one more mole of NADH than glucose. Fermentation of sorbitol was shown in this study (and by others) to produce significantly more succinate than fermentation of glucose, due to increased NADH availability. Xylose fermentation conserves the intracellular PEP pool, since its transport does not require the phosphotransferase system normally used for glucose transport. The extra PEP can then be assimilated in the succinate pathway to improve production. In this study, fermentation of xylose did yield higher succinate production than glucose fermentation. Subsequent inactivation of the acetate and lactate pathways was performed to study metabolite redistribution and the effect on succinate production. With the acetate pathway inactivated, significant carbon flux shifted toward lactate rather than succinate. When both acetate and lactate pathways were inactivated, succinate yield ultimately increased with a concomitant increase in ethanol yield.

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.

Regulation of the aggregation state of maize phosphoenolpyruvate carboxylase: evidence from dynamic light-scattering measurements

The molecular weights of different aggregational states of phosphoenolpyruvate carboxylase purified from the leaves of Zea mays have been determined by measurement of the molecular diameter using a Malvern dynamic light scattering spectrometer. Using these data to identify the monomer, dimer, tetramer, and larger aggregate(s) the effect of pH and various ligands on the aggregational equilibria of this enzyme have been determined. At neutral pH the enzyme favored the tetrameric form. At both low and high pH the tetramer dissociated, followed by aggregation to a "large" inactive form. The order of dissociation at least at low pH appeared to be two-step: from tetramer to dimers followed by dimer to monomers. The monomers then aggregate to a large aggregate, which is inactive. The presence of EDTA at pH 8 protected the enzyme against both inactivation and large aggregate formation. Dilution of the enzyme at pH 7 at room temperature results in driving the equilibrium from tetramer to dimer. The presence of malate with EDTA stabilizes the dimer as the predominant form at low protein concentrations. The presence of the substrate phosphoenolpyruvate alone and with magnesium and bicarbonate induced formation of the tetramer, and decreased the dissociation constant (Kd) of the tetrameric form. The inhibitor malate, however, induced dissociation of the tetramer as evidenced by an increase in the Kd of the tetramer.

The influence of pH on substrate form specificity of phosphoenolpyruvate carboxylase purified from Crassula argentea

Purified phosphoenolpyruvate carboxylase from both the crassulacean acid metabolism plant Crassula argentea and the C4 plant Zea mays was shown by kinetic studies at saturating fixed-varying concentrations of free mg2+ to selectively use the metal-complexed form of phosphoenolpyruvate when assayed at pH 8.0. A similar response to added magnesium at high free phosphoenolpyruvate concentrations was obtained for both enzymes, consistent with the use of the complex as the substrate. Kinetic studies at pH 7.0 indicated that at this pH the total concentration of phosphoenolpyruvate (including both free and metal-complexed forms) could be used by the enzyme from C.argentea while the C4 enzyme still utilized the complex. The loss of specificity induced by the decrease in the pH of the assay medium was accompanied by a decrease in the Km of this enzyme for phosphoenolpyruvate whatever the form considered and an increase in Vmax/Km. In contrast, a similar decrease of pH led to an increased Km of the C4 enzyme for phosphoenolpyruvate and a decrease of Vmax/Km. For the enzyme from C. argentea (previously shown to contain an essential arginine at the active site), protection of activity by the different forms of substrate against inactivation by the specific arginyl reagent 2,3-butanedione changes markedly with pH. At pH 8.1, the metal complex is the better protector while at pH 7.0 free phosphoenolpyruvate gives the best protection consistent with the observed kinetic changes in substrate form utilization. The relationship between the enzyme affinity for substrate, substrate specificity, and the requirement for magnesium for substrate turnover is discussed.

Purification, oligomerization state and malate sensitivity of maize leaf phosphoenolpyruvate carboxylase

A method was developed for the purification of phosphoenolpyruvate carboxylase from darkened maize leaves so that the enzyme retained its sensitivity to inhibition by malate. The procedure depended on the prevention of proteolysis by the inclusion of chymostatin in the buffers used during the purification. The purified enzyme was indistinguishable from that in crude extracts as judged by native polyacrylamide-gel electrophoresis. SDS/polyacrylamide-gel electrophoresis followed by immunoblotting, and Superose 6 gel filtration. Gel-filtration studies showed that the purified enzyme and the enzyme in extracts of darkened or illuminated leaves showed a concentration-dependent dissociation of tetrameric into dimeric forms. Purified phosphoenolpyruvate carboxylase and enzyme in crude extracts from darkened leaves were equally sensitive to inhibition by malate (Ki approx. 0.30 mM) under conditions where it existed in the tetrameric or dimeric forms, but the enzyme in crude extracts from illuminated leaves was less sensitive to malate inhibition (Ki approx. 0.95 mM) whether it was present as a tetramer or as a dimer. It is concluded that changes in the oligomerization state of phosphoenolpyruvate carboxylase are not directly involved in its regulation by light.

A kinetic study of the effects of phosphate and organic phosphates on the activity of phosphoenolpyruvate carboxylase from Crassula argentea

The effects of phosphate and several phosphate-containing compounds on the activity of purified phosphoenolpyruvate carboxylase (PEPC) from the crassulacean acid metabolism plant, Crassula argentea, were investigated. When assayed at subsaturating phosphoenolpyruvate (PEP) concentrations, low concentrations of most of the compounds tested were found to stimulate PEPC activity. This activation, variable in extent, was found in all cases to be competitive with glucose 6-phosphate (Glc-6-P) stimulation, suggesting that these effectors bind to the Glc-6-P site. At higher concentrations, depending upon the effector molecule studied, deactivation, inhibition, or no response was observed. More detailed studies were performed with Glc-6-P, AMP, phosphoglycolate, and phosphate. AMP had previously been shown to be a specific ligand for the Glc-6-P site. The main effect of Glc-6-P and AMP on the kinetic parameters was to decrease the apparent Km and increase Vmax/Km. AMP also caused a decrease in the Vmax of the reaction. In contrast, phosphoglycolate acted essentially as a competitive inhibitor increasing the apparent Km for PEP and decreasing Vmax/Km. Inorganic phosphate had a biphasic effect on the kinetic parameters, resulting in a transient decrease in Km followed by an increase of the apparent Km for PEP with increasing concentration of phosphate. The Vmax also was decreased with increasing phosphate concentrations. Further, the enzyme appeared to respond to the complex of phosphate with magnesium. In the presence of a saturating concentration of AMP, no activation but rather inhibition was observed with increasing phosphate concentration. This is consistent with the binding of phosphate to two separate sites--the Glc-6-P activation site and an inhibitory site, a phenomenon that may be occurring with other phosphate containing compounds. High concentrations of phosphate with magnesium were found to protect enzyme activity when PEPC, previously shown to contain an essential arginine at the active site, was incubated with the specific arginyl reagent 2,3-butanedione, consistent with the binding of phosphate at the active site. Data were successfully fitted to a rapid equilibrium model allowing for binding of the phosphate-magnesium complex to both the activation site and the active site which accounts for the activation/deactivation observed at low substrate concentrations. Effects on the Vmax of the reaction are also addressed. Factors controlling the differential affinity of various effectors to the active site or activation site appear to include charge distribution, size, and other steric factors.

Properties of phosphoenolpyruvate carboxylase in desalted extracts from isolated guard-cell protoplasts

Properties of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) obtained from isolated guard-cell protoplasts of Vicia faba L. were determined following rapidly desalting of the extract on a Sephadex G 25 column. The activity of PEP carboxylase was measured as a function of PEP and malate concentration, pH and K(+) concentration within 2-3 min after homogenization of the guard-cell protoplasts. The activity of this enzyme was stimulated by PEP concentrations of 0.1 to 0.75 mM and by K(+) ions (12 mM), but inhibited by PEP concentrations above 1 mM and by malate. Changes in the Km(PEP) and Vmax values with increasing malate concentrations (2.5 and 5 mM) indicate that the malate level, varying in relation to the physiological state of guard cells, plays an important role in regulating the properties of phosphoenolpyruvate carboxylase.

The circadian rhythm of carbon-dioxide metabolism in Bryophyllum: the mechanism of phase-shift induction by thermal stimuli

The detailed characteristics have been established for the phase shifts induced by high-temperature (35° C) stimuli in the circadian rhythm of phosphoenolpyruvate-carboxylase activity in leaves of Bryophyllum fedtschenkoi otherwise kept under constant environmental conditions. The magnitude and direction of the shifts depend upon the duration of the stimulus and its position in the cycle, and are closely similar to those induced by light. An hypothesis is advanced which accounts for all the characteristics of the phase shifts induced by both high-temperature and light stimuli in terms of the leakage of malate from the vacuole to the cytoplasm though "gates" in the tonoplast which are open only during exposure to these stimuli.

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.

Photoperiodism and Crassulacean acid metabolism : III. Different characteristics of the photoperiod-sensitive and non-sensitive isoforms of phosphoenolpyruvate carboxylase and Crassulacean acid metabolism operation

Sensitivity to glucose-6-P, inorganic phosphate and malate, Km phosphoenolpyruvate (PEP), and the effect of pH were comparatively investigated for phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) extracted along the day-night cycle from leaves of Kalanchoe blossfeldiana v. Poelln. Tom Thumb, grown under different photoperiodic conditions. Under long days, sensitivity to effectors is weak and varies only slightly during the 24h cycle, together with small variations of Km (PEP), ranging from 1.2 to 1.8 mM. The response-curve to pH shows two peaks for pH 7.4 and 8.4. Transfer of the plants to short days established an increase in the sensitivity of the enzyme to the effectors together with the appearance of a day-night variation of this effect, maximum during the day. A clear diurnal oscillation of Km (PEP) is observed from 3.1 mM at the beginning of day (09.00 h) to 0.9 mM at midnight (00.00 h), at pH 7.4. These results complement previous electrophoretic and immunological data by affording enzyme kinetic evidence that short days induce a PEP carboxylase form responsible for full-Crassulacean acid metabolism (CAM) operation which is kinetically different from the photoperiod-sensitive, C3-like form prevailing in young leaves under long days. The results indicate that diurnal enzymic rhythms would improve the efficiency of the metabolic regulatory mechanisms and act as a coordinating factor for the daily and seasonal adaptive operation of CAM.

Crassulacean acid metabolism (CAM) in Kalanchoë daigremontiana: Temperature response of phosphoenolpyruvate (PEP)-carboxylase in relation to allosteric effectors

Net CO2 dark fixation of Kalanchoë daigremontiana varies with night temperature. We found an optimum of fixation at about 15° C; with increasing night temperature fixation decreased. We studied the temperature dependence of the activity of phosphoenolpyruvate (PEP)-carboxylase, the key enzyme for CO2 dark fixation. We varied the pH, the substrate concentration (PEP), and the L-malate and glucose-6-phosphate (G-6-P) concentration in the assay. Generally, lowering the pH and reducing the amount of substrate resulted in an increase in activation by G-6-P and in an increase in malate inhibition of the enzyme. Furthermore, malate inhibition and G-6-P activation increased with increasing temperature. Activity measurements between 10° C and 45°C at a given concentration of the effectors revealed that the temperature optimum and maximum activities at that optimum varied with the effector applied. Under the influence of 5 mol m(-3) L-malate the temperature optimum and maximum activity dropped drastically, especially when the substrate level was low (at 0.5 mol m(-3) PEP from 32° C to 20° C). G-6-P raised the temperature optimum and maximum activity when the substrate level was low. If both malate and G-6-P were present, intermediate values were measured. We suggest that changes in metabolite levels in K. daigremontiana leaves can alter the temperature features of PEP-carboxylase so that the observed in vivo CO2 dark fixation can be explained on the basis of PEP-carboxylase activity.