Activation of higher plant phosphoenolpyruvate carboxylases by glucose-6-phosphate

Structural and biochemical evidence of the glucose 6-phosphate-allosteric site of maize C4-phosphoenolpyruvate carboxylase: its importance in the overall enzyme kinetics

Activation of phosphoenolpyruvate carboxylase (PEPC) enzymes by glucose 6-phosphate (G6P) and other phospho-sugars is of major physiological relevance. Previous kinetic, site-directed mutagenesis and crystallographic results are consistent with allosteric activation, but the existence of a G6P-allosteric site was questioned and competitive activation-in which G6P would bind to the active site eliciting the same positive homotropic effect as the substrate phosphoenolpyruvate (PEP)-was proposed. Here, we report the crystal structure of the PEPC-C4 isozyme from Zea mays with G6P well bound into the previously proposed allosteric site, unambiguously confirming its existence. To test its functionality, Asp239-which participates in a web of interactions of the protein with G6P-was changed to alanine. The D239A variant was not activated by G6P but, on the contrary, inhibited. Inhibition was also observed in the wild-type enzyme at concentrations of G6P higher than those producing activation, and probably arises from G6P binding to the active site in competition with PEP. The lower activity and cooperativity for the substrate PEP, lower activation by glycine and diminished response to malate of the D239A variant suggest that the heterotropic allosteric activation effects of free-PEP are also abolished in this variant. Together, our findings are consistent with both the existence of the G6P-allosteric site and its essentiality for the activation of PEPC enzymes by phosphorylated compounds. Furthermore, our findings suggest a central role of the G6P-allosteric site in the overall kinetics of these enzymes even in the absence of G6P or other phospho-sugars, because of its involvement in activation by free-PEP.

Phosphoenolpyruvate carboxylase from cherimoya fruit: properties, kinetics and effects of high CO(2)

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) regulatory properties were studied in non-photosynthetic (mesocarp) and photosynthetic (peel) tissues from cherimoya (Annona cherimola Mill.) fruit stored in air, in order to gain a better understanding of in vivo enzyme regulation. Analyses were also performed with fruit treated with 20% CO(2)-20% O(2) to define the role of PEPC as part of an adaptive mechanism to high external carbon dioxide levels. The results revealed that the special kinetic characteristics of the enzyme from mesocarp--high V(max) and low sensibility to L-malate inhibition - are related to the active acid metabolism of these fruits and point to a high rate of reassimilation of respired CO(2) into keto-acids. With respect to fruit stored in air, PEPC in crude extracts from CO(2)-treated cherimoyas gave a similar V(max) (1.12+/-0.03 microkat x mg(-1) protein), a lower apparent K(m) (68+/-9 microM for PEP) and a higher I(50) of L-malate (5.95+/-0.3 mM). These kinetic values showed the increase in the affinity of this enzyme toward one of its substrate, PEP, by elevated external CO(2) concentrations. The lower K(m) value and lower sensitivity to L-malate are consistent with higher in vivo carboxylation reaction efficiency in CO(2)-treated cherimoyas, while pointing to an additional enzyme regulation system via CO(2).

Studies of the allosteric properties of maize leaf phosphoenolpyruvate carboxylase with the phosphoenolpyruvate analog phosphomycin as activator

The antibiotic phosphomycin (1,2-epoxypropylphosphonic acid), an analog of phosphoenolpyruvate (PEP), behaved not as an inhibitor, but as an activator, of the enzyme phosphoenolpyruvate carboxylase (PEPC) from maize leaves. Multiple activation studies indicated that the analog binds to the Glc6P-allosteric site producing a more activated enzyme than Glc6P itself. Because of this, we used phosphomycin as a tool to further extend our understanding of the mechanisms of allosteric regulation of C4-PEPC. Initial velocity data from detailed kinetic studies, in which the concentrations of free and Mg-complexed PEP and phosphomycin were controlled, are consistent with: (1) the true activator is free phosphomycin, which competes with free PEP for the Glc6P-allosteric site; and (2) the Mg-phosphomycin complex caused inhibition by binding to the active site in competition with MgPEP. Therefore, although the Glc6P-allosteric site and the active site are able to bind the same ligands, they differ in the form of substrate and activator they bind. This important difference allows the full expression of the potential of activation and prevents inhibition by the activators, including the physiological ones, which are mostly uncomplexed at physiological free Mg2+ concentrations. At fixed low substrate concentrations, the saturation kinetics of the enzyme by phosphomycin showed positive cooperativity at pH 7.3 and 8.3, although at the latter pH, the kinetics of saturation by the substrate was hyperbolic. The cosolute glycerol greatly increased the affinity of the enzyme for phosphomycin and abolished the cooperativity in its binding, but did not eliminate the heterotropic effects of the activator. Therefore, the heterotropic and homotropic effects of the activator are not always coupled to the homotropic effects of the substrate, which argues against the two-state model previously proposed to explain the allosteric properties of maize-leaf PEPC.

Desensitization to glucose 6-phosphate of phosphoenolpyruvate carboxylase from maize leaves by pyridoxal 5'-phosphate

Incubation of the nonphosphorylated form of maize-leaf phospho enol pyruvate carboxylase (orthophosphate: oxaloacetate carboxy-lyase (phosphorylating), PEPC, EC 4.1.1.31) with the reagent pyridoxal 5'-phosphate (PLP) resulted in time-dependent, reversible inactivation and desensitization to the activator glucose 6-phosphate (Glc6P) and other related phosphorylated compounds. Both processes are not connected, since (i) when the PLP-modification was carried out in the presence of saturating ligands of the active site, which prevents inactivation, the desensitization to Glc6P is still observed, and (ii) under some experimental conditions the desensitization reaction is 4-times faster than the inactivation. Desensitization to Glc6P is first order with respect to PLP and has a second-order forward rate constant of 4.7 +/- 0.3 s-1 M-1 and a first-order reverse rate constant of 0.0046 +/- 0.0002 s-1. Correlation studies between the remaining Glc6P sensitivity and mol of PLP residues incorporated per mol of enzyme subunit indicate that one lysyl group for enzyme monomer is involved in the sensitivity of the enzyme to Glc6P. The reactivity of this group is increased by polyethylene glycol and glycerol, while the reactivity of the lysyl group of the active site is not affected by these organic cosolutes. In the presence but not in the absence of the organic cosolutes, Glc6P by itself offers significant protection against desensitization, while increases the extent of inactivation. Free PEP or PEP-Mg have opposite effects, protecting the enzyme against inactivation and increasing the degree of desensitization. They also increases the protection against desensitization afforded by Glc6P. Finally, the PEPC inhibitor malate provides some protection against both inactivation and desensitization. Taken together, these results are consistent with PLP-modification of a highly reactive lysyl group at or near the allosteric Glc6P-site.

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.

Light/dark modulation of phosphoenolpyruvate carboxylase in C3 and C 4 species

In this report, the effects of light on the activity and allosteric properties of phosphoenolpyruvate (PEP) carboxylase were examined in newly matured leaves of several C3 and C4 species. Illumination of previously darkened leaves increased the enzyme activity 1.1 to 1.3 fold in C3 species and 1.4 to 2.3 fold in C4 species, when assayed under suboptimal conditions (pH 7) without allosteric effectors. The sensitivities of PEP carboxylase to the allosteric effectors malate and glucose-6-phosphate were markedly different between C3 and C4 species. In the presence of 5 mM malate, the activity of the enzyme extracted from illuminated leaves was 3 to 10 fold higher than that from darkened leaves in C4 species due to reduced malate inhibition of the enzyme from illuminated leaves, whereas it increased only slightly in C3 species. The Ki(malate) for the enzyme increased about 3 fold by illumination in C4 species, but increased only slightly in C3 species. Also, the addition of the positive effector glucose-6-phosphate provided much greater protection against malate inhibition of the enzyme from C4 species than C3 species. Feeding nitrate to excised leaves of nitrogen deficient plants enhanced the degree of light activation of PEP carboxylase in the C4 species maize, but had little or no effect in the C3 species wheat. These results suggest that post-translational modification by light affects the activity and allosteric properties of PEP carboxylase to a much greater extend in C4 than in C3 species.

Oligomerization and the Affinity of Maize Phosphoenolpyruvate Carboxylase for Its Substrate

When two different forms of phosphoenolpyruvate carboxylase (PEPC) from maize (Zea mays L.) leaves are present in an assay it is possible to estimate the ratio of Vmax to Km (V/K) for the two forms separately. This measure of the binding of the substrate by the enzyme permits evaluation of the effects of various treatments on the relative substrate-binding velocity of the enzyme. PEPC diluted 1/20 is present in a mixture of a tetrameric form with a high affinity for phosphoenolpyruvate and a dimeric form with a low affinity (M.-X. Wu, C.R. Meyer, K.O. Willeford, R.T. Wedding [1990] Arch Biochem Biophys 281: 324-329). Malate at 5 mM reduced (V/K)1,[mdash]the V/K of the probable tetrameric form[mdash]almost to zero, but reduced (V/K)2[mdash]the V/K of the probable dimer[mdash]by only about 80%. Glucose-6-phosphate (Glc-6-P) at 5 mM increased (V/K)1 to 155% of the control but had no effect on (V/K)2. Glycerol (20%) alone increased both V/Ks, and its effects are additive to the Glc-6-P effects, implying different mechanisms for activation by Glc-6-P and glycerol.

Metabolite activation of crassulacean Acid metabolism and c(4) phosphoenolpyruvate carboxylase

The effects of glycine, alanine, serine, and various phosphorylated metabolites on the activity of phosphoenolpyruvate (PEP) carboxylase from Zea mays and Crassula argentea were studied. The maize enzyme was found to be activated by amino acids at a site that is separate from the glucose 6-phosphate binding site. The combination of glycine and glucose 6-phosphate synergistically reduced the apparent K(m) of the enzyme for PEP and increased the apparent V(max). Of the amino acids tested, glycine showed the lowest apparent K(a) and caused the greatest activation. d-Isomers of alanine and serine were more effective activators than the l-isomers. Unlike the maize enzyme, the Crassula enzyme was not activated by amino acids. Activation of either the Crassula or maize enzyme by glucose 6-phosphate occurred without dephosphorylation of the activator molecule. Furthermore, the Crassula enzyme was activated by two compounds containing phosphonate groups whose carbon-phosphorus bonds were not cleaved by the enzyme. A study of analogs of glucose 6-phosphate with Crassula PEP carboxylase revealed that the identity of the ring heteroatom was a significant structural feature affecting activation. Activation was not highly sensitive to the orientation of the hydroxyl group at the second or fourth carbon positions or to the presence of a hydroxyl group at the second position. However, the position of the phosphate group was found to be a significant factor.

Inactivation of maize leaf phosphoenolpyruvate carboxylase by the binding to chloroplast membranes

Phosphoenolpyruvate carboxylase (PEPC) purified from maize (Zea mays L.) leaves associates with maize leaf chloroplast membrane in vitro. The binding of PEPC to the membrane results in enzyme inactivation. A protein isolated from a maize leaf chloroplast membrane preparation inactivated PEPC. Treatment with membrane preparation or with partially purified inactivating protein accelerates PEPC inactivation at low temperature (4 degrees C). Interaction of PEPC with chloroplast membrane or inactivating protein may inactivate the enzyme by influencing dissociation of the enzyme active tetramer.

Regulation of Crassula argentea phosphoenolpyruvate carboxylase in relation to temperature

The effect of temperature on the kinetic parameters of phosphoenolpyruvate carboxylase purified from Crassula argentea was such that both the Vmax and Km(MgPEP) values tended upward over the range from 11 to 35 degrees C. The increased rate at low temperatures due to the low Km is at least partially offset by the increased Vmax at higher temperatures, potentially leading to a broad plateau of enzyme activity and a relatively small effect of temperature on the enzyme. The cooperativity was negative at 11 degrees C, but above 15 degrees C it became positive. The presence of 5 mM glucose-6-phosphate has relatively little effect on Vmax but it clearly reduces Km and overcomes any effect of temperature on this parameter in the range studied. Positive cooperativity is observed only at temperatures above 25 degrees C. The size of the native enzyme, as determined by dynamic light scattering, was strongly toward the tetrameric form. At a temperature of 40 degrees C and above, a considerable oligomerization takes place. No loss of activity can be observed in this range of temperature. In the presence of either glucose-6-phosphate or magnesium phosphoenolpyruvate, at temperatures under 25 degrees C, the equilibrium is displaced toward higher levels of aggregation. Maximal accumulation of lead malate occurred at 10 to 12 degrees C in vivo with reduction to about 25% at 35 degrees C. Glucose-6-phosphate followed a similar curve in response to temperature, but the overall difference was about 50%. The sum of phosphoenolpyruvate plus pyruvate is level at night temperatures below 25 degrees C, doubling at 35 degrees C. Calculated concentrations of malate, glucose-6-phosphate, and phosphoenolpyruvate plus pyruvate indicate that the concentrations present are equal to or greater than Ki, Ka, and Km values for these metabolites, respectively.

Effect of Light and NO(3) on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity: Evidence for Covalent Modulation of the C(3) Enzyme

Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO(3) (-) leaves) or 40 millimolar KNO(3) (high-NO(3) (-) leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO(3) (-). In contrast, the lower rate of increase recorded for low-NO(3) (-) leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO(3) (-) effect increased linearly with the level of NO(3) (-) uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO(3) (-) leaves, illuminated high-NO(3) (-) leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, (32)P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO(3) (-) conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO(3) (-), an interpretation of the role of NO(3) (-) as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO(3) (-), the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO(3) (-) modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis.

Fluorescence Study of Chemical Modification of Phosphoenolpyruvate Carboxylase from Crassula argentea

The chemical modification of phosphoenolpyruvate carboxylase purified from Crassula argentea leaves was studied using the fluorescence of the extrinsic probe 8-anilino-1-naphalenesulfonate. The effects of ligands on kinetic parameters of phosphoenolpyruvate carboxylase activity, and its response to pH and metal cations, were associated with the binding of the ligands to the enzyme as measured by fluorescence. Binding of the ligands phosphoenolpyruvate, malate, and glucose-6-phosphate revealed by fluorescence measurements corresponds to competitive phenomena observed in kinetic studies. The fluorescence measurements also suggest the involvement of specific amino acids in the binding of a given ligand. Arginyl residues modified by 2,3-butanedione appear to be directly involved in the binding of phosphoenolpyruvate and malate to the active and the inhibition sites, respectively. A histidyl residue was involved in the binding of malate, accounting for the lack of inhibition by malate in kinetic studies of the enzyme treated with diethylpyrocarbonate. Although activity was lost, there was no decrease in the ability of the treated enzyme to bind phosphoenolpyruvate, suggesting that additional histidyl residues are essential for activity although not directly involved in the binding of phosphoenolpyruvate. The lysine reagent trinitrobenzenesulfonate caused a loss of activity and a reduction in malate inhibition and glucose-6-phosphate activation, but these modifications were not related to changes in the ability of the enzyme to bind any of the three ligands. This suggests that lysine residues were not directly involved in the binding of these ligands.

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.

Effect of Diethylpyrocarbonate on the Allosteric Properties of Phosphoenolpyruvate Carboxylase from Crassula argentea

Phosphoenolpyruvate carboxylase from the Crassulacean acid metabolism plant Crassula argentea was substantially desensitized to the effects of regulatory ligands by treatment with diethylpyrocarbonate, a reagent which selectively modifies histidyl residues. Desensitization of the enzyme to the inhibitor malate and the activator glucose 6-phosphate was accompanied by the appearance of a peak in the ultraviolet difference spectrum at 240 nanometers, indicating the formation of ethoxyformylhistidyl derivatives. Hydroxylamine reversed part of the spectral change under native conditions, and almost all of the change under denaturing conditions, but failed to restore sensitivity to effectors. The pH profiles of desensitization to malate and glucose 6-phosphate indicated the involvement of groups on the enzyme with pK, values of 6.8 and 6.4, respectively. Under denaturing conditions, a total of 15 histidine residues per subunit were modified by diethylpyrocarbonate, whereas for the native enzyme nine histidines were modified per subunit. Effector desensitization occurs after the modification of two to three histidyl residues per subunit. The presence of malate reduced the apparent rate constant for desensitization by 60%, suggesting that the modification occurred at the malate binding site. Diethylpyrocarbonate treatment also eliminated the kinetic lag caused by malate. Glucose 6-phosphate did not protect the enzyme against diethylpyrocarbonate-induced desensitization.

Effects of pH on inactivation of maize phosphoenolpyruvate carboxylase

Maize leaf phosphoenolpyruvate carboxylase (PEPC) is inactivated by incubation at pH's above neutrality. Both the amount and the rapidity of inactivation increase as the pH rises. The presence of phosphoenolpyruvate (PEP), malate, glucose 6-phosphate and dithiothreitol in the incubation medium give protection to the enzyme. While the presence of PEP during incubation at pH 8 prevents inactivation, the level of PEP in the assay after incubation has no effect on the relative inactivation. When the enzyme is incubated at pH 7 with 5 mM malate (a treatment known to cause dimerization) subsequent assay at saturating levels of MgPEP completely restores activity while assay at less than Km MgPEP produces greater than 99% inhibition of the same sample, showing that high PEP concentration has reconverted the PEPC to the malate-resistant tetramer. Thus the protective effect of PEP against inactivation at high pH probably is not related to its effect on the aggregation state of the enzyme but rather is due to the presence of PEP at the active site. Protection of PEPC at pH 8 by EDTA and its inactivation by low concentrations of Cu2- indicates that the loss of activity at high pH probably is in a sense an artifact resulting from the binding to a deprotinated cysteine of heavy metal ions contaminating the enzyme preparation or present in reagents. This suggests that caution should be used in the interpretation of experiments involving PEPC activity at alkaline pH's.

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.

Inhibition of phosphoenolpyruvate carboxylase by malate

Malate has been noted to be a ;mixed' inhibitor of phosphoenolpyruvate (PEP) carboxylase. The competitive portion of this inhibition appears to be fairly constant regardless of the condition of the enzyme being measured, but the noncompetitive (V-type) inhibition is subject to variation depending on the source of the enzyme, its storage condition, the presence or absence of various ligands, and differences in pH. In the case of the maize (Zea mays L.) phosphoenolpyruvate carboxylase (PEPC), the V-type inhibition by malate is much less pronounced at pH 8 than at pH 7. Examination of the response of the maize PEPC to PEP concentration reveals a pronounced cooperativity at pH 8 which is not present at pH 7, and which results in the disappearance of the V-type inhibition at pH 8. The ability of high concentrations of PEP to convert PEPC from a form readily inhibited by malate to one resistant to malate inhibition has been previously demonstrated and we attribute the cooperativity shown at pH 8 to this response to high levels of PEP. Support for this proposal is provided by studies of the enzyme at pH 7 and pH 8 run in 20% glycerol. In this case there was no V-type inhibition of PEPC at either pH. Treatment with 20% glycerol has been shown to result in the aggregation of maize PEPC.