Evidence for a multiple subunit composition of plant NAD malic enzyme

Chimeric Structure of Plant Malic Enzyme Family: Different Evolutionary Scenarios for NAD- and NADP-Dependent Isoforms

Malic enzyme (ME) comprises a family of proteins with multiple isoforms located in different compartments of eukaryotic cells. In plants, cytosolic and plastidic enzymes share several characteristics such as NADP specificity (NADP-ME), oxaloacetate decarboxylase (OAD) activity, and homo-oligomeric assembly. However, mitochondrial counterparts are NAD-dependent proteins (mNAD-ME) lacking OAD activity, which can be structured as homo- and hetero-oligomers of two different subunits. In this study, we examined the molecular basis of these differences using multiple sequence analysis, structural modeling, and phylogenetic approaches. Plant mNAD-MEs show the lowest identity values when compared with other eukaryotic MEs with major differences including short amino acid insertions distributed throughout the primary sequence. Some residues in these exclusive segments are co-evolutionarily connected, suggesting that they could be important for enzymatic functionality. Phylogenetic analysis indicates that eukaryotes from different kingdoms used different strategies for acquiring the current set of NAD(P)-ME isoforms. In this sense, while the full gene family of vertebrates derives from the same ancestral gene, plant NADP-ME and NAD-ME isoforms have a distinct evolutionary history. Plant NADP-ME genes may have arisen from the α-protobacterial-like mitochondrial ancestor, a characteristic shared with major eukaryotic taxa. On the other hand, plant mNAD-ME genes were probably gained through an independent process involving the Archaeplastida ancestor. Finally, several residue signatures unique to all plant mNAD-MEs could be identified, some of which might be functionally connected to their exclusive biochemical properties. In light of these results, molecular evolutionary scenarios for these widely distributed enzymes in plants are discussed.

Possible involvement of phosphoenolpyruvate carboxylase and NAD-malic enzyme in response to drought stress. A case study: a succulent nature of the C4-NAD-ME type desert plant, Salsola lanata (Chenopodiaceae)

The co-ordination between the primary carboxylating enzyme phosphoenolpyruvate carboxylase (PEPC) and the further decarboxylating enzymes is crucial to the efficiency of the CO2-concentrating mechanism in C4 plants, and investigations on more types of C4 plants are needed to fully understand their adaptation mechanisms. In this study we investigated the effect of drought on carboxylating enzyme PEPC, and the further decarboxylating NAD-malic enzyme (NAD-ME) of Salsola lanata Pall. (Chenopodiaceae) - an annual succulent C4-NAD-ME subtype desert plant. We investigated enzyme activity at the transcriptional level with real-time quantitative PCR and at the translational level by immunochemical methods, and compared S. lanata with other forms of studied C4 plants under drought stress. Results showed that only severe stress limited PEPC enzyme activity (at pH 8.0) of S. lanata significantly. Considering that PEPC enzyme activity (at pH 8.0) was not significantly affected by phosphorylation, the decrease of PEPC enzyme activity (at pH 8.0) of S. lanata under severe stress may be related with decreased PEPC mRNA. The suggestion of increased phosphorylation of the PEPC enzyme in plants under moderate stress was supported by the ratio of PEPC enzyme activity at pH 7.3/8.0, as PEPC enzyme is inhibited by L-malate and the evidence of the 50% inhibiting concentration of L-malate. NAD-ME activity decreased significantly under moderate and severe stress, and coincided with a change of leaf water content rather than the amount of α-NAD-ME mRNA and protein. Leaf dehydration may cause the decrease of NAD-ME activity under water stress. Compared with other C4 plants, the activities of PEPC and NAD-ME of S. lanata under drought stress showed distinct features.

2-Hydroxy Acids in Plant Metabolism

Glycolate, malate, lactate, and 2-hydroxyglutarate are important 2-hydroxy acids (2HA) in plant metabolism. Most of them can be found as D- and L-stereoisomers. These 2HA play an integral role in plant primary metabolism, where they are involved in fundamental pathways such as photorespiration, tricarboxylic acid cycle, glyoxylate cycle, methylglyoxal pathway, and lysine catabolism. Recent molecular studies in Arabidopsis thaliana have helped elucidate the participation of these 2HA in in plant metabolism and physiology. In this chapter, we summarize the current knowledge about the metabolic pathways and cellular processes in which they are involved, focusing on the proteins that participate in their metabolism and cellular/intracellular transport in Arabidopsis.

Allosteric substrate inhibition of Arabidopsis NAD-dependent malic enzyme 1 is released by fumarate

Plant mitochondria can use L-malate and fumarate, which accumulate in large levels, as respiratory substrates. In part, this property is due to the presence of NAD-dependent malic enzymes (NAD-ME) with particular biochemical characteristics. Arabidopsis NAD-ME1 exhibits a non-hyperbolic behavior for the substrate L-malate, and its activity is strongly stimulated by fumarate. Here, the possible structural connection between these properties was explored through mutagenesis, kinetics, and fluorescence studies. The results indicated that NAD-ME1 has a regulatory site for L-malate that can also bind fumarate. L-Malate binding to this site elicits a sigmoidal and low substrate-affinity response, whereas fumarate binding turns NAD-ME1 into a hyperbolic and high substrate affinity enzyme. This effect was also observed when the allosteric site was either removed or altered. Hence, fumarate is not really an activator, but suppresses the inhibitory effect of l-malate. In addition, residues Arg50, Arg80 and Arg84 showed different roles in organic acid binding. These residues form a triad, which is the basis of the homo and heterotrophic effects that characterize NAD-ME1. The binding of L-malate and fumarate at the same allosteric site is herein reported for a malic enzyme and clearly indicates an important role of NAD-ME1 in processes that control flow of C4 organic acids in Arabidopsis mitochondrial metabolism.

The study of NAD-malic enzyme in Amaranthus cruentus L. under drought

Decarboxylating NAD-malate dehydrogenase (NAD-malic enzyme, NAD-ME, EC 1.1.1.39) has been investigated under a long-term drought during pre-anthesis, anthesis and seed-formation phases of ontogenesis of a NAD-ME type C4 plant Amaranthus cruentus L. using cytosol, chloroplast and mitochondrial fractions of mesophyll (M) and bundle sheath (BS) cells. We detected several molecular forms of NAD-ME with different subcellular localization patterns in the studied phases of amaranth ontogenesis. However, no enzyme activity was observed experimentally in chloroplasts of M and BS cells. In the pre-anthesis phase NAD-ME isoform with molecular weight of ∼115 kDa was found in cytosol of M and BS cells of control and drought-exposed plants. One of NAD-ME isoforms with molecular weight of 110 kDa was located in mitochondria of BS cells of control and drought-exposed plants, and a new isoform of ∼121 kDa was formed in mitochondria of BS cells under the influence of drought. After resuming watering this isoform (∼121 kDa) disappeared again. Approximately 90.6% and 9.4% of the total NAD-ME activity were localized in mitochondrial stroma and cytosol of BS cells, respectively, while in mesophyll cells 100% activity was found in cytosol fractions. The reaction catalyzed by NAD-ME follows Michaelis-Menten equation. NAD(+), l-malate and Mn(2+) activate this enzyme in mitochondria. Appearance of the ∼121 kDa isoform of NAD-ME in the mitochondrial fraction of BS cells under drought and its disappearance after resuming watering could be attributed to one of the protection functions of plants.

Enzymatic properties of Populus α- and β-NAD-ME recombinant proteins

Plant mitochondrial NAD-malic enzyme (NAD-ME), which is composed of α- and β-subunits in many species, participates in many plant biosynthetic pathways and in plant respiratory metabolism. However, little is known about the properties of woody plant NAD-MEs. In this study, we analyzed four NAD-ME genes (PtNAD-ME1 through PtNAD-ME4) in the genome of Populus trichocarpa. PtNAD-ME1 and -2 encode putative α-subunits, while PtNAD-ME3 and -4 encode putative β-subunits. The Populus NAD-MEs were expressed in Escherichia coli cells as GST-tagged fusion proteins. Each recombinant GST-PtNAD-ME protein was purified to near homogeneity by glutathione-Sepharose 4B affinity chromatography. Milligram quantities of each native protein were obtained from 1 L bacterial cultures after cleavage of the GST tag. Analysis of the enzymatic properties of these proteins in vitro indicated that α-NAD-MEs are more active than β-NAD-MEs and that α- and β-NAD-MEs presented different kinetic properties (V_max, k_cat and k_cat/K_m). The effect of different amounts of metabolites on the activities of Populus α- and β-NAD-MEs was assessed in vitro. While none of the metabolites evaluated in our assays activated Populus NAD-ME, oxalacetate and citrate inhibited all α- and β-NAD-MEs and glucose-6-P and fructose inhibited only the α-NAD-MEs.

Differential fumarate binding to Arabidopsis NAD+-malic enzymes 1 and -2 produces an opposite activity modulation

Arabidopsis mitochondria contain two NAD(+)-malic enzymes, NAD-ME1 and NAD-ME2. These proteins have similar affinity for their substrates but display opposite regulation by fumarate, which strongly stimulates NAD-ME1 but inhibits NAD-ME2 activity. Here, the interaction of NAD-ME1 and -2 with fumarate was investigated by kinetic approaches, urea denaturation assays and intrinsic fluorescence quenching, in the absence and presence of NAD(+). Fumarate inhibited NAD-ME2 at saturating, but not at low, levels of NAD(+), and it behaved as competitive inhibitor with respect to L-malate. In contrast, NAD-ME1 fumarate activation was higher at suboptimal NAD(+) concentrations. In the absence of cofactor, the fluorescence of both NAD-ME1 and -2 is quenched by fumarate. However, for NAD-ME2 the quenching arises from a collisional phenomenon, while in NAD-ME1 the fluorescence decay can be explained by a static process that involves fumarate binding to the protein. Furthermore, the residue Arg84 of NAD-ME1 is essential for fumarate binding, as the mutant protein R84A exhibits a collisional quenching by this metabolite. Together, the results indicate that the differential fumarate regulation of Arabidopsis NAD-MEs, which is further modulated by NAD(+) availability, is related to the gaining of an allosteric site for fumarate in NAD-ME1 and an active site-associated inhibition by this C(4)-organic acid in NAD-ME2.

The role of proteins in C(3) plants prior to their recruitment into the C(4) pathway

Our most productive crops and native vegetation use a modified version of photosynthesis known as the C(4) pathway. Leaves of C(4) crops have increased nitrogen and water use efficiencies compared with C(3) species. Although the modifications to leaves of C(4) plants are complex, their faster growth led to the proposal that C(4) photosynthesis should be installed in C(3) crops in order to increase yield potential. Typically, a limited set of proteins become restricted to mesophyll or bundle sheath cells, and this allows CO(2) to be concentrated around the primary carboxylase RuBisCO. The role that these proteins play in C(3) species prior to their recruitment into the C(4) pathway is addressed here. Understanding the role of these proteins in C(3) plants is likely to be of use in predicting how the metabolism of a C(3) leaf will alter as components of the C(4) pathway are introduced as part of efforts to install characteristics of C(4) photosynthesis in leaves of C(3) crops.

Malate decarboxylases: evolution and roles of NAD(P)-ME isoforms in species performing C(4) and C(3) photosynthesis

In the C(4) pathway of photosynthesis two types of malate decarboxylases release CO(2) in bundle sheath cells, NADP- and NAD-dependent malic enzyme (NADP-ME and NAD-ME), located in the chloroplasts and the mitochondria of these cells, respectively. The C(4) decarboxylases involved in C(4) photosynthesis did not evolve de novo; they were recruited from existing housekeeping isoforms. NADP-ME housekeeping isoforms would function in the control of malate levels during hypoxia, pathogen defence responses, and microspore separation, while NAD-ME participates in the respiration of malate in the tricarboxylic acid cycle. Recently, the existence of three enzymatic NAD-ME entities in Arabidopsis, occurring by alternative association of two subunits, was described as a novel mechanism to regulate NAD-ME activity under changing metabolic environments. The C(4) NADP-ME is thought to have evolved from a C(3) chloroplastic ancestor, which in turn would have evolved from an ancient cytosolic enzyme. In this way, the C(4) NADP-ME would have emerged through gene duplication, acquisition of a new promoter, and neo-functionalization. In contrast, there would exist a unique NAD-ME in C(4) plants, which would have been adapted to perform a dual function through changes in the kinetic and regulatory properties of the C(3) ancestors. In addition to this, for the evolution of C(4) NAD-ME, insertion of promoters or enhancers into the single-copy genes of the C(3) ancestors would have changed the expression without gene duplication.

NAD-malic enzymes of Arabidopsis thaliana display distinct kinetic mechanisms that support differences in physiological control

The Arabidopsis thaliana genome contains two genes encoding NAD-MEs [NAD-dependent malic enzymes; NAD-ME1 (TAIR accession number At4G13560) and NAD-ME2 (TAIR accession number At4G00570)]. The encoded proteins are localized to mitochondria and assemble as homo- and hetero- dimers in vitro and in vivo. In the present work, the kinetic mechanisms of NAD-ME1 and -ME2 homodimers and NAD-MEH (NAD-ME heterodimer) were studied as an approach to understand the contribution of these enzymes to plant physiology. Product-inhibition and substrate-analogue analyses indicated that NAD-ME2 follows a sequential ordered Bi-Ter mechanism, NAD being the leading substrate followed by L-malate. On the other hand, NAD-ME1 and NAD-MEH can bind both substrates randomly. However, NAD-ME1 shows a preferred route that involves the addition of NAD first. As a consequence of the kinetic mechanism, NAD-ME1 showed a partial inhibition by L-malate at low NAD concentrations. The analysis of a protein chimaeric for NAD-ME1 and -ME2 indicated that the first 176 amino acids are associated with the differences observed in the kinetic mechanisms of the enzymes. Furthermore, NAD-ME1, -ME2 and -MEH catalyse the reverse reaction (pyruvate reductive carboxylation) with very low catalytic activity, supporting the notion that these isoforms act only in L-malate oxidation in plant mitochondria. The different kinetic mechanism of each NAD-ME entity suggests that, for a metabolic condition in which the mitochondrial NAD level is low and the L-malate level is high, the activity of NAD-ME2 and/or -MEH would be preferred over that of NAD-ME1.

Three different and tissue-specific NAD-malic enzymes generated by alternative subunit association in Arabidopsis thaliana

The Arabidopsis thaliana genome contains two genes encoding the mitochondrial NAD-malic enzyme (NAD-ME), NAD-ME1 (At2g13560) and NAD-ME2 (At4g00570). The characterization of recombinant NAD-ME1 and -2 indicated that both enzymes assemble as active homodimers; however, a heterodimeric enzyme (NAD-MEH) can also be detected by electrophoretic studies. To analyze the metabolic contribution of each enzymatic entity, NAD-MEH was obtained by a co-expression-based recombinant approach, and its kinetic and regulatory properties were analyzed. The three NAD-MEs show similar kinetic properties, although they differ in the regulation by several metabolic effectors. In this regard, whereas fumarate activates NAD-ME1 and CoA activates NAD-ME2, both compounds act synergistically on NAD-MEH activity. The characterization of two chimeric enzymes between NAD-ME1 and -2 allowed specific domains of the primary structure, which are involved in the differential allosteric regulation, to be identified. NAD-ME1 and -2 subunits showed a distinct pattern of accumulation in the separate components of the floral organ. In sepals, the NAD-ME1 subunit is present at a slightly higher proportion than the NAD-ME2 subunit, and thus, NAD-MEH and NAD-ME1 act in concert in this tissue. On the other hand, NAD-ME2 is the only isoform present in anthers. In view of the different properties of NAD-ME1, -2, and -H, we suggest that mitochondrial NAD-ME activity may be regulated by varying native association in vivo, rendering enzymatic entities with distinct allosteric regulation to fulfill specific roles. The presence of three different NAD-ME entities, which originate by alternative associations of two subunits, is suggested to be a novel phenomenon unique to plant mitochondria.

C acid decarboxylases required for C photosynthesis are active in the mid-vein of the C species Arabidopsis thaliana, and are important in sugar and amino acid metabolism

Cells associated with veins of petioles of C(3) tobacco possess high activities of the decarboxylase enzymes required in C(4) photosynthesis. It is not clear whether this is the case in other C(3) species, nor whether these enzymes provide precursors for specific biosynthetic pathways. Here, we investigate the activity of C(4) acid decarboxylases in the mid-vein of Arabidopsis, identify regulatory regions sufficient for this activity, and determine the impact of removing individual isoforms of each protein on mid-vein metabolite profiles. This showed that radiolabelled malate and bicarbonate fed to the xylem stream were incorporated into soluble and insoluble material in the mid-vein of Arabidopsis leaves. Compared with the leaf lamina, mid-veins possessed high activities of NADP-dependent malic enzyme (NADP-ME), NAD-dependent malic enzyme (NAD-ME) and phosphoenolpyruvate carboxykinase (PEPCK). Transcripts derived from both NAD-ME, one PCK and two of the four NADP-ME genes were detectable in these veinal cells. The promoters of each decarboxylase gene were sufficient for expression in mid-veins. Analysis of insertional mutants revealed that cytosolic NADP-ME2 is responsible for 80% of NADP-ME activity in mid-veins. Removing individual decarboxylases affected the abundance of amino acids derived from pyruvate and phosphoenolpyruvate. Reducing cytosolic NADP-ME activity preferentially affected the sugar content, whereas abolishing NAD-ME affected both the amino acid and the glucosamine content of mid-veins.

Arabidopsis NAD-malic enzyme functions as a homodimer and heterodimer and has a major impact on nocturnal metabolism

Although the nonphotosynthetic NAD-malic enzyme (NAD-ME) was assumed to play a central role in the metabolite flux through the tricarboxylic acid cycle, the knowledge on this enzyme is still limited. Here, we report on the identification and characterization of two genes encoding mitochondrial NAD-MEs from Arabidopsis (Arabidopsis thaliana), AtNAD-ME1 and AtNAD-ME2. The encoded proteins can be grouped into the two clades found in the plant NAD-ME phylogenetic tree. AtNAD-ME1 belongs to the clade that includes known alpha-subunits with molecular masses of approximately 65 kD, while AtNAD-ME2 clusters with the known beta-subunits with molecular masses of approximately 58 kD. The separated recombinant proteins showed NAD-ME activity, presented comparable kinetic properties, and are dimers in their active conformation. Native electrophoresis coupled to denaturing electrophoresis revealed that in vivo AtNAD-ME forms a dimer of nonidentical subunits in Arabidopsis. Further support for this conclusion was obtained by reconstitution of the active heterodimer in vitro. The characterization of loss-of-function mutants for both AtNAD-MEs indicated that both proteins also exhibit enzymatic activity in vivo. Neither the single nor the double mutants showed a growth or developmental phenotype, suggesting that NAD-ME activity is not essential for normal autotrophic development. Nevertheless, metabolic profiling of plants completely lacking NAD-ME activity revealed differential patterns of modifications in light and dark periods and indicates a major role for NAD-MEs during nocturnal metabolism.

Cloning and functional characterization of the 5′ flanking region of the human mitochondrial malic enzyme gene

This work reports the molecular cloning and functional characterization of the 5' flanking region of the human mitochondrial malic enzyme (mME) gene. The proximal promoter region has features of housekeeping genes like high G + C-content and absence of TATA or CCAAT boxes. Deletion analysis of the 5' region of the mME showed that maximal transcriptional activity is located within the -205/+86 region. Footprinting analysis showed two protected regions, one comprising potential overlapped AP-1, CREB, and AP-4 sites and a second one encompassing AP-2 and several Sp1 ci-acting elements. Mutation of putative AP-1/AP-4/CREB sites reduced basal promoter activity to less than 50%. Supershift assays demonstrated the specific binding of Sp1 and AP-2 proteins. Moreover, experiments in Drosophila SL2 cells lacking endogenous Sp1 demonstrated that the Sp1 site(s) is essential to maintain a normal basal rate of transcription of this gene. A low-level expression of AP-2 enhanced the activity of a mME promoter construct in HepG2 cells and this effect was prevented by disruption of the putative AP-2 element. In contrast, higher levels of expression of AP-2 induced a DNA-independent inhibitory response. A biphasic regulation of endogenous mME gene is also shown in HepG2 cells transfected with an AP-2 expression plasmid, suggesting that availability of AP-2 protein may control this gene under physiological conditions. A recombinant lambda genomic clone containing a mME pseudogene was also isolated. The high degree of sequence conservation seems to indicate a recent emergency of this human pseudogene.

NAD malic enzyme and the control of carbohydrate metabolism in potato tubers

Potato (Solanum tuberosum) plants were transformed with a cDNA encoding the 59-kD subunit of the potato tuber NAD-dependent malic enzyme (NADME) in the antisense orientation. Measurements of the maximum catalytic activity of NADME in tubers revealed a range of reductions in the activity of this enzyme down to 40% of wild-type activity. There were no detrimental effects on plant growth or tuber yield. Biochemical analyses of developing tubers indicated that a reduction in NADME activity had no detectable effects on flux through the tricarboxylic acid cycle. However, there was an effect on glycolytic metabolism with significant increases in the concentration of 3-phosphoglycerate and phosphoenolpyruvate. These results suggest that alterations in the levels of intermediates toward the end of the glycolytic pathway may allow respiratory flux to continue at wild-type rates despite the reduction in NADME. There was also a statistically significant negative correlation between NADME activity and tuber starch content, with tubers containing reduced NADME having an increased starch content. The effect on plastid metabolism may result from the observed glycolytic perturbations.

Tansley Review No. 37 Circadian rhythms: their origin and control

This article reviews the circadian rhythm of carbon dioxide metabolism in leaves of the Crassulacean plant Bryophyllum (Kalanchoë) fedtsckenkoi which persists both in continuous darkness and a CO₂ -free atmosphere, and in continuous light and normal air. Under both conditions the rhythm is due to the periodic activity of the enzyme phosphoenolpyruvate carboxylase (PEPc). The physiological characteristics of the rhythm are described in detail and, from these characteristics, hypotheses are advanced to account for both the generation of the rhythm and the regulation of its phase and period by environmental factors. The periodic activity of PEPc is ascribed to the periodic accumulation of an allosteric inhibitor, malate, in the cytoplasm and its subsequent removal either to the vacuole in continuous darkness, or by metabolism in continuous light. Also involved in the generation of the rhythm is a periodic change in the sensitivity of PEPc to malate inhibition due to the periodic phosphorylation and dephosphorylation of PEPc which changes its K₁ by a factor of 10 from 30 to 0.3 mM and vice versa. This periodic phosphorylation of PEPc is apparently achieved by the periodic synthesis and breakdown of a PEPc kinase which phosphorylates the enzyme on a serine residue; dephosphorylation is achieved by a type 2A phosphatase which shows no rhythmic variation. The induction of phase shifts in the rhythm in continuous darkness and CO₂ -free air has been explained in terms of light and high-temperature activated gates or channels in the tonoplast which, when open, allow malate to diffuse between the vacuole and cytoplasm. For the rhythm in continuous light and normal air phase, control by environmental signals can be attributed to changes in the malate levels in critical cell compartments, or in particular cell populations such as the stomatal guard cells, due to regulation of the malate synthesizing enzyme system involving PEPc, and malic enzyme which is responsible for malate metabolism. The role of the stomata in the generation of the rhythm is also discussed. The biochemical events which appear to give rise to the well-studied circadian rhythms in leaf movement in Samanea and Albizza, in luminescence in Gonyaulax polyedra and in the synthesis of the chlorophyll a/b binding protein are also reviewed in an attempt to identify similarities between these events and those involved in the Bryophyllum rhythm. Finally, the somewhat similar nature of the genes apparently responsible for circadian rhythmicity in Neurospora and Drosophila are discussed, and suggestions made for utilizing anti-sense nucleic acid technology in the further elucidation of the critical biochemical events involved in the basic, temperature-compensated circadian oscillator in living organisms. CONTENTS Summary 347 I. Introduction 348 II. Occurrence of circadian rhythms 348 III. Physiological characteristics of circadian rhythms 349 IV. Biochemical and molecular events involved in the circadian rhythm in Bryophyllum leaves 362 V. Biochemical and molecular events involved in the origin and control of circadian rhythmicity in other organisms 366 VI. Genetic studies 370 VII. Conclusion 371 References 372.

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.

Oligomeric enzymes in the C4 pathway of photosynthesis

This review deals with the factors controlling the aggregation-state of several enzymes involved in C4 photosynthesis, namely phosphoenolpyruvate carboxylase, NAD-and NADP-malic enzyme, NADP-malic dehydrogenase and pyruvate, phosphate dikinase and its regulatory protein. All of these enzymes are oligomeric and have been shown to undergo changes in their quaternary structure in vitro under different conditions. The activity changes linked to variations in aggregation-state are discussed in terms of their putative physiological role in the regulation of C4 metabolism.

Oxalacetate decarboxylase and pyruvate carboxylase activities, and effect of sulfhydryl reagents in malic enzyme from Sulfolobus solfataricus

Malic enzyme (S)-malate: NADP+ oxidoreductase (oxaloacetate-decarboxylating, EC 1.1.1.40) purified from the thermoacidophilic archaebacterium Sulfolobus solfataricus, strain MT-4, catalyzed the metal-dependent decarboxylation of oxaloacetate at optimum pH 7.6 at a rate comparable to the decarboxylation of L-malate. The oxaloacetate decarboxylase activity was stimulated about 50% by NADP but only in the presence of MgCl2, and was strongly inhibited by L-malate and NADPH which abolished the NADP activation. In the presence of MnCl2 and in the absence of NADP, the Michaelis constant and Vm for oxaloacetate were 1.7 mM and 2.3 mumol.min-1.mg-1, respectively. When MgCl2 replaced MnCl2, the kinetic parameters for oxaloacetate remained substantially unvaried, whereas the Km and Vm values for L-malate have been found to vary depending on the metal ion. The enzyme carried out the reverse reaction (malate synthesis) at about 70% of the forward reaction, at pH 7.2 and in the presence of relatively high concentrations of bicarbonate and pyruvate. Sulfhydryl residues (three cysteine residues per subunit) have been shown to be essential for the enzymatic activity of the Sulfolobus solfataricus malic enzyme. 5,5'-Dithiobis(2-nitrobenzoic acid), p-hydroxymercuribenzoate and N-ethylmaleimide caused the inactivation of the oxidative decarboxylase activity, but at different rates. The inactivation of the overall activity by p-hydroxymercuribenzoate was partially prevented by NADP singly or in combination with both L-malate and MnCl2, and strongly enhanced by the carboxylic acid substrates; NADP + malate + MnCl2 afforded total protection. The inactivation of the oxaloacetate decarboxylase activity by p-hydroxymercuribenzoate treatment was found to occur at a slower rate than that of the oxidative decarboxylase activity.