Kinetic and Structural Properties of NADP-Malic Enzyme from Sugarcane Leaves

Increased [CO2] Causes Changes in Physiological and Genetic Responses in C4 Crops: A Brief Review

Climate change not only worries government representatives and organizations, but also attracts the attention of the scientific community in different contexts. In agriculture specifically, the cultivation and productivity of crops such as sugarcane, maize, and sorghum are influenced by several environmental factors. The effects of high atmospheric concentration of carbon dioxide ([CO₂]) have been the subject of research investigating the growth and development of C₄ plants. Therefore, this brief review presents some of the physiological and genetic changes in economically important C₄ plants following exposure periods of increased [CO₂] levels. In the short term, with high [CO₂], C₄ plants change photosynthetic metabolism and carbohydrate production. The photosynthetic apparatus is initially improved, and some responses, such as stomatal conductance and transpiration rate, are normally maintained throughout the exposure. Protein-encoding genes related to photosynthesis, such as the enzyme phosphoenolpyruvate carboxylase, to sucrose accumulation and to biomass growth and are differentially regulated by [CO₂] increase and can variably participate owing to the C₄ species and/or other internal and external factors interfering in plant development. Despite the consensus among some studies, mainly on physiological changes, further studies are still necessary to identify the molecular mechanisms modulated under this condition. In addition, considering future scenarios, the combined effects of high environmental and [CO₂] stresses need to be investigated so that the responses of maize, sugarcane, and sorghum are better understood.

Posttranslational Modification of the NADP-Malic Enzyme Involved in C4 Photosynthesis Modulates the Enzymatic Activity during the Day

Evolution of the C4 photosynthetic pathway involved in some cases recruitment of housekeeping proteins through gene duplication and their further neofunctionalization. NADP-malic enzyme (ME), the most widespread C4 decarboxylase, has increased its catalytic efficiency and acquired regulatory properties that allowed it to participate in the C4 pathway. Here, we show that regulation of maize (Zea mays) C4-NADP-ME activity is much more elaborate than previously thought. Using mass spectrometry, we identified phosphorylation of the Ser419 residue of C4-NADP-ME in protein extracts of maize leaves. The phosphorylation event increases in the light, with a peak at Zeitgeber time 2. Phosphorylation of ZmC4-NADP-ME drastically decreases its activity as shown by the low residual activity of the recombinant phosphomimetic mutant. Analysis of the crystal structure of C4-NADP-ME indicated that Ser419 is involved in the binding of NADP at the active site. Molecular dynamics simulations and effective binding energy computations indicate a less favorable binding of the cofactor NADP in the phosphomimetic and the phosphorylated variants. We propose that phosphorylation of ZmC4-NADP-ME at Ser419 during the first hours in the light is a cellular mechanism that fine tunes the enzymatic activity to coordinate the carbon concentration mechanism with the CO2 fixation rate, probably to avoid CO2 leakiness from bundle sheath cells.

Molecular adaptations of NADP-malic enzyme for its function in C4 photosynthesis in grasses

In C₄ grasses of agronomical interest, malate shuttled into the bundle sheath cells is decarboxylated mainly by nicotinamide adenine dinucleotide phosphate (NADP)-malic enzyme (C₄-NADP-ME). The activity of C₄-NADP-ME was optimized by natural selection to efficiently deliver CO₂ to Rubisco. During its evolution from a plastidic non-photosynthetic NADP-ME, C₄-NADP-ME acquired increased catalytic efficiency, tetrameric structure and pH-dependent inhibition by its substrate malate. Here, we identified specific amino acids important for these C₄ adaptions based on strict differential conservation of amino acids, combined with solving the crystal structures of maize and sorghum C₄-NADP-ME. Site-directed mutagenesis and structural analyses show that Q503, L544 and E339 are involved in catalytic efficiency; E339 confers pH-dependent regulation by malate, F140 is critical for the stabilization of the oligomeric structure and the N-terminal region is involved in tetramerization. Together, the identified molecular adaptations form the basis for the efficient catalysis and regulation of one of the central biochemical steps in C₄ metabolism.

Plastidial NADP-malic enzymes from grasses: unraveling the way to the C4 specific isoforms

Malic enzyme is present in many plant cell compartments such as plastids, cytosol and mitochondria. Particularly relevant is the plastidial isoform that participates in the C(4) cycle providing CO(2) to RuBisCO in C(4) species. This type of photosynthesis is more frequent among grasses where anatomical preconditioning would have facilitated the evolution of the C(4) syndrome. In maize (C(4) grass), the photosynthetic NADP dependent Malic enzyme (ZmC(4)-NADP-ME, l-malate:NADP oxidoreductase, E.C. 1.1.1.40) and the closest related non-photosynthetic isoform (ZmnonC(4)-NADP-ME, l-malate:NADP oxidoreductase, E.C. 1.1.1.40) are both plastidial but differ in expression pattern, kinetics and structure. Features like high catalytic efficiency, inhibition by high malate concentration at pH 7.0, redox modulation and tetramerization are characteristic of the photosynthetic NADP-ME. In this work, the proteins encoded by sorghum (C(4) grass) and rice (C(3) grass) NADP-ME genes, orthologues of the plastidial NADP-MEs from maize, were recombinantly expressed, purified and characterized. In a global comparison, we could identify a small group of residues which may explain the special features of C(4) enzymes. Overall, the present work presents biochemical and molecular data that helps to elucidate the changes that took place in the evolution of C(4) NADP-ME in grasses.

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.

The activities of PEP carboxylase and the C4 acid decarboxylases are little changed by drought stress in three C4 grasses of different subtypes

The C4 photosynthetic pathway involves the assimilation of CO2 by phosphoenolpyruvate carboxylase (PEPC) and the subsequent decarboxylation of C4 acids. The enzymes of the CO2 concentrating mechanism could be affected under water deficit and limit C4 photosynthesis. Three different C4 grasses were submitted to gradually induced drought stress conditions: Paspalum dilatatum (NADP-malic enzyme, NADP-ME), Cynodon dactylon (NAD-malic enzyme, NAD-ME) and Zoysia japonica (PEP carboxykinase, PEPCK). Moderate leaf dehydration affected the activity and regulation of PEPC in a similar manner in the three grasses but had species-specific effects on the C4 acid decarboxylases, NADP-ME, NAD-ME and PEPCK, although changes in the C4 enzyme activities were small. In all three species, the PEPC phosphorylation state, judged by the inhibitory effect of L-malate on PEPC activity, increased with water deficit and could promote increased assimilation of CO2 by the enzyme under stress conditions. Appreciable activity of PEPCK was observed in all three species suggesting that this enzyme may act as a supplementary decarboxylase to NADP-ME and NAD-ME in addition to its role in other metabolic pathways.

Nicotiana tabacum NADP-malic enzyme: cloning, characterization and analysis of biological role

NADP-malic enzyme (NADP-ME) catalyzes the oxidative decarboxylation of L-malate, producing pyruvate, CO2 and NADPH. The photosynthetic role of this enzyme in C(4) and Crassulacean acid metabolism (CAM) plants has been well established; however, the biological role of several non-photosynthetic isoforms described in C(3), C(4) and CAM plants is still speculative. In this study, the characterization of the NADP-ME isoforms from Nicotiana tabacum was performed. Three different nadp-me transcripts were identified in this C(3) plant, two of which encode for putative cytosolic isoforms (DQ923118 and EH663836), while the third encodes for a plastidic counterpart (DQ923119). Although the three transcripts are expressed in vegetative as well as in reproductive tissues, they display different levels of expression. With regards to enzyme activity, root is the tissue that displays the highest NADP-ME activity. Recombinant NADP-MEs encoded by DQ923118 and DQ923119 were expressed in Escherichia coli and their kinetic parameters and response to different metabolic effectors were analyzed. Studies carried out with crude extracts and with the recombinant proteins indicate that the cytosolic and plastidic isoforms aggregate as tetramers of subunits of 65 and 63 kDa, respectively. Real-time reverse transcription-PCR studies show that the three nadp-me tobacco transcripts respond differently to several biotic and abiotic stress stimuli. Finally, the physiological role of each isoform is discussed in terms of the occurrence, kinetic properties and response to stress. The structure of the NADP-ME family in tobacco is compared with those of other C(3) species.

Conformational changes of maize and wheat NADP-malic enzyme studied by quenching of protein native fluorescence

Quenching of tryptophan fluorescence of maize and wheat NADP-malic enzyme by KI and acrylamide was studied after denaturating proteins with guanidine hydrochloride, and subjecting them to different pH values or temperatures. Protein unfolding by guanidine hydrochloride resulted in a red shift of the fluorescence spectrum, providing further support for the motion that several of the tryptophan residues evolved from an apolar to a polar environment. Protein denaturation was accompanied by an increase in the effective dynamic quenching constant values and by loss of the enzyme's activities. Thermal denaturation gave results consistent with the ones observed for chemical denaturation suggesting that a putative intermediate is involved in the denaturation process. Finally, exposure of both enzymes at various pH values allowed us to infer the number of accessible tryptophan residues in the different oligomeric conformations. The results suggest that the aggregation process seems to be different for each enzyme. Thus, as the maize enzyme associated from monomer to tetramer, one tryptophan residue would change from a polar to an apolar environment, while the association of the wheat enzyme would cause that two tryptophan residues to be excluded from quenching. Hitherto, quenching of the tryptophan fluorescence provides a good tool for studying conformational changes of proteins. The future availability of the crystal structures of plant NADP-malic enzymes will offer a good validation point for our model and the technology used.

The Enzyme Kinetics of the NADP-Malic Enzyme from Tobacco Leaves

Malic enzyme (L-malate: NADP+ oxidoreductase (oxaloacetate-decarboxylating), EC 1.1.1.40, NADP-ME), which was found in chloroplasts, was isolated from tobacco leaves (Nicotiana tabacum L.) almost homogenous. The specific enzyme activity was 0.95 μmol min-1 mg-1. The enzyme pH optimum was found between pH 7.1 and 7.4. The affinity of NADP-ME to substrates (L-malate and NADP+) was evaluated in the presence of divalent metal ions (Mg2+, Mn2+, Co2+, Ni2+). The value of the apparent Michaelis constant of NADP-ME for L-malate was dependent on the ion cofactor, while no such relationship was found for NADP+. The dependence of the reaction rate on concentration of Mg2+ indicates the presence of more than one binding site for these ions in NADP-ME. Likewise, the sigmoidal dependence of the reaction rate on Mn2+ concentration and the value of Hill coefficient 7.5 indicate the positive cooperativity of the reaction kinetics in the presence of the ions. The effect of Co2+ and Ni2+ ions was analogous to that of Mn2+ ions; however, the cooperativity was lower (the values of Hill coefficients were 3.0 and 1.3 for Co2+ and Ni2+, respectively). Regulation of NADP-ME from tobacco leaves by divalent metal ions is discussed.

Identification of domains involved in tetramerization and malate inhibition of maize C4-NADP-malic enzyme

C(4) photosynthetic NADP-malic enzyme (ME) has evolved from non-C(4) isoforms and gained unique kinetic and structural properties during this process. To identify the domains responsible for the structural and kinetic differences between maize C(4) and non-C(4)-NADP-ME several chimeras between these isoforms were constructed and analyzed. By using this approach, we found that the region flanked by amino acid residues 102 and 247 is critical for the tetrameric state of C(4)-NADP-ME. In this way, the oligomerization strategy of these NADP-ME isoforms differs markedly from the one that present non-plant NADP-ME with known crystal structures. On the other hand, the region from residue 248 to the C-terminal end of the C(4) isoform is involved in the inhibition by high malate concentrations at pH 7.0. The inhibition pattern of the C(4)-NADP-ME and some of the chimeras suggested an allosteric site responsible for such behavior. This pH-dependent inhibition could be important for regulation of the C(4) isoform in vivo, with the enzyme presenting maximum activity while photosynthesis is in progress.

Characterization of NADP-dependent malic enzyme from developing castor oil seed endosperm

Metabolic pathways sequestered within the leucoplast of developing oilseeds ensure a balanced supply of substrates and cofactors for fatty acid biosynthesis. NADP-dependent malic enzyme (NADP-ME) may be important in supplying both carbon and NADPH for fatty acid biosynthesis in the developing endosperm of the oilseed Ricinus communis. NADP-ME was purified 5160-fold to a specific activity of 18.2 U/mg protein. NADP-ME is a homotetramer with a native mass of 254 kDa and a subunit size of approximately 63 kDa. Effectors of castor NADP-ME are typical of the NADP-malic enzymes, with the exception of acetyl-CoA and its derivatives, which were found to act as activators. This is consistent with a regulatory role for these molecules during fatty acid biosynthesis in vivo. NADP-ME was found to have maximal activity at stage 7 of endosperm development, coincident with maximal lipid accumulation.

Non-photosynthetic 'malic enzyme' from maize: a constituvely expressed enzyme that responds to plant defence inducers

The characterization of a non-photosynthetic isoform of NADP-malic enzyme (NADP-ME) from maize roots, which represents nearly 7% of the total soluble protein of this tissue, was performed. The molecular properties of the purified protein, as well as the kinetic parameters determined, indicate that the NADP-ME isoform present in maize roots differs from the photosynthetic enzyme implicated in the C4 cycle, but is similar, or identical, to the enzyme previously characterized from etiolated maize leaves (Maurino, Drincovich and Andreo, Biochem. Mol. Biol. Int. 38 (1996) 239-250). A full-length ORF encoding a plastidic NADP-ME (almost identical to the maize root NADP-ME, GenBank accession number U39958) was cloned from a root cDNA library as well as isolated by reverse transcription (RT)-PCR using green leaves mRNA as template. These results indicate that root NADP-ME does not constitute a root-specific isoform, but represents a protein with a constitutive pattern of expression in plastids of the C4 plant maize. The amount of NADP-ME measured by activity, western and northern blot was modified when different stress conditions (including treatments with cellulase, fungal elicitors, jasmonate and hypoxic treatment) were applied to maize roots, indicating that the enzyme from maize roots is under transcriptional or post-transcriptional regulation by effectors related to plant defence responses. It is deduced that the induction of housekeeping genes, like non-photosynthetic NADP-ME, whose constitutive role may be the provision of reductive power in non-photosynthetic plastids, is likely to accompany the defence response.

Ontogenic changes in enzymes of carbon metabolism in relation to carbohydrate status in developing mungbean reproductive structures

The content of free sugars and the activities of enzymes involved in carbon metabolism-sucrose synthase, acid and alkaline invertase, phosphoenol pyruvate carboxylase, malic enzyme and isocitrate dehydrogenase were determined during seed development in mungbean pods. A decrease in carbohydrate content of pod wall from 10 to 25 days after flowering (DAF) and a concomitant increase in the seed till 20 DAF was observed. Sucrose remained the dominant soluble sugar in the pod wall and seed. In the branch of inflorescence and pod wall, the activities of sucrose metabolizing enzymes, viz. acid and alkaline invertase, sucrose synthase (synthesis and cleavage) and sucrose phosphate synthase were higher at 5-10 DAF, whereas in seed the maximum activities of these enzymes were observed at the time of maximum seed filling stage (10-20 DAF). High activities of sucrose synthase at the time of rapid seed filling can be correlated to its sink strength. Higher activities of phosphoenol pyruvate carboxylase in the branch of inflorescence and pod wall than in seed may indicate the involvement of the fruiting structure for recapturing respired CO2. High activities of isocitrate dehydrogenase and malic enzyme in the seed at the time of rapid seed filling could provide NADPH and carbon skeletons required for the synthesis of various seed reserves.

Factors affecting the oligomeric state of NADP-malic enzyme from maize and wheat tissues: a chemical crosslinking study

The different aggregational states of maize and wheat NADP-malic enzyme as affected by pH, temperature and various metabolites have been studied by the combined use of intersubunit crosslinking and denaturing polyacrylamide gel electrophoresis. The association/dissociation equilibrium is a pH-dependent process: pH values above 8.0 promote the tetramer formation, while lowering the pH shifts the equilibria towards dimers and monomers. Below pH 6.0, most molecules exist as monomers. In the same way, the temperature governs the equilibria between the different oligomeric states. As the temperature is lowered from 42 to 0 degrees C, a progressive dissociation into dimers and monomers is observed. Excess enthalpies are negative in all cases, but the overall process demands an input of Gibb's free energy. Consequently, the protein dissociation is an entropy-driven process. The presence of Mg2+ or glycerol induces aggregation in both enzymes, while increasing the ionic strength produces the opposite effect. The results suggest that changes in the equilibria between monomer, dimer and tetramer of NADP-malic enzyme could be the molecular basis for an effective regulation of the enzyme activity in vivo.

NADP-malic enzyme from the C4 plant Flaveria bidentis: nucleotide substrate specificity

NADP-malic enzyme (NADP-ME, EC 1.1.1.40) was purified to near-homogeneity from leaves of the C4 dicot Flaveria bidentis and shown to possess intrinsic NAD-dependent malic enzyme activity. The NAD-dependent activity is optimal at pH 7.5 and in the presence of Mn2+. The Km for NAD is very high (20 mM), while the Vmax is 50% greater than the Vmax with NADP under the same conditions. The NAD-dependent activity is competitively inhibited by micromolar concentrations of NADP and NADPH (Ki approximately 2 microM). This very low Ki reflects the high affinity of malic enzyme for NADP(H) under these conditions. When utilizing NADP, the Km for NADP is 1.5 microM while the Ki for NADPH is 2 microM. Chicken liver NADP-ME also has NAD-dependent activity that is inhibited by low concentrations of NADPH. These results indicate that the NAD- and NADP-dependent activities are likely catalyzed by the same active site. The use of NAD as an alternative coenzyme revealed interactions between the binding of coenzyme and metal ions on the Km values of each of the other participants in the malic enzyme reaction. Thus, the affinity of malic enzyme for the divalent metal ions Mg2+ and particularly Mn2+ as well as the other substrate L-malate is also dependent on the nucleotide coenzyme substrate. In turn, the divalent metal ion influences the affinity of the enzyme for the coenzyme as well as L-malate. With NADP as substrate the Km for Mn2+ is 4 microM, whereas with NAD the Km is 300 microM. The relatively high affinity of the enzyme for Mn2+ and low affinity for NAD required the use of metal ion buffers when determining these values because of the substantial depletion of free Mn2+ caused by binding to NADP.

Kinetic mechanism of NADP-malic enzyme from maize leaves

The kinetic mechanism of NADP-dependent malic enzyme purified from maize leaves was studied in the physiological direction. Product inhibition and substrate analogues studies with 3' aminopyridine dinucleotide phosphate and tartrate indicate that the enzyme reaction follows a sequential ordered Bi-Ter kinetic mechanism. NADP is the leading substrate followed by L-malate and the products are released in the order of CO2, pyruvate and NADPH. The enzyme also catalyzes a slow, magnesium-dependent decarboxylation of oxaloacetate and reduction of pyruvate and oxaloacetate in the presence of NADPH to produce L-lactate and L-malate, respectively.

Interaction of analogues of substrate with NADP-malic enzyme from maize leaves

The effect of structural analogues of L-malate was studied on NADP-malic enzyme purified from Zea mays L. leaves. Among the compounds tested, the organic acids behaved as more potent inhibitors at pH 7.0 than at pH 8.0, suggesting that the dimeric form was more susceptible to the inhibition than the tetrameric form of the enzyme.Oxalate, ketomalonate, hydroxymalonate, malonate, oxaloacetate, tartrate, α-hydroxybutyrate, α-ketobutyrate, α-ketoglutarate and α-hydroxyglutarate exhibited linear competitive inhibition with respect to the substrate L-malate at pH 8.0. On the other hand, glyoxylate and glycolate turned out to be non-competitive inhibitors, while glycolaldehyde, succinate, fumarate, maleate and β- and γ-hydroxybutyrate had no effect on the enzyme activity, at the concentrations assayed. These results suggest that the extent of inhibition was dependent on the size of the analogues and that the presence of an 1-carboxyl group along with a 2-hydroxyl or 2-keto group was important for binding of the substrate analogue to the enzyme.

Hysteretic properties of NADP-malic enzyme from sugarcane leaves

NADP-malic enzyme highly purified from sugarcane leaves exhibited hysteretic properties. This behavior resulted in a lag phase during activity measurement of the enzyme preincubated in the absence of substrates. The lag was inversely proportional to the protein concentration during preincubation, which suggests that changes in the aggregational state of the enzyme are responsible for hysteresis. The pH conditions as well as the presence of different compounds in the preincubation medium modified the hysteretic properties of the enzyme. Mg(2+) eliminated the lag period and increased the enzyme activity by nearly 2-fold. NADP(+), 3-phosphoglycerate, ATP and dithiothreitol shortened the lag phase. The substrate L-malate inhibited the enzyme by decreasing the steady state velocity and increasing the lag time in a concentration-dependent manner. NADPH, triose-phosphates and high ionic strength increased the lag phase. Results are consistent with the view that the level of different metabolites and the pH conditions at the chloroplast regulate the activity of NADP-malic enzyme in a coordinate and effective manner.

Purification and characterization of the cytosolic NADP(+)-dependent malic enzyme from human breast cancer cell line

Cytosolic NADP(+)-dependent malic enzyme from a cultured human breast cancer cell line was purified to near homogeneity by two highly efficient chromatography systems: Pharmacia-LKB Q-Sepharose anion-exchange chromatography and adenosine-2',5'-bisphosphate-agarose affinity chromatography. The overall yield was 27%. The enzyme is presumably a tetramer composed of four probably identical subunits of Mr 65,000, which is similar to the enzyme from other sources. The pI and optimum reaction pH values for the tumor malic enzyme are 5.5 and 7.2, respectively. At pH 6.9, most of the enzyme exists as monomers. Activation energy for the enzyme-catalyzed oxidative-decarboxylation reaction is 57.4 kJ/mol. The enzyme is strictly NADP+ dependent, as NAD+ cannot support the oxidative-decarboxylation reaction. ATP at low concentration inhibits the enzyme activity. Fumarate at concentrations up to 5 mM does not affect the enzymatic reaction rate. Therefore the tumor cytosolic malic enzyme, unlike the mitochondrial malic enzyme, is not an allosteric regulatory enzyme.

NADP(+)-malic enzyme from sugarcane leaves: structural properties studied by thermal inactivation

The irreversible thermal inactivation of the sugarcane leaf NADP(+)-malic enzyme was studied at 50 degrees C and pH 7.0 and 8.0. Depending on the preincubation conditions, thermal inactivation followed mono- or biphasic first-order kinetics. A two-step behavior in the irreversible denaturation process was found when protein concentration was sufficiently low. The protein concentration necessary to obtain monlphasic thermal inactivation kinetics was lower at pH 8.0 than at pH 7.0. The results suggest that biphasic inactivation kinetics are the consequence of the existence of two different oligomeric forms of the enzyme (dimer and tetramer), with the dimer being more stable in regards to thermal inactivation. The effects of the substrate and essential cofactors on the thermostability and equilibrium between the dimeric and tetrameric enzyme forms were also studied. Depending on the pH, NADP+, L-malate, and Mg2+ all had a protective effect on the stability of the dimeric and tetrameric species during thermal treatment. However, these ligands showed different effects on the aggregation state of the enzyme. NADP+ and L-malate induced dissociation, especially at pH 8.0, whereas Mg2+ induced aggregation of the protein. By studying the thermal inactivation kinetics at 50 degrees C and different pH values it was observed that the equilibrium between dimers and tetramers was dramatically affected in the range of pH 7.0-8.0. These results suggest that an amino acid residue(s) in the protein with an apparent pKa value of 7.7 needs to be deprotonated to stabilize aggregation of the enzyme to the tetrameric form.

Analogues of NADP(+) as inhibitors and coenzymes for NADP(+) malic enzyme from maize leaves

Structural analogues of the NADP(+) were studied as potential coenzymes and inhibitors for NADP(+) dependent malic enzyme from Zea mays L. leaves. Results showed that 1, N(6)-etheno-nicotinamide adenine dinucleotide phosphate (∈ NADP(+)), 3-acetylpyridine-adenine dinucleotide phosphate (APADP(+)), nicotinamide-hypoxanthine dinucleotide phosphate (NHDP(+)) and β-nicotinamide adenine dinucleotide 2': 3'-cyclic monophosphate (2'3'NADPc(+)) act as alternate coenzymes for the enzyme and that there is little variation in the values of the Michaelis constants and only a threefold variation in Vmax for the five nucleotides. On the other hand, thionicotinamide-adenine dinucleotide phosphate (SNADP(+)), 3-aminopyridine-adenine dinucleotide phosphate (AADP(+)), adenosine 2'-monophosphate (2'AMP) and adenosine 2': 3'-cyclic monophosphate (2'3'AMPc) were competitive inhibitors with respect to NADP(+), while β-nicotinamide adenine dinucleotide 3'-phosphate (3'NADP(+)), NAD(+), adenosine 3'-monophosphate (3'AMP), adenosine 2': 5'-cyclic monophosphate (2'5'AMPc), 5'AMP, 5'ADP, 5'ATP and adenosine act as non-competitive inhibitors. These results, together with results of semiempirical self-consistent field-molecular orbitals calculations, suggest that the 2'-phosphate group is crucial for the nucleotide binding to the enzyme, whereas the charge density on the C4 atom of the pyridine ring is the major factor that governs the coenzyme activity.

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.

NADP-dependent malate dehydrogenase (decarboxylating) from sugar cane leaves. Kinetic properties of different oligomeric structures

NADP-dependent malate dehydrogenase (decarboxylating) from sugar cane leaves was inhibited by increasing the ionic strength in the assay medium. The inhibitory effect was higher at pH 7.0 than 8.0, with median inhibitory concentrations (IC50) of 89 mM and 160 mM respectively, for inhibition by NaCl. Gel-filtration experiments indicated that the enzyme dissociated into dimers and monomers when exposed to high ionic strength (0.3 M NaCl). By using the enzyme-dilution approach in the absence and presence of 0.3 M NaCl, the kinetic properties of each oligomeric species of the protein was determined at pH 7.0 and 8.0. Tetrameric, dimeric and monomeric structures were shown to be active but with different V and Km values. The catalytic efficiency of the oligomers was tetramer greater than dimer greater than monomer, and each quaternary structure exhibited higher activity at pH 8.0 than 7.0. Dissociation constants for the equilibria between the different oligomeric forms of the enzyme were determined. It was established that Kd values were affected by pH and Mg2+ levels in the medium. Results suggest that the distinct catalytic properties of the different oligomeric forms of NADP-dependent malate dehydrogenase and changes in their equilibrium could be the molecular basis for an efficient physiological regulation of the decarboxylation step of C4 metabolism.