Cloning and structural characterization of the 6-phosphogluconate dehydrogenase locus of the medfly Ceratitis capitata and the olive fruit fly Bactrocera oleae

The pentose phosphate cycle is considered as a major source of NADPH and pentose needed for nucleic acid biosynthesis. 6-Phosphogluconate dehydrogenase (6PGD), an enzyme participating in this cycle, catalyzes the oxidative decarboxylation of 6PGD to ribulose 5-phosphate with the subsequent release of CO(2) and the reduction of NADP. We have determined the genomic sequences of 6PGD of two species of Tephritidae, the medfly Ceratitis capitata and olive fruit fly Bactrocera oleae, and constructed a three-dimensional model of 6PGD of C. capitata based on the homologous known sheep structure. In a comparative study of 6PGD sequences from seven species, all the conserved and variable sites of the enzyme were analyzed and the regions of functional importance were localized, an attempt promoted also by the direct involvement of the enzyme in various human diseases. The enzymes between the two species of Tephritidae have a very high homology and further examination of the variable positions with respect to the highly conserved binding site residues enabled their grouping in three distinct categories, with possible association to dimer formation, functional specificity, and antigenicity. Moreover, placement of sequence differences on the 3-D model suggests probable sites accommodating variations appearing at the allozymic variants of both species.     

The synthesis of N‐benzoylindoles as inhibitors of rat erythrocyte glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase

Glucose‐6‐phosphate dehydrogenase (G6PD) and 6‐phosphogluconate dehydrogenase (6PGD) play an important function in various biochemical processes as they generate reducing power of the cell. Thus, metabolic reprogramming of reduced nicotinamide adenine dinucleotide phosphate (NADPH) homeostasis is reported to be a vital step in cancer progression as well as in combinational therapeutic approaches. In this study, N‐benzoylindoles 9a‐ ‐ 9d , which form the main framework of many natural indole derivatives such as indomethacin and N‐benzoylindoylbarbituric acid, were synthesized through three easy and effective steps as an in vitro inhibitor effect of G6PD and 6PGD. The N‐benzoylindoles inhibited the enzymatic activity with IC₅₀ in the range of 3.391505 μM for G6PD and 2.19–990 μM for 6PGD.     

Conformational changes associated with cofactor/substrate binding of 6-phosphogluconate dehydrogenase from Escherichia coli and Klebsiella pneumoniae: Implications for enzyme mechanism

6-Phosphogluconate dehydrogenase (6PGDH), the third enzyme of the pentose phosphate pathway, catalyzes the oxidative decarboxylation of 6-phosphogluconate, making ribulose 5-phosphate, along with the reduction of NADP⁺ to NADPH and the release of CO₂. Here, we report the first apo-form crystal structure of the pathogenic Klebsiella pneumoniae 6PGDH (Kp6PGDH) and the structures of the highly homologous Escherichia coli K12 6PGDH (Ec6PGDH) complexed with substrate, substrate/NADPH and glucose at high resolution. The binding of NADPH to one subunit of the homodimeric structure triggered a 10° rotation and resulting in a 7 Å movement of the coenzyme-binding domain. The coenzyme was thus trapped in a closed enzyme conformation, in contrast to the open conformation of the neighboring subunit. Comparison of our Ec/Kp6PGDH structures with those of other species illustrated how the domain conformation can be affected upon binding of the coenzyme, which in turn gives rise to concomitant movements of two important NADP⁺-interacting amino acids, M14 and N102. We propose that the catalysis follows an ordered binding mechanism with alternating conformational changes in the corresponding subunits, involving several related amino acid residues.     

Xylose reductase from the thermophilic fungus Talaromyces emersonii: cloning and heterologous expression of the native gene (Texr) and a double mutant (TexrK271R + N273D) with altered coenzyme specificity

Xylose reductase is involved in the first step of the fungal pentose catabolic pathway. The gene encoding xylose reductase (Texr) was isolated from the thermophilic fungus Talaromyces emersonii, expressed in Escherichia coli and purified to homogeneity. Texr encodes a 320 amino acid protein with a molecular weight of 36 kDa, which exhibited high sequence identity with other xylose reductase sequences and was shown to be a member of the aldoketoreductase (AKR) superfamily with a preference for reduced nicotinamide adenine dinucleotide phosphate (NADPH) as coenzyme. Given the potential application of xylose reductase enzymes that preferentially utilize the reduced form of nicotinamide adenine dinucleotide (NADH) rather than NADPH in the fermentation of five carbon sugars by genetically engineered microorganisms, the coenzyme selectivity of TeXR was altered by site-directed mutagenesis. The TeXR^K271R+N273D double mutant displayed an altered coenzyme preference with a 16-fold improvement in NADH utilization relative to the wild type and therefore has the potential to reduce redox imbalance of xylose fermentation in recombinant S. cerevisiae strains. Expression of Texr was shown to be inducible by the same carbon sources responsible for the induction of genes encoding enzymes relevant to lignocellulose hydrolysis, suggesting a coordinated expression of intracellular and extracellular enzymes relevant to hydrolysis and metabolism of pentose sugars in T. emersonii in adaptation to its natural habitat. This indicates a potential advantage in survival and response to a nutrient-poor environment.     

Purification and Kinetic Properties of 6-Phosphogluconate Dehydrogenase from Rat Small Intestine

6-Phosphogluconate dehydrogenase (6PG) was purified from rat small intestine with 36% yield and a specific activity of 15 U/mg. On SDS/PAGE, one band with a mass of 52 kDa was found. On native PAGE three protein and two activity bands were observed. The pH optimum was 7.35. Using Arrhenius plots, Ea, ΔH, Q₁₀ and Tm for 6PGD were found to be 7.52 kcal/mol, 6.90 kcal/mol, 1.49 and 49.4°C, respectively. The enzyme obeyed “Rapid Equilibrium Random Bi Bi” kinetic model with K_m values of 595 ± 213 μM for 6PG and 53.03±1.99 μM for NADP. 1/V_m versus 1/6PG and 1/NADP plots gave a V_m value of 8.91±1.92 U/mg protein. NADPH is the competitive inhibitor with a K_i of 31.91±1.31 μM. The relatively small K_i for the 6PGD:NADPH complex indicates the importance of NADPH in the regulation of the pentose phosphate pathway through G6PD and 6PGD.     

Disorders in NADPH generation via pentose phosphate pathway influence the reproductive potential of the Saccharomyces cerevisiae yeast due to changes in redox status

Intermediary metabolites have a crucial impact on basic cell functions. There is a relationship between cellular metabolism and redox balance. To maintain redox homoeostasis, the cooperation of both glutathione and nicotine adenine dinucleotides is necessary. Availability of nicotinamide adenine dinucleotide phosphate (NADPH) as a major electron donor is critical for many intracellular redox reactions. The activity of glucose-6-phosphate dehydrogenase (Zwf1p) and 6-phosphogluconate dehydrogenase (Gnd1p and Gnd2p) is responsible for NADPH formation in a pentose phosphate (PP) pathway. In this study, we examine the impact of redox homoeostasis on cellular physiology and proliferation. We have noted that the Δzwf1 mutant lacking the rate-limiting enzyme of the PP pathway shows changes in the cellular redox status caused by disorders in NADPH generation. This leads to a decrease in reproductive potential but without affecting the total lifespan of the cell. The results presented in this paper show that nicotine adenine dinucleotides play a central role in cellular physiology.     

Phenylpyruvic Acid decreases glucose-6-phosphate dehydrogenase activity in rat brain

Phenylketonuria is a recessive autosomal disorder that is caused by a deficiency in the activity of phenylalanine-4-hydroxylase, which converts phenylalanine to tyrosine, leading to the accumulation of phenylalanine and its metabolites phenyllactic acid, phenylacetic acid, and phenylpyruvic acid in the blood and tissues of patients. Phenylketonuria is characterized by severe neurological symptoms, but the mechanisms underlying brain damage have not been clarified. Recent studies have shown the involvement of oxidative stress in the neuropathology of hyperphenylalaninemia. Glucose-6-phosphate dehydrogenase plays an important role in antioxidant defense because it is the main source of reduced nicotinamide adenine dinucleotide phosphate (NADPH), providing a reducing power that is essential in protecting cells against oxidative stress. Therefore, the present study investigated the in vitro effect of phenylalanine (0.5, 1, 2.5, and 5?mM) and its metabolites phenyllactic acid, phenylacetic acid, and phenylpyruvic acid (0.2, 0.6, and 1.2?mM) on the activity of enzymes of the pentose phosphate pathway, which is involved in the oxidative phase in rat brain homogenates. 6-Phosphogluconate dehydrogenase activity was not altered by any of the substances tested. Phenylalanine, phenyllactic acid, and phenylacetic acid had no effect on glucose-6-phosphate dehydrogenase activity. Phenylpyruvic acid significantly reduced glucose-6-phosphate dehydrogenase activity without pre-incubation and after 1?h of pre-incubation with the homogenates. The inhibition of glucose-6-phosphate dehydrogenase activity caused by phenylpyruvic acid could elicit an impairment of NADPH production and might eventually alter the cellular redox status. The role of phenylpyruvic acid in the pathophysiological mechanisms of phenylketonuria remains unknown.     

植物戊糖磷酸途径及其两个关键酶的研究进展

戊糖磷酸途径是植物体中糖代谢的重要途径,主要生理功能是产生供还原性生物合成需要的NADPH,可供核酸代谢的磷酸戊糖以及一些中间产物可参与氨基酸合成和脂肪酸合成等.葡萄糖-6-磷酸脱氢酶和6-磷酸葡萄糖酸脱氢酶是戊糖磷酸途径的两个关键酶,广泛的分布于高等植物的胞质和质体中.本文综述了植物戊糖磷酸途径及其两个关键酶的分子生物学的研究进展,讨论了该途径在植物生长发育和环境胁迫应答中的作用.     

植物戊糖磷酸途径及其两个关键酶的研究进展

戊糖磷酸途径是植物体中糖代谢的重要途径,主要生理功能是产生供还原性生物合成需要的NADPH,可供核酸代谢的磷酸戊糖以及一些中间产物可参与氨基酸合成和脂肪酸合成等。葡萄糖-6-磷酸脱氢酶和6-磷酸葡萄糖酸脱氢酶是戊糖磷酸途径的两个关键酶,广泛的分布于高等植物的胞质和质体中。本文综述了植物戊糖磷酸途径及其两个关键酶的分子生物学的研究进展,讨论了该途径在植物生长发育和环境胁迫应答中的作用。     

Purification and Characterization of the Two 6-Phosphogluconate Dehydrogenase Species from Pseudomonas multivorans

The two species of 6-phosphogluconate dehydrogenase (EC 1.1.1.43) from Pseudomonas multivorans were resolved from extracts of gluconate-grown bacteria and purified to homogeneity. Each enzyme comprised between 0.1 and 0.2% of the total cellular protein. Separation of the two enzymes, one which is specific for nicotinamide adenine dinucleotide phosphate and the other which is active with nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate was facilitated by the marked difference in their respective isoelectric points, which were at pH 5.0 and 6.9. Comparison of the subunit compositions of the two enzymes indicated that they do not share common peptide chains. The enzyme active with nicotinamide adenine dinucleotide was composed of two subunits of about 40,000 molecular weight, and the nicotinamide adenine dinucleotide phosphate-specific enzyme was composed of two subunits of about 60,000 molecular weight. Immunological studies indicated that the two enzymes do not share common antigenic determinants. Reduced nicotinamide adenine dinucleotide phosphate strongly inhibited the 6-phosphogluconate dehydrogenase active with nicotinamide adenine dinucleotide by decreasing its affinity for 6-phosphogluconate. Guanosine-5'-triphosphate had a similar influence on the nicotinamide adenine dinucleotide phosphate-specific 6-phosphogluconate dehydrogenase. These results in conjunction with other data indicating that reduced nicotinamide adenine dinucleotide phosphate stimulates the conversion of 6-phosphogluconate to pyruvate by crude bacterial extracts suggest that in P. multivorans, the relative distribution of 6-phosphogluconate into the pentose phosphate and Entner-Doudoroff pathways might be determined by the intracellular concentrations of reduced nicotinamide adenine dinucleotide phosphate and purine nucleotides.     

Purification and kinetic characterization of 6-phosphogluconate dehydrogenase from Schizosaccharomyces pombe.

6-Phosphogluconate dehydrogenase is the pivotal enzyme that links the gluconate route and the oxidative phase of the pentose phosphate pathway in Schizosaccharomyces pombe. The enzyme differs from the known 6-phosphogluconate dehydrogenases of other sources in that the Schizosaccharomyces enzyme is tetrameric having a subunit mass of 38 kDa, that it requires NADP+ obligatorily for activity, and that it can be activated by divalent metal ions such as Co2+ and Mn2+. Steady-state kinetic studies were undertaken. Initial rate and product inhibition results suggest that 6-phosphogluconate dehydrogenase from Schizosaccharomyces pombe catalyzes NADP(+)-linked oxidative decarboxylation of 6-phosphogluconate by an equilibrium random mechanism with two independent binding sites, namely one site for the nicotinamide coenzyme, NADP+/NADPH, and another site for 6-phosphogluconate-D-ribulose-5-phosphate and for CO2. Studies of pH dependence implicated a basic residue with a pK value of 7.4 in the binding of 6-phosphogluconate and an acidic residue with a pK value of 6.7 in the cation-mediated interaction of NADP+ with the enzyme.     

The role of 6-phosphogluconate dehydrogenase in Rhizobium.

A nicotinamide adenine dinucleotide (NAD) linked 6-phosphogluconate (6-PG)dehydrogenase has been detected in Rhizobium. The enzyme activity is similar in both slow- and fast-growing rhizobia. The nicotinamide adenine dinucleotide phosphate (NADP) dependent 6-PG dehydrogenase was detected only in the fast growers and was more than twice as active as the NAD-linked enzyme. Partial characterization of the products of 6-PG oxidation in Rhizobium suggests that the NADP-linked enzyme is the decarboxylating enzyme of the pentose phosphate (PP) pathway (EC 1.1.1.44) whereas a phosphorylated six-carbon compound, containing ketonic group(s), is the product of the oxidation catalyzed by the NAD-linked enzyme.     

6-Phosphogluconate dehydrogenase regulates tumor cell migration in vitro by regulating receptor tyrosine kinase c-Met

6-Phosphogluconate dehydrogenase (6PGD) is the third enzyme in the oxidative pentose phosphate pathway (PPP). Recently, we reported that knockdown of 6PGD inhibited lung tumor growth in vitro and in a xenograft model in mice. In this study, we continued to examine the functional role of 6PGD in cancer. We show that 6PGD expression positively correlates with advancing stage of lung carcinoma. In search of functional signals related to 6PGD, we discovered that knockdown of 6PGD significantly inhibited phosphorylation of c-Met at tyrosine residues known to be critical for activity. This downregulation of c-Met phosphorylation correlated with inhibition of cell migration in vitro. Overexpression of a constitutively active c-Met specifically rescued the migration but not proliferation phenotype of 6PGD knockdown. Therefore, 6PGD appears to be required for efficient c-Met signaling and migration of tumor cells in vitro.     

Characterization of Plasmodium falciparum 6-Phosphogluconate Dehydrogenase as an Antimalarial Drug Target

The enzyme 6-phosphogluconate dehydrogenase (6PGD) of the malaria parasite Plasmodium falciparum catalyzes the third step of the pentose phosphate pathway converting 6-phosphogluconate (6PG) to ribulose 5-phosphate. The NADPH produced by 6PGD is crucial for antioxidant defense and redox regulation, and ribose 5-phosphate is essential for DNA and RNA synthesis in the rapidly growing parasite. Thus, 6PGD represents an attractive antimalarial drug target. In this study, we present the X-ray structures of Pf6PGD in native form as well as in complex with 6PG or nicotinamide adenine dinucleotide phosphate (NADP⁺) at resolutions of 2.8, 1.9, and 2.9?Å, respectively. The overall structure of the protein is similar to structures of 6PGDs from other species; however, a flexible loop close to the active site rearranges upon binding of 6PG and likely regulates the conformation of the cofactor NADP⁺. Upon binding of 6PG, the active site loop adopts a closed conformation. In the absence of 6PG, the loop opens and NADP⁺ is bound in a waiting position, indicating that the cofactor and 6PG bind independently from each other. This sequential binding mechanism was supported by kinetic studies on the homodimeric wild-type Pf6PGD. Furthermore, the function of the Plasmodium-specific residue W104L mutant was characterized by site-directed mutagenesis. Notably, the activity of Pf6PGD was found to be post-translationally redox regulated via S-nitrosylation, and screening the Medicines for Malaria Venture Malaria Box identified several compounds with IC₅₀s in the low micromolar range. Together with the three-dimensional structure of the protein, this is a promising starting point for further drug discovery approaches.     

Induction and Regulation of a Nicotinamide Adenine Dinucleotide-Specific 6-Phosphogluconate Dehydrogenase in Streptococcus faecalis

Streptococcus faecalis grown with glucose as the primary energy source contains a single, nicotinamide adenine dinucleotide phosphate (NADP)-specific 6-phosphogluconate dehydrogenase. Extracts of gluconate-adapted cells, however, exhibited 6-phosphogluconate dehydrogenase activity with either NADP or nicotinamide adenine dinucleotide (NAD). This was shown to be due to the presence of separate enzymes in gluconate-adapted cells. Although both enzymes catalyzed the oxidative decarboxylation of 6-phosphogluconate, they differed from one another with respect to their coenzyme specificity, molecular weight, pH optimum, K(m) values for substrate and coenzyme, and electrophoretic mobility in starch gels. The two enzymes also differed in their response to certain effector ligands. The NADP-linked enzyme was specifically inhibited by fructose-1,6-diphosphate, but was insensitive to adenosine triphosphate (ATP) and certain other nucleotides. The NAD-specific enzyme, in contrast, was insensitive to fructose-1,6-diphosphate, but was inhibited by ATP. The available data suggest that the NAD enzyme is involved primarily in the catabolism of gluconate, whereas the NADP enzyme appears to function in the production of reducing equivalents (NADPH) for use in various reductive biosynthetic reactions.     

Enzymes limiting butanol and acetone formation in continuous and batch cultures of Clostridium acetobutylicum

Summary The formation of butanol in continuous cultures of Clostridium acetobutylicum is regulated at the genetic level via expression of butyraldehyde dehydrogenase since increased in vitro activities of this key enzyme are associated with increased in vivo butanol formation rates in both acidogenic and solventogenic fermentations. Addition of glucose, butyric acid and carbon monoxide results in induction of butyraldehyde dehydrogenase. The production of acetone in continuous fermentation is also controlled at the genetic level through expression of coenzyme A (CoA)-transferase; this enzyme is induced by glucose. Carbon monoxide inactivates acetoacetate decarboxylase. In controlled-pH batch fermentation solventogenesis does not correlate with in vitro activities of butyraldehyde dehydrogenase. Instead, initiation of alcohol formation is accompanied by increased activities of both reduced nicotine adenine dinucleotide (NADH)- and reduced nicotine adenine dinucleotide phosphate (NADPH)-specific alcohol dehydrogenases. The production of acetone in batch fermentation is regulated at the genetic level through combined induction of both CoA-transferase and acetoacetate decarboxylase. These two enzymes are not detected in either batch or continuous culture at or above pH 6.0. This finding explains the inability of the cells to produce acetone at elevated culture pH.     

Pentose phosphate metabolism during dormancy breakage in Corylus avellana L.

Commercially obtained fruits of Corylus avellana exhibit the characteristic loss of dormancy of this seed following chilling under moist conditions. The activities of cytosolic and organellar enzymes of pentose phosphate pathway in cotyledonary tissue were assayed throughout stratification and over a similar period in damp vermiculite at 20° C. Glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconic acid dehydrogenase (6PGDH) were both found in cytosolic extracts in all treatments; only 6PGDH was present in the organellar fraction.The enzyme activities monitored in seeds at 20° C remained relatively constant over the course of the investigation except in the case of cytosolic 6PGDH where it is suggested an inhibitor of the enzyme accumulated. This inhibitor was removed by the partial purification procedure. Increases in the activities of the enzymes occurred during stratification, the major increase coinciding exactly with dormancy breakage but prior to the initiation of germination. The marked increase in G6PDH and 6PGDH concurrent with the change in germination potential of the chilled seed may have considerable biochemical significance in breaking down the dormant state.Abbreviations G6P glucose-6-phosphate - G6PDH glucose-6 phosphate dehydrogenase - NADP nicotinamide adenine dinucleotide phosphate - 6 PGDH 6-phosphogluconic acid dehydrogenase - PPP pentose phosphate pathway     

Glucose-6-phosphate dehydrogenase of Anabaena sp.

The kinetic and molecular properties of cyanobacterial glucose-6-phosphate dehydrogenase, partly purified from Anabaena sp. ATCC 27893, show that it undergoes relatively slow, reversible transitions between different aggregation states which differ in catalytic activity. Sucrose gradient centrifugation and polyacrylamide gel electrophoresis reveal three principal forms, with approximate molecular weights of 120 000 (M ₁), 240 000 (M ₂) and 345 000 (M ₃). The relative catalytic activities are: M ₁M ₂<M ₃. In concentrated solutions of the enzyme, the equilibrium favors the more active, oligomeric forms. Dilution in the absence of effectors shifts the equilibrium in favor of the M ₁ form, with a marked diminution of catalytic activity. This transition is prevented by a substrate, glucose-6-phosphate, and also by glutamine. The other substrate, nicotinamide adenine dinucleotide phosphate (NADP⁺), and (in crude cell-free extracts) ribulose-1,5-diphosphate are negative effectors, which tend to maintain the enzyme in the M ₁ form. The equilibrium state between different forms of the enzyme is also strongly dependent on hydrogen ion concentration. Although the optimal pH for catalytic activity is 7.4, dissociation to the hypoactive M ₁ form is favored at pH values above 7; a pH of 6.5 is optimal for maintenace of the enzyme in the active state. Reduced nicotamide adenine dinucleotide phosphate (NADPH) and adenosine 5-triphosphate (ATP), inhibit catalytic activity, but do not significantly affect the equilibrium state. The relevance of these findings to the regulation of enzyme activity in vivo is discussed.Abbreviations G6PD glucose-6-phosphate dehydrogenase - 6PGD 6-phosphogluconate dehydrogenase - RUDP ribulose-1,5-diphosphate - G6P glucose-6-phosphate - 6PG 6-phosphogluconate     

Neuroprotection by glucose-6-phosphate dehydrogenase and the pentose phosphate pathway

Glucose-6-phosphate dehydrogenase (G6PD), the rate limiting enzyme that channels glucose catabolism from glycolysis into the pentose phosphate pathway (PPP), is vital for the production of reduced nicotinamide adenine dinucleotide phosphate (NADPH) in cells. NADPH is in turn a substrate for glutathione reductase, which reduces oxidized glutathione disulfide to sulfhydryl glutathione. Best known for inherited deficiencies underlying acute hemolytic anemia due to elevated oxidative stress by food or medication, G6PD, and PPP activation have been associated with neuroprotection. Recent works have now provided more definitive evidence for G6PD's protective role in ischemic brain injury and strengthened its links to neurodegeneration. In Drosophila models, improved proteostasis and lifespan extension result from an increased PPP flux due to G6PD induction, which is phenocopied by transgenic overexpression of G6PD in neurons. Moderate transgenic expression of G6PD was also shown to improve healthspan in mouse. Here, the deciphered and implicated roles of G6PD and PPP in protection against brain injury, neurodegenerative diseases, and in healthspan/lifespan extensions are discussed together with an important caveat, namely NADPH oxidase (NOX) activity and the oxidative stress generated by the latter. Activation of G6PD with selective inhibition of NOX activity could be a viable neuroprotective strategy for brain injury, disease, and aging.     

Evidence for dimer/tetramer equilibrium in Trypanosoma brucei 6-phosphogluconate dehydrogenase

6-Phosphogluconate dehydrogenase (6PGDH), the third enzyme of the pentose phosphate pathway (PPP), is essential for biosyntheses and oxidative stress defence. It also has the function of depleting 6PG, whose accumulation induces cell senescence. 6PGDH is a proposed drug target for African trypanosomiasis caused by Trypanosoma brucei and for other microbial infections and cancer. Gel filtration, density gradient sedimentation, cross-linking and dynamic light scattering were used to assay the oligomerization state of T. brucei 6PGDH in the absence and presence of several ligands. The enzyme displays a dimer–tetramer equilibrium and NADPH (but not NADP) reduces the rate of approach to equilibrium, while 6PG is able to antagonize the NADPH effect. The different behaviour of the two forms of coenzyme appears to be related to the differences in ΔC_p, with NADP binding ΔC_p closer to what is expected of crystallographic structures, while NADPH ΔC_p is three times larger. The estimated dimer–tetramer association constant is 1.5 · 10⁶ M^? 1, and the specific activity of the tetramer is about 3 fold higher than the specific activity of the dimer. Thus, cellular conditions promoting tetramer formation could allow an efficient clearing of 6PG. Experiments carried out on sheep liver 6PGDH indicate that tetramerization is a specificity of the parasite enzyme.