A novel glucose-6-phosphate dehydrogenase in Plasmodium falciparum: cDNA and primary protein structure

Fused Enzyme Glucose-6-Phosphate Dehydrogenase::6-Phosphogluconolactonase (G6PD::6PGL) as a Potential Drug Target in Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum

Several microaerophilic parasites such as Giardia lamblia, Trichomonas vaginalis, and Plasmodium falciparum are major disease-causing organisms and are responsible for spreading infections worldwide. Despite significant progress made in understanding the metabolism and molecular biology of microaerophilic parasites, chemotherapeutic treatment to control it has seen limited progress. A current proposed strategy for drug discovery against parasitic diseases is the identification of essential key enzymes of metabolic pathways associated with the parasite's survival. In these organisms, glucose-6-phosphate dehydrogenase::6-phosphogluconolactonase (G6PD:: 6PGL), the first enzyme of the pentose phosphate pathway (PPP), is essential for its metabolism. Since G6PD:: 6PGL provides substrates for nucleotides synthesis and NADPH as a source of reducing equivalents, it could be considered an anti-parasite drug target. This review analyzes the anaerobic energy metabolism of G. lamblia, T. vaginalis, and P. falciparum, with a focus on glucose metabolism through the pentose phosphate pathway and the significance of the fused G6PD:: 6PGL enzyme as a therapeutic target in the search for new drugs.

Glucose-6-phosphate metabolism in Plasmodium falciparum

Malaria is still one of the most threatening diseases worldwide. The high drug resistance rates of malarial parasites make its eradication difficult and furthermore necessitate the development of new antimalarial drugs. Plasmodium falciparum is responsible for severe malaria and therefore of special interest with regard to drug development. Plasmodium parasites are highly dependent on glucose and very sensitive to oxidative stress; two observations that drew interest to the pentose phosphate pathway (PPP) with its key enzyme glucose-6-phosphate dehydrogenase (G6PD). A central position of the PPP for malaria parasites is supported by the fact that human G6PD deficiency protects to a certain degree from malaria infections. Plasmodium parasites and the human host possess a complete PPP, both of which seem to be important for the parasites. Interestingly, there are major differences between parasite and human G6PD, making the enzyme of Plasmodium a promising target for antimalarial drug design. This review gives an overview of the current state of research on glucose-6-phosphate metabolism in P. falciparum and its impact on malaria infections. Moreover, the unique characteristics of the enzyme G6PD in P. falciparum are discussed, upon which its current status as promising target for drug development is based.

NMR spectroscopic analysis of the first two steps of the pentose-phosphate pathway elucidates the role of 6-phosphogluconolactonase

The pentose-phosphate pathway provides reductive power and nucleotide precursors to the cell through oxidative and nonoxidative branches, respectively. 6-Phosphogluconolactonase is the second enzyme of the oxidative branch and catalyzes the hydrolysis of 6-phosphogluconolactones, the products of glucose 6-phosphate oxidation by glucose-6-phosphate dehydrogenase. The role of 6-phosphogluconolactonase was still questionable, because 6-phosphogluconolactones were believed to undergo rapid spontaneous hydrolysis. In this work, nuclear magnetic resonance spectroscopy was used to characterize the chemical scheme and kinetic features of the oxidative branch. We show that 6-phosphogluconolactones have in fact a nonnegligible lifetime and are highly electrophilic compounds. The delta form (1-5) of the lactone is the only product of glucose 6-phosphate oxidation. Subsequently, it leads to the gamma form (1-4) by intramolecular rearrangement. However, only the delta form undergoes spontaneous hydrolysis, the gamma form being a "dead end" of this branch. The delta form is the only substrate for 6-phosphogluconolactonase. Therefore, 6-phosphogluconolactonase activity accelerates hydrolysis of the delta form, thus preventing its conversion into the gamma form. Furthermore, 6-phosphogluconolactonase guards against the accumulation of delta-6-phosphogluconolactone, which may be toxic through its reaction with endogenous cellular nucleophiles. Finally, the difference between activity of human, Trypanosoma brucei, and Plasmodium falciparum 6-phosphogluconolactonases is reported and discussed.

Glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase. A novel bifunctional enzyme in malaria parasites

Plasmodium falciparum glucose 6-phosphate dehydrogenase (Pf Glc6PD), compared to other Glc6PDs has an additional 300 amino acids at the N-terminus. They are not related to Glc6PD but are similar to a family of proteins (devb) of unknown function, some of which are encoded next to Glc6PD in certain bacteria. The human devb homologue has recently been shown to have 6-phosphogluconolactonase (6PGL) activity. This suggests Pf Glc6PD may be a bifunctional enzyme, the evolution of which has involved the fusion of adjacent genes. Further functional analysis of Pf Glc6PD has been hampered because parts of the gene could not be cloned. We have isolated and sequenced the corresponding Plasmodium berghei gene and shown it encodes an enzyme (Pb Glc6PD) with the same structure as the P. falciparum enzyme. Pb Glc6PD is 950 amino acids long with significant sequence similarity in both the devb and Glc6PD domains with the P. falciparum enzyme. The P. berghei enzyme does not have an asparagine-rich segment between the N and C halves and it contains an insertion at the same point in the Glc6PD region as the P. falciparum enzyme but the insertion in the P. berghei is longer (110 versus 62 amino acids) and unrelated in sequence to the P. falciparum insertion. Though expression of this enzyme in bacteria produced largely insoluble protein, conditions were found where the full-length enzyme was produced in a soluble form which was purified via a histidine tag. We show that this enzyme has both Glc6PD and 6PGL activities. Thus the first two steps of the pentose phosphate pathway are catalysed by a single novel bifunctional enzyme in these parasites.

The Pseudomonas aeruginosa devB/SOL homolog, pgl, is a member of the hex regulon and encodes 6-phosphogluconolactonase

A cyclic version of the Entner-Doudoroff pathway is used by Pseudomonas aeruginosa to metabolize carbohydrates. Genes encoding the enzymes that catabolize intracellular glucose to pyruvate and glyceraldehyde 3-phosphate are coordinately regulated, clustered at 39 min on the chromosome, and collectively form the hex regulon. Within the hex cluster is an open reading frame (ORF) with homology to the devB/SOL family of unidentified proteins. This ORF encodes a protein of either 243 or 238 amino acids; it overlaps the 5' end of zwf (encodes glucose-6-phosphate dehydrogenase) and is followed immediately by eda (encodes the Entner-Doudoroff aldolase). The devB/SOL homolog was inactivated in P. aeruginosa PAO1 by recombination with a suicide plasmid containing an interrupted copy of the gene, creating mutant strain PAO8029. PAO8029 grows at 9% of the wild-type rate using mannitol as the carbon source and at 50% of the wild-type rate using gluconate as the carbon source. Cell extracts of PAO8029 were specifically deficient in 6-phosphogluconolactonase (Pgl) activity. The cloned devB/SOL homolog complemented PAO8029 to restore normal growth on mannitol and gluconate and restored Pgl activity. Hence, we have identified this gene as pgl and propose that the devB/SOL family members encode 6-phosphogluconolactonases. Interestingly, three eukaryotic glucose-6-phosphate dehydrogenase (G6PDH) isozymes, from human, rabbit, and Plasmodium falciparum, contain Pgl domains, suggesting that the sequential reactions of G6PDH and Pgl are incorporated in a single protein. 6-Phosphogluconolactonase activity is induced in P. aeruginosa PAO1 by growth on mannitol and repressed by growth on succinate, and it is expressed constitutively in P. aeruginosa PAO8026 (hexR). Taken together, these results establish that Pgl is an essential enzyme of the cyclic Entner-Doudoroff pathway encoded by pgl, a structural gene of the hex regulon.

Glucose metabolism in Chlamydia trachomatis: the 'energy parasite' hypothesis revisited

Chlamydia trachomatis is an obligate intracellular eubacteria that is dependent on a eukaryotic host cell for a variety of metabolites. For years, it has been speculated that chlamydiae are energy parasites, totally dependent on their host cell for ATP and other high-energy intermediates. To determine whether C. trachomatis contains functional enzymes that produce energy or reducing power, four enzymes involved in glycolysis or the pentose phosphate pathway, specifically pyruvate kinase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase, were cloned, sequenced and expressed as recombinant proteins in Escherichia coli. The deduced amino acid sequences obtained show high homology to other pyruvate kinase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase enzymes. In contrast to numerous other bacterial species, chlamydial glycolytic genes are not arranged in an operon, but are dispersed throughout the genome. Results from reverse transcriptase-polymerase chain reaction (RT-PCR) analysis indicate that all four genes are maximally expressed in the middle of the chlamydial developmental cycle. The chlamydial genes are capable of complementing mutant E. coli strains lacking the respective enzyme activities. In vitro enzyme analysis indicates that recombinant chlamydial enzymes expressed in E. coli are active and, interestingly, recombinant chlamydial pyruvate kinase is not regulated allosterically by fructose 1,6 bisphosphate or AMP, as found with other bacterial pyruvate kinases. In summary, identification and characterization of these glucose-catabolizing enzymes indicate that chlamydia contains the functional capacity to produce its own ATP and reducing power.

Purification, localisation and characterisation of glucose-6-phosphate dehydrogenase of Trypanosoma brucei

Cell-fractionation and digitonin titration of procyclic trypomastigotes of Trypanosoma brucei, revealed that almost half of the total NADP+ -dependent glucose-6-phosphate dehydrogenase (G6PDH) activity, the first enzyme of the pentose phosphate pathway (PPP), is associated with glycosomes. The specific activity of G6PDH in the purified organelles was increased 4-fold relative to a total cell extract and showed latency. Moreover, in the absence of detergents this activity was totally resistant to the action of trypsin. The cytosolic counterpart was neither latent, nor was it resistant to trypsin. Both cytosolic and glycosomal G6PDH activities behaved identically on phenyl-, CM-, heparin-, and Affigel-blue-Sepharose columns. Both isoenzymes had a subunit Mr of 62 000 and an isoelectric point of 6.85, while kinetic studies carried out on the partially purified G6PDH from both cell compartments did not reveal any differences. The purified enzyme had an apparent Km of 138 and 5.3 microM for glucose 6-phosphate (G6P), and for NADP+, respectively, and had a specific activity of 14 micromol. (min mg of protein)(-1). We conclude that while in procyclic stages of T. brucei G6PDH activity is present in two different cell compartments, i.e. the cytosol and the glycosomes, these two activities most likely represent one and the same isoenzyme.

Amino acid conservation and clinical severity of human glucose-6-phosphate dehydrogenase mutations

More than a hundred naturally occurring mutations of human glucose-6-phosphate dehydrogenase (G6PD) have been identified at the amino acid level. The abundance of distinct mutation sites and their clinical manifestations make this enzyme ideal for structure-function analysis studies. We present here a sequence and structure combined analysis by which the severity of clinical symptoms resulting from point mutations of this enzyme is correlated with quantified degrees of amino acid conservation within 23 G6PD sequences from different organisms. Our analysis verifies, on a quantitative basis, a widely held notion that clinically severer mutations of G6PD usually occur at conserved amino acids. However, marked exceptions to this general trend exist which are most notably revealed by a number of mutations associated with chronic nonspherocytic hemolytic anemia (class I variants). When mapped onto a homology-derived structural model of human G6PD, these class I mutational sites of low amino acid conservation appear to localize in two spatially distinct clusters, both of which are populated with mutations consisting mainly of clinically severer variants (i.e. class I and class II). These results of computer-assisted analyses contribute to a further understanding of the structure-function relationships of human G6PD deficiency.