Purification and characterization of glutamine:fructose 6-phosphate amidotransferase from rat liver

Microplastics weaken the exoskeletal mechanical properties of Pacific whiteleg shrimp Litopenaeus vannamei

The ubiquitous presence of microplastics (MPs) in aquatic environments poses a significant threat to crustaceans. Although exoskeleton quality is critical for crustacean survival, the impact of MPs on crustacean exoskeletons remains elusive. Our study represents a pioneering effort to characterize the effects of MPs exposure on crustacean exoskeletons. In this study, the mechanical properties of whiteleg shrimp Litopenaeus vannamei exoskeletons were analyzed after exposure to environmentally realistic levels of MPs. Nanoindentation data demonstrated that MPs exposure significantly increased the hardness and modulus of both the carapace and abdominal segments of L. vannamei. Moreover, fractures and embedded MPs were detected on the exoskeleton surface using SEM-EDS analysis. Further analysis demonstrated that the degree of chitin acetylation (DA) in the shrimp exoskeleton, as indicated by FTIR peaks, was reduced by MPs exposure. In addition, exposure to MPs significantly inhibited the muscle Ca2+-ATPase activity and hemolymph calcium levels. Transcriptome and metabolome analyses revealed that the expression levels of genes encoding key enzymes and metabolites in the chitin biosynthetic pathway were significantly affected by MPs exposure. In conclusion, MPs at environmentally relevant concentrations may affect the exoskeletal mechanical properties of L. vannamei through a comprehensive mechanism involving the disruption of the crystalline structure of chitin, assimilation into the exoskeleton, and dysregulation of exoskeleton biosynthesis-related pathways.

Glucose transport, transporters and metabolism in diabetic retinopathy

Diabetic retinopathy (DR) is the most common reason for blindness in working-age individuals globally. Prolonged high blood glucose is a main causative factor for DR development, and glucose transport is prerequisite for the disturbances in DR caused by hyperglycemia. Glucose transport is mediated by its transporters, including the facilitated transporters (glucose transporter, GLUTs), the "active" glucose transporters (sodium-dependent glucose transporters, SGLTs), and the SLC50 family of uniporters (sugars will eventually be exported transporters, SWEETs). Glucose transport across the blood-retinal barrier (BRB) is crucial for nourishing the neuronal retina in the context of retinal physiology. This physiological process primarily relies on GLUTs and SGLTs, which mediate the glucose transportation across both the cell membrane of retinal capillary endothelial cells and the retinal pigment epithelium (RPE). Under diabetic conditions, increased accumulation of extracellular glucose enhances the retinal cellular glucose uptake and metabolism via both glycolysis and glycolytic side branches, which activates several biochemical pathways, including the protein kinase C (PKC), advanced glycation end-products (AGEs), polyol pathway and hexosamine biosynthetic pathway (HBP). These activated biochemical pathways further increase the production of reactive oxygen species (ROS), leading to oxidative stress and activation of Poly (ADP-ribose) polymerase (PARP). The activated PARP further affects all the cellular components in the retina, and finally resulting in microangiopathy, neurodegeneration and low-to-moderate grade inflammation in DR. This review aims to discuss the changes of glucose transport, glucose transporters, as well as its metabolism in DR, which influences the retinal neurovascular unit (NVU) and implies the possible therapeutic strategies for treating DR.

Enzymatic and structural properties of human glutamine:fructose-6-phosphate amidotransferase 2 (hGFAT2)

Glycoconjugates play a central role in several cellular processes, and alteration in their composition is associated with numerous human pathologies. Substrates for cellular glycosylation are synthesized in the hexosamine biosynthetic pathway, which is controlled by the glutamine:fructose-6-phosphate amidotransfera-se (GFAT). Human isoform 2 GFAT (hGFAT2) has been implicated in diabetes and cancer; however, there is no information about structural and enzymatic properties of this enzyme. Here, we report a successful expression and purification of a catalytically active recombinant hGFAT2 (rhGFAT2) in Escherichia coli cells fused or not to a HisTag at the C-terminal end. Our enzyme kinetics data suggest that hGFAT2 does not follow the expected ordered bi–bi mechanism, and performs the glucosamine-6-phosphate synthesis much more slowly than previously reported for other GFATs. In addition, hGFAT2 is able to isomerize fructose-6-phosphate into glucose-6-phosphate even in the presence of equimolar amounts of glutamine, which results in unproductive glutamine hydrolysis. Structural analysis of a three-dimensional model of rhGFAT2, corroborated by circular dichroism data, indicated the presence of a partially structured loop in the glutaminase domain, whose sequence is present in eukaryotic enzymes but absent in the E. coli homolog. Molecular dynamics simulations suggest that this loop is the most flexible portion of the protein and plays a key role on conformational states of hGFAT2. Thus, our study provides the first comprehensive set of data on the structure, kinetics, and mechanics of hGFAT2, which will certainly contribute to further studies on the (patho)physiology of hGFAT2.

Azide- and Alkyne-Bearing Metabolic Chemical Reporters of Glycosylation Show Structure-Dependent Feedback Inhibition of the Hexosamine Biosynthetic Pathway

Metabolic chemical reporters (MCRs) of protein glycosylation are analogues of natural monosaccharides that bear reactive groups, like azides and alkynes. When they are added to living cells and organisms, these small molecules are biosynthetically transformed into nucleotide donor sugars and then used by glycosyltransferases to modify proteins. Subsequent installation of tags by bioorthogonal chemistries can then enable the visualization and enrichment of these glycoproteins. Although this two-step procedure is powerful, the use of MCRs has the potential to change the endogenous production of the natural repertoire of donor sugars. A major route for the generation of these glycosyltransferase substrates is the hexosamine biosynthetic pathway (HBP), which results in uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). Interestingly, the rate-determining enzyme of the HBP, glutamine fructose-6-phosphate amidotransferase (GFAT), is feedback inhibited by UDP-GlcNAc. This raises the possibility that a build-up of UDP-MCRs would block the biosynthesis of UDP-GlcNAc, resulting in off target effects. Here, we directly test this possibility with recombinant human GFAT and a small panel of synthetic UDP-MCRs. We find that MCRs with larger substitutions at the N-acetyl position do not inhibit GFAT, whereas those with modifications of the 2- or 6-hydroxy group do. These results further illuminate the considerations that should be applied to the use of MCRs.

Glucosamine: fructose-6-phosphate amidotransferase in the white shrimp Litopenaeus vannamei: characterization and regulation under alkaline and cadmium stress

Heavy metal residues and chemical contaminators considered as relevant sources of aquatic environmental pollutants have a generally immunosuppressive effect on aquatic organisms, depressing metabolic activities and immune response. Glutamine: fructose-6-phosphate aminotransferase (GFAT, EC2.6.1.16) is the first, and rate-limiting, enzyme in the hexosamine biosynthetic pathway, and is involved in the regulation of chitin biosynthesis and glycosylation of proteins. We have isolated and characterized GFAT from the white shrimp Litopenaeus vannamei. Amino acid sequence similarity of the Lv-GFAT (L.vannamei-GFAT) was highest to GFATs isolated from insects and mammals (83 % similarity to that of Haemaphysalis longicornis). The open-reading frame of the Lv-GFAT codes for a protein of 41.6 kDa with a calculated isoelectric point of 5.03. RT-PCR assays showed that endogenous Lv-GFAT mRNA is most strongly expressed in the intestine. Further analysis of Lv-GFAT gene expression in hepatopancreas by quantitative real-time PCR demonstrated that Lv-GFAT transcript levels increased when the shrimp were exposed to alkaline pH (9.3) and cadmium stress, but the time when its mRNA expression level peaked differed under these stresses. We also first expressed the recombinant protein of GFAT from shrimps in Escherichia coli. Western blot analyses confirmed that the Lv-GFAT protein was strongly expressed in the hepatopancreas after exposure to the LC-Cd stress. These results suggest that Lv-GFAT expression is stimulated by alkaline pH and cadmium stress and that it may play important roles in resistance of shrimp to environmental stresses.

Structural basis for morpheein-type allosteric regulation of Escherichia coli glucosamine-6-phosphate synthase: equilibrium between inactive hexamer and active dimer

The amino-terminal cysteine of glucosamine-6-phosphate synthase (GlmS) acts as a nucleophile to release and transfer ammonia from glutamine to fructose 6-phosphate through a channel. The crystal structure of the C1A mutant of Escherichia coli GlmS, solved at 2.5 Å resolution, is organized as a hexamer, where the glutaminase domains adopt an inactive conformation. Although the wild-type enzyme is active as a dimer, size exclusion chromatography, dynamic and quasi-elastic light scattering, native polyacrylamide gel electrophoresis, and ultracentrifugation data show that the dimer is in equilibrium with a hexameric state, in vitro and in cellulo. The previously determined structures of the wild-type enzyme, alone or in complex with glucosamine 6-phosphate, are also consistent with a hexameric assembly that is catalytically inactive because the ammonia channel is not formed. The shift of the equilibrium toward the hexameric form in the presence of cyclic glucosamine 6-phosphate, together with the decrease of the specific activity with increasing enzyme concentration, strongly supports product inhibition through hexamer stabilization. Altogether, our data allow us to propose a morpheein model, in which the active dimer can rearrange into a transiently stable form, which has the propensity to form an inactive hexamer. This would account for a physiologically relevant allosteric regulation of E. coli GlmS. Finally, in addition to cyclic glucose 6-phosphate bound at the active site, the hexameric organization of E. coli GlmS enables the binding of another linear sugar molecule. Targeting this sugar-binding site to stabilize the inactive hexameric state is therefore suggested for the development of specific antibacterial inhibitors.

Transcriptome atlas of glutamine family amino acid metabolism-related genes in eight regenerating liver cell types

To explore glutamine family amino acid metabolism of eight liver cell types in rat liver regeneration, eight kinds of rat regenerating liver cells were isolated by using the combination of Percoll density gradient centrifugation and immunomagnetic bead methods, then Rat Genome 230 2.0 Array was used to detect the expression profiles of the genes associated with metabolism of glutamine family amino acid in rat liver regeneration and finally how these genes involved in activities of eight regenerating liver cell types were analysed by the methods of bioinformatics and systems biology. The results showed that in the priming stage of liver regeneration, hepatic stellate cells and sinusoidal endothelial cells transformed proline and glutamine into glutamate; hepatocytes, hepatic stellate cells, sinusoidal endothelial cells and dendritic cells catabolized glutamate to 2-oxoglutarate or succinate; hepatic stellate cells and sinusoidal endothelial cells catalysed glutamate into glutamyl-tRNA for protein synthesis; urea cycle, which degraded from arginine, was enhanced in biliary epithelia cells, sinusoidal endothelial cells and dendritic cells; synthesis of polyamines from arginine was enhanced in biliary epithelia cells, sinusoidal endothelial cells, Kupffer cells and dendritic cells; the content of NO was increased in sinusoidal endothelial cells and dendritic cells; degradation of proline was enhanced in hepatocytes and biliary epithelia cells. In the progress stage, biliary epithelia cells converted glutamine into GMP and glucosamine 6-phosphate; oval cells converted glutamine into glucosamine 6-phosphate; hepatic stellate cells converted glutamine into NAD; the content of NO, which degraded from arginine, was increased in biliary epithelia cells, oval cells, pit cells and dendritic cells. In the termination stage, oval cells converted proline into glutamate; glutamate degradation, which degraded from arginine, was enhanced in hepatocytes and dendritic cells; the content of NO was increased in oval cells, sinusoidal endothelial cells, pit cells and dendritic cells. The synthesis of creatine phosphate was enhanced in hepatocytes, biliary epithelia cells, pit cells and dendritic cells in both progress and termination stages. In summary, glutamine family amino acid metabolism has some differences in liver regeneration in different liver cells.

Molecular cloning, sequencing, and expression of a L -glutamine D-fructose 6-phosphate amidotransferase gene from Volvariella volvacea

Using 3'-RACE and 5'-RACE, we have cloned and sequenced the genomic gene and complete cDNA encoding L: -glutamine D: -fructose 6-phosphate amidotransferase (GFAT) from the edible straw mushroom, Volvariella volvacea. Gfat contains five introns, and encodes a predicted protein of 697 amino acids that is homologous to other reported GFAT sequences. Southern hybridization indicated that a single gfat gene locus exists in the V. volvacea genome. Recombinant native V. volvacea GFAT enzyme, over-expressed using Escherichia coli and partially purified, had an estimated molecular mass of 306 kDa and consisted of four equal-sized subunits of 77 kD. Reciprocal plots revealed K (m) values of 0.55 and 0.75 mM for fructose 6-phosphate and L: -glutamine, respectively. V. volvacea GFAT activity was inhibited by the end-product of the hexosamine pathway, UDP-GlcNAc, and by the glutamine analogues N (3)-(4-methoxyfumaroyl)-L: -2,3-diaminopropanoic acid and 2-amino-2-deoxy-D: -glucitol-6-phosphate.

Structural analysis of human glutamine:fructose-6-phosphate amidotransferase, a key regulator in type 2 diabetes

Glutamine:fructose-6-phosphate amidotransferase (GFAT) is a rate-limiting enzyme in the hexoamine biosynthetic pathway and plays an important role in type 2 diabetes. We now report the first structures of the isomerase domain of the human GFAT in the presence of cyclic glucose-6-phosphate and linear glucosamine-6-phosphate. The C-terminal tail including the active site displays a rigid conformation, similar to the corresponding Escherichia coli enzyme. The diversity of the CF helix near the active site suggests the helix is a major target for drug design. Our study provides insights into the development of therapeutic drugs for type 2 diabetes.

Cytoplasmic steps of peptidoglycan biosynthesis

The biosynthesis of bacterial cell wall peptidoglycan is a complex process that involves enzyme reactions that take place in the cytoplasm (synthesis of the nucleotide precursors) and on the inner side (synthesis of lipid-linked intermediates) and outer side (polymerization reactions) of the cytoplasmic membrane. This review deals with the cytoplasmic steps of peptidoglycan biosynthesis, which can be divided into four sets of reactions that lead to the syntheses of (1) UDP-N-acetylglucosamine from fructose 6-phosphate, (2) UDP-N-acetylmuramic acid from UDP-N-acetylglucosamine, (3) UDP-N-acetylmuramyl-pentapeptide from UDP-N-acetylmuramic acid and (4) D-glutamic acid and dipeptide D-alanyl-D-alanine. Recent data concerning the different enzymes involved are presented. Moreover, special attention is given to (1) the chemical and enzymatic synthesis of the nucleotide precursor substrates that are not commercially available and (2) the search for specific inhibitors that could act as antibacterial compounds.

Highlights of glucosamine-6P synthase catalysis

L-Glutamine:d-fructose-6-phosphate amidotransferase, also known as glucosamine-6-phosphate synthase (GlcN6P synthase), which catalyzes the first step in a pathway leading to the formation of uridine 5'-diphospho-N-acetyl-d-glucosamine (UDP-GlcNAc), is a key point in the metabolic control of the biosynthesis of amino sugar-containing macromolecules. The molecular mechanism of the reaction catalyzed by GlcN6P synthase is complex and involves amide bond cleavage followed by ammonia channeling and sugar isomerization. This article provides a comprehensive overview of the present knowledge on this multi-faceted enzyme emphasizing the progress made during the last five years.

Characterization of glutamine: fructose-6-phosphate aminotransferase from the ixodid tick, Haemaphysalis longicornis, and its critical role in host blood feeding

Glutamine: fructose-6-phosphate aminotransferase (GFAT, EC2.6.1.16) is the first, and rate-limiting, enzyme in the hexosamine biosynthetic pathway, and is involved in the regulation of chitin biosynthesis and glycosylation of proteins. We report here the molecular characterization and potential functions of a novel GFAT (HlGFAT) from the ixodid tick Haemaphysalis longicornis. HlGFAT consists of 696 amino acids, possesses a class II glutamine aminotransferase domain and two sugar isomerase motifs, and has a close phylogenetic relationship to insect GFAT. HlGFAT was expressed at all stages of development and in multiple organs. The transcription levels in the cuticle and midgut were enhanced significantly by blood feeding during the first 3 days and decreased on the fifth day, while those in salivary glands maintained almost the same level during the first 3 days, and decreased to a rather low level at 5 days postinfestation. Endogenous HlGFAT was identified at all developmental stages and in multiple organs, such as epidermis, midgut epithelium, salivary gland, ovary, Malpigian's tubule and trachea. It was identified as a protein of 78.4 kDa using Western blot analysis. Following RNA interference of HlGFAT, engorgement by adult females was reduced significantly. One of the potential mechanisms for this effect may be that the inhibition of HlGFAT limits chitin biosynthesis, so disrupting cuticle growth and possibly peritrophic matrix formation during blood feeding.

Expression and purification of active human internal His(6)-tagged L-glutamine: D-Fructose-6P amidotransferase I

Human L-glutamine: D-fructose-6-phosphate amidotransferase (Gfat1), a recognized target in type 2 diabetes complications, was expressed in Sf9 insect cells with an internal His(6)-tag and purified to homogenity. Two different microplate assays that quantify, respectively D-glucosamine-6-phosphate and L-glutamate were used to analyze the enzyme kinetic properties. The recombinant human L-glutamine: D-fructose-6-phosphate amidotransferase isoform 1 exhibits Michaelis parameters K(m)(Fru-6P)=0.98 mM and K(m)(Gln)=0.84 mM which are similar to the values reported for the same enzyme from different sources. The stimulation of hydrolysis of the alternate substrate L-glutamine para-nitroanilide by D-fructose-6P (Fru-6P) afforded a K(d) of 5 microM for Fru-6P.

Proteomic response to intracellular proteins of Monascus pilosus grown under phosphate-limited complex medium with different growth rates and pigment production

Monascus pigments are important colorings in food applications. Rice containing potassium phosphate and sodium nitrate was reported as a good pigment-producing medium for Monascus in previous studies. We found that the lack of potassium phosphate in this medium depressed red pigment production in cultivated Monascus pilosus. However, the influence of phosphate limitation on the biochemical metabolisms concerning culture growth and pigment production in Monascus remains unknown. Here, we used proteomic analysis by two-dimensional gel electrophoresis, matrix-assisted laser desorption ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS), tandem mass spectrometry (MS/MS), and database interrogation to separate and identify the proteins of M. pilosus grown between the lack of potassium phosphate and the control media. Phosphate limitation to this complex medium induced an up-regulation of aldehyde dehydrogenase and several glycolytic enzymes in Monascus relative to the control. In contrast, the metabolic enzymes such as glucosamine:fructose-6-phosphate aminotransferase and ADP-ribosylation factor 1 were up-regulated in the control.

Directed evolution and characterization of Escherichia coli glucosamine synthase

Glucosamine synthase (GlmS) converts fructose-6-phosphate to glucosamine-6-phosphate. Overexpression of GlmS in Escherichia coli increased synthesis of glucosamine-6-P, which was dephosphorylated and secreted as glucosamine into the growth medium. The E. coli glmS gene was improved through error-prone polymerase chain reaction (PCR) in order to develop microbial strains for fermentation production of glucosamine. Mutants producing higher levels of glucosamine were identified by a plate cross-feeding assay and confirmed in shake flask cultures. Over 10 mutants were characterized and all showed significantly reduced sensitivity to inhibition by glucosamine-6-phosphate. Ki of mutants ranged from 1.4 to 4.0 mM as compared to 0.56 mM for the wild type enzyme. Product resistance resulted from single mutations (L468P, G471S) and/or combinations of mutations in the sugar isomerase domain. Most overexpressed GlmS protein was found in the form of inclusion bodies. Cell lysate from mutant 2123-72 contained twice as much soluble GlmS protein and enzyme activity as the strain overexpressing the wild type gene. Using the product-resistant mutant, glucosamine production was increased 60-fold.

Regulatory mechanisms of chitin biosynthesis and roles of chitin in peritrophic matrix formation in the midgut of adult Aedes aegypti

In mosquitoes, the peritrophic matrix is formed in response to blood feeding and can be a physical barrier when pathogens ingested with blood meal attempt to reach and transverse the midgut epithelium. The main components of the peritrophic matrix are chitin-biding-domain containing proteins, glycosylated proteins, and chitin fibrils. Chitin is synthesized from fructose-6-phosphate by a series of five enzymatic reactions. We previously found that blood feeding induces transcriptional up-regulation of glutamine: fructose-6-phosphate amidotransferase-1 (AeGfat-1) and chitin synthase (AeCs), the first and last enzymes of the biosynthetic pathway, respectively, in the midgut of Aedes aegypti. In this study, we demonstrated that formation of the peritrophic matrix is disrupted when the transcript abundance of either gene is knocked-down using RNAi methodologies. We also have shown that enzymatic activity of recombinant AeGFAT-1 is sensitive to feedback inhibition by UDP-N-acetylglucosamine, a substrate of chitin synthase. These findings demonstrate that in the midgut of adult Ae. aegypti, (1) chitin is synthesized de novo in response to blood feeding and is an essential component of the peritrophic matrix, and (2) chitin biosynthesis is negatively regulated, in part, by inhibitory sensitivity of AeGFAT-1 to UDP-N-acetylglucosamine.

Purification and Characterization of Dioscin-.ALPHA.-L-rhamnosidase from Pig Liver

Dioscin-alpha-L-rhamnosidase was isolated, purified and partially characterized from pig liver. The maximum activity was reached at pH 7, 42 degrees C, 24 h, and 2% of substrate concentration. Fe3+ and Cu2+ inhibited the enzyme; the ion Ca2+ activated it. Mg2+ was an inhibitor at 100 mM, but it was an activator at 200 mM. Zn2+ could be a weak activator of the enzyme. The molecular weight of dioscin-alpha-L-rhamnosidase was about 47 kDa as determined by the method of SDS-polyacrylamide gel electrophoresis.

Microarray analysis of somitogenesis reveals novel targets of different WNT signaling pathways in the somitic mesoderm

WNT signaling plays a major role in patterning the dermomyotome of the somitic mesoderm. However, knowledge of downstream target genes and their regulation is limited. To identify new genes involved in the development and early patterning of the somite, we performed a comparison of gene expression by microarray between the presomitic mesoderm and the 5 most recently formed somites of the mouse at embryonic day 9.5. We identified 207 genes upregulated and 120 genes downregulated in somite formation. Expression analysis and functional categorization of these genes demonstrate this to be a diverse pool that provides a valuable resource for studying somite development. Thus far, we have found three genes expressed in the dermomyotome of the early somite. Consistent with their expression patterns, these genes are transcriptional targets of WNT signals, but display differential activation by different WNTs. We further demonstrate that 1 of these genes, Troy, is a direct target of canonical WNT signaling, while the other 2 genes, Selp and Arl4, are not. Thus, our microarray study using microdissected tissues not only provides global information on gene expression during somite development, it also provides novel targets to study the inductive signaling pathways that direct somite patterning.

Glucosamine-6-phosphate synthase--the multi-facets enzyme

L-Glutamine: D-fructose-6-phosphate amidotransferase, known under trivial name of glucosamine-6-phosphate synthase, as the only member of the amidotransferase subfamily of enzymes, does not display any ammonia-dependent activity. This enzyme, catalysing the first committed step in a pathway leading to the eventual formation of uridine 5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc), is an important point of metabolic control in biosynthesis of amino sugar-containing macromolecules. The molecular mechanism of reaction catalysed by GlcN-6-P synthase is complex and involves both amino transfer and sugar isomerisation. Substantial alterations to the enzyme structure and properties have been detected in different neoplastic tissues. GlcN-6-P synthase is inflicted in phenomenon of hexosamine-induced insulin resistance in diabetes. Finally, this enzyme has been proposed as a promising target in antifungal chemotherapy. Most of these issues, especially their molecular aspects, have been extensively studied in recent years. This article provides a comprehensive overview of the present knowledge on this multi-facets enzyme.

A radiometric assay for glutamine:fructose-6-phosphate amidotransferase

Glutamine:fructose-6-phosphate amidotransferase (GFAT) catalyzes the first step in the biosynthesis of amino sugars by transferring the amino group from l-glutamine to the acceptor substrate, fructose 6-phosphate, generating the products glucosamine 6-phosphate and glutamic acid. We describe a method for the synthesis and purification of the substrate, fructose 6-phosphate, and methods for a radiometric assay of human GFAT1 that can be performed in either of two formats: a small disposable-column format and a high-throughput 96-well-plate format. The method performed in the column format can detect 1 pmol of glucosamine 6-phosphate, much less than that required by previously published assays that measure GlcN 6-phosphate. The column assay demonstrates a broad linear range with low variability. In both formats, the assay is linear with time and enzyme concentration and is highly reproducible. This method greatly improves the sensitivity and speed with which GFAT1 activity can be measured and facilitates direct kinetic measurement of the transferase activity.

Glucosamine:fructose-6-phosphate aminotransferase: gene characterization, chitin biosynthesis and peritrophic matrix formation in Aedes aegypti

Glucosamine:fructose-6-phosphate aminotransferase (GFAT) catalyses the formation of glucosamine 6-phosphate and is the first and rate-limiting enzyme of the hexosamine biosynthetic pathway. The final product of the hexosamine pathway, UDP-N-acetyl glucosamine, is an active precursor of numerous macromolecules containing amino sugars, including chitin in fungi and arthropods. Chitin is one of the essential components of insect cuticle and peritrophic matrix. The peritrophic matrix is produced in the midgut of mosquitoes in response to bloodfeeding, and may affect vector competence by serving as a physical barrier to pathogens. It is hypothesized that GFAT plays a regulatory role in biosynthesis of chitin and peritrophic matrix formation in insects. We cloned and sequenced the GFAT gene (AeGfat-1) and its 5' regulatory region from Aedes aegypti. There is no intron in AeGfat-1 and there are two potential transcription start sites. AeGfat-1 cDNA is 3.4 kb in length and its putative translation product is 75.4 kDa. The amino acid sequence of GFAT is highly conserved in lower and higher eukaryotes, as well as in bacteria. AeGfat-1 message is constitutively expressed but is gradually up-regulated in the midgut after bloodfeeding. The putative regulatory region of the gene contains the ecdysone response element, E74, and Broad complex motifs, similar to what is found in the glutamine synthetase gene in Ae. aegypti. Results suggest that Ae. aegypti GFAT-1 may have a regulatory role in chitin biosynthesis and peritrophic matrix formation, and probably is under the regulation of ecdysteroids.

Kinetic characterization of human glutamine-fructose-6-phosphate amidotransferase I: potent feedback inhibition by glucosamine 6-phosphate

Glutamine-fructose-6-phosphate amidotransferase (GFAT) catalyzes the first committed step in the pathway for biosynthesis of hexosamines in mammals. A member of the N-terminal nucleophile class of amidotransferases, GFAT transfers the amino group from the L-glutamine amide to D-fructose 6-phosphate, producing glutamic acid and glucosamine 6-phosphate. The kinetic constants reported previously for mammalian GFAT implicate a relatively low affinity for the acceptor substrate, fructose 6-phosphate (Fru-6-P, K(m) 0.2-1 mm). Utilizing a new sensitive assay that measures the production of glucosamine 6-phosphate (GlcN-6-P), purified recombinant human GFAT1 (hGFAT1) exhibited a K(m) for Fru-6-P of 7 microm, and was highly sensitive to product inhibition by GlcN-6-P. In a second assay method that measures the stimulation of glutaminase activity, a K(d) of 2 microm was measured for Fru-6-P binding to hGFAT1. Further, we report that the product, GlcN-6-P, is a potent competitive inhibitor for the Fru-6-P site, with a K(i) measured of 6 microm. Unlike other members of the amidotransferase family, where glutamate production is loosely coupled to amide transfer, we have demonstrated that hGFAT1 production of glutamate and GlcN-6-P are strictly coupled in the absence of inhibitors. Similar to other amidotransferases, competitive inhibitors that bind at the synthase site may inhibit the synthase activity without inhibiting the glutaminase activity at the hydrolase domain. GlcN-6-P, for example, inhibited the transfer reaction while fully activating the glutaminase activity at the hydrolase domain. Inhibition of hGFAT1 by the end product of the pathway, UDP-GlcNAc, was competitive with a K(i) of 4 microm. These data suggest that hGFAT1 is fully active at physiological levels of Fru-6-P and may be regulated by its product GlcN-6-P in addition to the pathway end product, UDP-GlcNAc.