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

An update on current type 2 diabetes mellitus (T2DM) druggable targets and drugs targeting them

Type 2 diabetes mellitus (T2DM) is characterized by hyperglycemia and affects millions of people globally. Even after advancement and development in medical science, it is a big task to achieve victory over type 2 diabetes mellitus (T2DM). T2DM can be a reason for fatal events like stroke, cardiac failure, nephropathy, and retinopathy. Many advanced antidiabetic drugs have been introduced in the market in the past two decades, leading researchers to hunt for new target proteins and their potential modulators that can help develop newer antidiabetic drugs. This review article comprises a broad literature of the latest developments in the management of T2DM concerning new target proteins, their inhibitors, or drugs from the clinical arena employed for the successful management of symptoms of T2DM using mono, dual, or triple combination medication therapy. The review categorizes antidiabetic drugs into three general classes that include conventional drug targets, currently explored targets, and upcoming emerging targets. The review aims to merge information on the medicines affecting these targets, their mechanisms, followed by the chemical structures, and recent advancements.

Multireceptor Analysis for Evaluating the Antidiabetic Efficacy of Karanjin: A Computational Approach

BACKGROUND

Diabetes mellitus, notably type 2, is a rising global health challenge, prompting the need for effective management strategies. Common medications such as metformin, insulin, repaglinide and sitagliptin can induce side effects like gastrointestinal disturbances, hypoglycemia, weight gain and specific organ risks. Plant-derived therapies like Karanjin from Pongamia pinnata present promising alternatives due to their historical use, holistic health benefits and potentially fewer adverse effects. This study employs in silico analysis to explore Karanjin's interactions with diabetes-associated receptors, aiming to unveil its therapeutic potential while addressing the limitations and side effects associated with conventional medications.

METHODOLOGY

The research encompassed the selection of proteins from the Protein Data Bank (PDB), followed by structural refinement processes and optimization. Ligands such as Karanjin and standard drugs were retrieved from PubChem, followed by a comprehensive analysis of their ADMET profiling and pharmacokinetic properties. Protein-ligand interactions were evaluated through molecular docking using AutoDockTools 1.5.7, followed by the analysis of structural stability using coarse-grained simulations with CABS Flex 2.0. Molecular dynamics simulations were performed using Desmond 7.2 and the OPLS4 force field to explore how Karanjin interacts with proteins over 100 nanoseconds, focusing on the dynamics and structural stability.

RESULTS

Karanjin, a phytochemical from Pongamia pinnata, shows superior drug candidate potential compared to common medications, offering advantages in efficacy and reduced side effects. It adheres to drug-likeness criteria and exhibits optimal ADMET properties, including moderate solubility, high gastrointestinal absorption and blood-brain barrier penetration. Molecular docking revealed Karanjin's highest binding energy against receptor 3L2M (Pig pancreatic alpha-amylase) at -9.1 kcal/mol, indicating strong efficacy potential. Molecular dynamics simulations confirmed stable ligand-protein complexes with minor fluctuations in RMSD and RMSF, suggesting robust interactions with receptors 3L2M.

CONCLUSION

Karanjin demonstrates potential in pharmaceutical expansion for treating metabolic disorders such as diabetes, as supported by computational analysis. Prospects for Karanjin in pharmaceutical development include structural modifications for enhanced efficacy and safety. Nanoencapsulation may improve bioavailability and targeted delivery to pancreatic cells, while combination therapies could optimize treatment outcomes in diabetes management. Clinical trials and experimental studies are crucial to validate its potential as a novel therapeutic agent.

Pharmacotherapy of type 2 diabetes: An update and future directions

Type 2 diabetes (T2D) is a widely prevalent disease with substantial economic and social impact for which multiple conventional and novel pharmacotherapies are currently available; however, the landscape of T2D treatment is constantly changing as new therapies emerge and the understanding of currently available agents deepens. This review aims to provide an updated summary of the pharmacotherapeutic approach to T2D. Each class of agents is presented by mechanism of action, details of administration, side effect profile, cost, and use in certain populations including heart failure, non-alcoholic fatty liver disease, obesity, chronic kidney disease, and older individuals. We also review targets of novel therapeutic T2D agent development. Finally, we outline an up-to-date treatment approach that starts with identification of an individualized goal for glycemic control then selection, initiation, and further intensification of a personalized therapeutic plan for T2D.

Biosynthesis of uridine diphosphate N-Acetylglucosamine: An underexploited pathway in the search for novel antibiotics?

Although the prevalence of antibiotic resistance is increasing at an alarming rate, there are a dwindling number of effective antibiotics available. Thus, the development of novel antibacterial agents should be of utmost importance. Peptidoglycan biosynthesis has been and is still an attractive source for antibiotic targets; however, there are several components that remain underexploited. In this review, we examine the enzymes involved in the biosynthesis of one such component, UDP-N-acetylglucosamine, an essential building block and precursor of bacterial peptidoglycan. Furthermore, given the presence of a similar biosynthesis pathway in eukaryotes, we discuss the current knowledge on the differences and similarities between the bacterial and eukaryotic enzymes. Finally, this review also summarises the recent advances made in the development of inhibitors targeting the bacterial enzymes.

In Silico Approaches to Identify Polyphenol Compounds as α-Glucosidase and α-Amylase Inhibitors against Type-II Diabetes

Type-II diabetes mellitus (T2DM) results from a combination of genetic and lifestyle factors, and the prevalence of T2DM is increasing worldwide. Clinically, both α-glucosidase and α-amylase enzymes inhibitors can suppress peaks of postprandial glucose with surplus adverse effects, leading to efforts devoted to urgently seeking new anti-diabetes drugs from natural sources for delayed starch digestion. This review attempts to explore 10 families e.g., Bignoniaceae, Ericaceae, Dryopteridaceae, Campanulaceae, Geraniaceae, Euphorbiaceae, Rubiaceae, Acanthaceae, Rutaceae, and Moraceae as medicinal plants, and folk and herb medicines for lowering blood glucose level, or alternative anti-diabetic natural products. Many natural products have been studied in silico, in vitro, and in vivo assays to restrain hyperglycemia. In addition, natural products, and particularly polyphenols, possess diverse structures for exploring them as inhibitors of α-glucosidase and α-amylase. Interestingly, an in silico discovery approach using natural compounds via virtual screening could directly target α-glucosidase and α-amylase enzymes through Monte Carto molecular modeling. Autodock, MOE-Dock, Biovia Discovery Studio, PyMOL, and Accelrys have been used to discover new candidates as inhibitors or activators. While docking score, binding energy (Kcal/mol), the number of hydrogen bonds, or interactions with critical amino acid residues have been taken into concerning the reliability of software for validation of enzymatic analysis, in vitro cell assay and in vivo animal tests are required to obtain leads, hits, and candidates in drug discovery and development.

Advanced Bioinformatics Tools in the Pharmacokinetic Profiles of Natural and Synthetic Compounds with Anti-Diabetic Activity

Diabetes represents a major health problem, involving a severe imbalance of blood sugar levels, which can disturb the nerves, eyes, kidneys, and other organs. Diabes management involves several synthetic drugs focused on improving insulin sensitivity, increasing insulin production, and decreasing blood glucose levels, but with unclear molecular mechanisms and severe side effects. Natural chemicals extracted from several plants such as Gymnema sylvestre, Momordica charantia or Ophiopogon planiscapus Niger have aroused great interest for their anti-diabetes activity, but also their hypolipidemic and anti-obesity activity. Here, we focused on the anti-diabetic activity of a few natural and synthetic compounds, in correlation with their pharmacokinetic/pharmacodynamic profiles, especially with their blood-brain barrier (BBB) permeability. We reviewed studies that used bioinformatics methods such as predicted BBB, molecular docking, molecular dynamics and quantitative structure-activity relationship (QSAR) to elucidate the proper action mechanisms of antidiabetic compounds. Currently, it is evident that BBB damage plays a significant role in diabetes disorders, but the molecular mechanisms are not clear. Here, we presented the efficacy of natural (gymnemic acids, quercetin, resveratrol) and synthetic (TAK-242, propofol, or APX3330) compounds in reducing diabetes symptoms and improving BBB dysfunctions. Bioinformatics tools can be helpful in the quest for chemical compounds with effective anti-diabetic activity that can enhance the druggability of molecular targets and provide a deeper understanding of diabetes mechanisms.

Multi-Targeted Molecular Docking, Pharmacokinetics, and Drug-Likeness Evaluation of Okra-Derived Ligand Abscisic Acid Targeting Signaling Proteins Involved in the Development of Diabetes

Diabetes mellitus is a global threat affecting millions of people of different age groups. In recent years, the development of naturally derived anti-diabetic agents has gained popularity. Okra is a common vegetable containing important bioactive components such as abscisic acid (ABA). ABA, a phytohormone, has been shown to elicit potent anti-diabetic effects in mouse models. Keeping its anti-diabetic potential in mind, in silico study was performed to explore its role in inhibiting proteins relevant to diabetes mellitus- 11β-hydroxysteroid dehydrogenase (11β-HSD1), aldose reductase, glucokinase, glutamine-fructose-6-phosphate amidotransferase (GFAT), peroxisome proliferator-activated receptor-gamma (PPAR-gamma), and Sirtuin family of NAD(+)-dependent protein deacetylases 6 (SIRT6). A comparative study of the ABA-protein docked complex with already known inhibitors of these proteins relevant to diabetes was compared to explore the inhibitory potential. Calculation of molecular binding energy (ΔG), inhibition constant (pKi), and prediction of pharmacokinetics and pharmacodynamics properties were performed. The molecular docking investigation of ABA with 11-HSD1, GFAT, PPAR-gamma, and SIRT6 revealed considerably low binding energy (ΔG from −8.1 to −7.3 Kcal/mol) and predicted inhibition constant (pKi from 6.01 to 5.21 µM). The ADMET study revealed that ABA is a promising drug candidate without any hazardous effect following all current drug-likeness guidelines such as Lipinski, Ghose, Veber, Egan, and Muegge.

Biological and computational studies provide insights into Caesalphinia digyna Rottler stems

Caesalpinia digyna (Rottl.) (Family: Fabaceae) is an essential medicinal plant for it's conventional uses against a kind of human disorders. This research aims to investigate the antidiarrheal, antibacterial and antifungal properties of the methanol extract of the stems extracts of the C. digyna (MECD). The in vivo antidiarrheal activity of the stem extracts were evaluated by using castor oil-induced diarrhea, castor oil-induced enteropooling and charcoal induced intestinal transit in mice model. Besides, in vitro antimicrobial potentiality of MECD was investigated by the disc diffusion method. In silico activity of the isolated compounds were performed by Schrödinger-Maestro (Version 11.1) software. In addition, The ADME/T analysis and PASS prediction were implemented by using pass online tools. In the antidiarrheal investigation, the MECD exhibited a notable inhibition rate in all test approaches which were statistically significant (p < 0.05, p < 0.1, p < 0.01). MECD 400 mg/kg showed the maximum antidiarrheal potency in all the test methods. In vitro antimicrobial analysis unveiled that, MECD revealed higher potentiality against almost all pathogens and indicates dose-dependent activity against almost all the bacteria and fungi. In the case of in silico evaluation of anti-diarrheal, anti-bacterial and anti-fungal activity, all three isolated compounds met the pre-conditions of Lipinski's five rules for drug discovery. Pass predicted study also employed for all compounds. However, The chemical constituents of the C. digyna can be a potent source of anti-diarrheal, anti-bacterial and anti-fungal medicine and further modification and simulation studies are required to establish the effectiveness of bioactive compounds.

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Caesalpinia digyna is used to prepare therapeutic product "Geriforte".

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The pods, bark and seed pods of C. digyna contains high amount of tannins and flavonoids.

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Caesalpinia digyna is used to treat diarrhea, chronic fluxes, senile pruritis, tuberculosis, tonic disorder, and diabetes.

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This study is to validate the ethnopharmacological values of C. digyna stems.

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This is the first research work till now where, the stems of C. digyna have been studied.

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The results of in vivo, in vitro and in silico studies is comparatively closer to the standard drugs.

Phytochemicals as Potential Epidrugs in Type 2 Diabetes Mellitus

Type 2 diabetes Mellitus (T2DM) prevalence has significantly increased worldwide in recent years due to population age, obesity, and modern sedentary lifestyles. The projections estimate that 439 million people will be diabetic in 2030. T2DM is characterized by an impaired β-pancreatic cell function and insulin secretion, hyperglycemia and insulin resistance, and recently the epigenetic regulation of β-pancreatic cells differentiation has been underlined as being involved. It is currently known that several bioactive molecules, widely abundant in plants used as food or infusions, have a key role in histone modification and DNA methylation, and constituted potential epidrugs candidates against T2DM. In this sense, in this review the epigenetic mechanisms involved in T2DM and protein targets are reviewed, with special focus in studies addressing the potential use of phytochemicals as epidrugs that prevent and/or control T2DM in vivo and in vitro. As main findings, and although some controversial results have been found, bioactive molecules with epigenetic regulatory function, appear to be a potential replacement/complementary therapy of pharmacological hypoglycemic drugs, with minimal side effects. Indeed, natural epidrugs have shown to prevent or delay the T2DM development and the morbidity associated to dysfunction of blood vessels, eyes and kidneys due to sustained hyperglycemia in T2DM patients.

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.

Drosophila GFAT1 and GFAT2 enzymes encode obligate developmental functions

Glutamine: fructose-6-phosphate amidotransferase (GFAT) enzymes catalyse the first committed step of the hexosamine biosynthesis pathway (HBP) using glutamine and fructose-6-phosphate to form glucosamine-6-phosphate (GlcN6P). Numerous species (e.g. mouse, rat, zebrafish, chicken) including humans and Drosophila encode two broadly expressed copies of this enzyme but whether these perform redundant, partially overlapping or distinct functions is not known. To address this question, we produced single gene null mutations in the fly counterparts of gfat1 and gfat2. Deletions for either enzyme were fully lethal and homozygotes lacking either GFAT1 or GFAT2 died at or prior to the first instar larval stage. Therefore, when genetically eliminated, neither isoform was able to compensate for the other. Importantly, dietary supplementation with D-glucosamine-6-phosphate rescued GFAT2 deficiency and restored viability to gfat2^-/- mutants. In contrast, glucosamine-6-phosphate did not rescue gfat1^-/- animals.

Loss of GFAT-1 feedback regulation activates the hexosamine pathway that modulates protein homeostasis

Glutamine fructose-6-phosphate amidotransferase (GFAT) is the key enzyme in the hexosamine pathway (HP) that produces uridine 5′-diphospho-N-acetyl-d-glucosamine (UDP-GlcNAc), linking energy metabolism with posttranslational protein glycosylation. In Caenorhabditis elegans, we previously identified gfat-1 gain-of-function mutations that elevate UDP-GlcNAc levels, improve protein homeostasis, and extend lifespan. GFAT is highly conserved, but the gain-of-function mechanism and its relevance in mammalian cells remained unclear. Here, we present the full-length crystal structure of human GFAT-1 in complex with various ligands and with important mutations. UDP-GlcNAc directly interacts with GFAT-1, inhibiting catalytic activity. The longevity-associated G451E variant shows drastically reduced sensitivity to UDP-GlcNAc inhibition in enzyme activity assays. Our structural and functional data point to a critical role of the interdomain linker in UDP-GlcNAc inhibition. In mammalian cells, the G451E variant potently activates the HP. Therefore, GFAT-1 gain-of-function through loss of feedback inhibition constitutes a potential target for the treatment of age-related proteinopathies.

Mutations in the hexosamine pathway key enzyme glutamine fructose-6-phosphate amidotransferase (GFAT-1) improve protein quality control and extend C. elegans lifespan. Here the authors present the crystal structures of full-length human GFAT-1 alone and with bound ligands and perform activity assays, which show that gain-of-function in the longevity-associated G451E variant is caused by a loss of feedback regulation.

A Recent Achievement In the Discovery and Development of Novel Targets for the Treatment of Type-2 Diabetes Mellitus

Type 2 diabetes (T2DM) is a chronic metabolic disorder. Impaired insulin secretion, enhanced hepatic glucose production, and suppressed peripheral glucose use are the main defects responsible for developing the disease. Besides, the pathophysiology of T2DM also includes enhanced glucagon secretion, decreased incretin secretion, increased renal glucose reabsorption, and adipocyte, and brain insulin resistance. The increasing prevalence of T2DM in the world beseeches an urgent need for better treatment options. The antidiabetic drugs focus on control of blood glucose concentration, but the future treatment goal is to delay disease progression and treatment failure, which causes poorer glycemic regulation. Recent treatment approaches target on several novel pathophysiological defects present in T2DM. Some of the promising novel targets being under clinical development include those that increase insulin sensitization (antagonists of glucocorticoids receptor), decreasing hepatic glucose production (glucagon receptor antagonist, inhibitors of glycogen phosphorylase and fructose-1,6-biphosphatase). This review summarizes studies that are available on novel targets being studied to treat T2DM with an emphasis on the small molecule drug design. The experience gathered from earlier studies and knowledge of T2DM pathways can guide the anti-diabetic drug development toward the discovery of drugs essential to treat T2DM.

Metabolic and Epigenetic Action Mechanisms of Antidiabetic Medicinal Plants

Diabetes is a predominant metabolic disease nowadays due to the off-beam lifestyle of diet and reduced physical activity. Complications of the illness include the gene-environment interactions and the downstream genetic and epigenetic consequences, e.g., cardiovascular diseases, tumor progression, retinopathy, nephropathy, neuropathy, polydipsia, polyphagia, polyuria, and weight loss. This review sheds the light on the mechanistic insights of antidiabetic medicinal plants in targeting key organs and tissues involved in regulating blood glucose homeostasis including the pancreas, liver, muscles, adipose tissues, and glucose absorption in the intestine. Diabetes is also involved in modulating major epigenetic pathways such as DNA methylation and histone modification. In this respect, we will discuss the phytochemicals as current and future epigenetic drugs in the treatment of diabetes. In addition, several proteins are common targets for the treatment of diabetes. Some phytochemicals are expected to directly interact with these targets. We lastly uncover modeling studies that predict such plausible interactions. In conclusion, this review article presents the mechanistic insight of phytochemicals in the treatment of diabetes by combining both the cellular systems biology and molecular modeling.

A half-site multimeric enzyme achieves its cooperativity without conformational changes

Cooperativity is a feature many multimeric proteins use to control activity. Here we show that the bacterial heptose isomerase GmhA displays homotropic positive and negative cooperativity among its four protomers. Most similar proteins achieve this through conformational changes: GmhA instead employs a delicate network of hydrogen bonds, and couples pairs of active sites controlled by a unique water channel. This network apparently raises the Lewis acidity of the catalytic zinc, thus increasing the activity at one active site at the cost of preventing substrate from adopting a reactive conformation at the paired negatively cooperative site – a “half-site” behavior. Our study establishes the principle that multimeric enzymes can exploit this cooperativity without conformational changes to maximize their catalytic power and control. More broadly, this subtlety by which enzymes regulate functions could be used to explore new inhibitor design strategies.

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Structure of uridine diphosphate N-acetylglucosamine pyrophosphorylase from Entamoeba histolytica

Uridine diphosphate N-acetylglucosamine pyrophosphorylase (UAP) catalyzes the final step in the synthesis of UDP-GlcNAc, which is involved in cell-wall biogenesis in plants and fungi and in protein glycosylation. Small-molecule inhibitors have been developed against UAP from Trypanosoma brucei that target an allosteric pocket to provide selectivity over the human enzyme. A 1.8 Å resolution crystal structure was determined of UAP from Entamoeba histolytica, an anaerobic parasitic protozoan that causes amoebic dysentery. Although E. histolytica UAP exhibits the same three-domain global architecture as other UAPs, it appears to lack three α-helices at the N-terminus and contains two amino acids in the allosteric pocket that make it appear more like the enzyme from the human host than that from the other parasite T. brucei. Thus, allosteric inhibitors of T. brucei UAP are unlikely to target Entamoeba UAPs.

Glutamine: fructose-6-phosphate amidotransferase (GFAT): homology modelling and designing of new inhibitors using pharmacophore and docking based hierarchical virtual screening protocol

Glutamine: fructose-6-phosphate amidotransferase (GFAT), also termed GFPT1 and GFAT1, catalyzes the first committed step of the hexosamine biosynthesis pathway in mammals and consequently plays an important role in type 2 diabetes. In the present study, a combination of pharmacophore modelling, homology modelling, and molecular docking analysis was performed to design new glutamine competitive inhibitors of human GFAT, and to investigate important interaction details of inhibitor molecules. A pharmacophore model of GFAT inhibitors was developed, subsequently validated, and utilized for the screening by the PHASE database to identify new molecules. Afterwards, homology modelling was performed to construct the glutamine-binding site of the GFAT protein. The modelled active site was utilized to dock the studied molecules to investigate important receptor-ligand interactions and to scrutinize database-screened molecules on the basis of essential interactions. This systematic in silico protocol helped us to identify new molecules that would be explored for the treatment of type 2 diabetes and its complications.

Study of global transcriptional changes of N-GlcNAc2 proteins-producing T24 bladder carcinoma cells under glucose deprivation

Increased levels of N-linked (β-N- acetylglucosamine)₂ [N-GlcNAc₂]-modified proteins have been recognized to be an effective response to glucose deprivation. In the first step of this study, using a next generation sequencer, we investigated the global transcriptional changes induced by glucose deprivation in a T24 bladder carcinoma cell line, producing N-GlcNAc₂-modified proteins under glucose deprivation. Our transcriptome analysis revealed significant up-regulation of the UDP-GlcNAc biosynthesis pathway and unfolded protein response genes, and down-regulation of G2/M transition-related genes containing mitotic kinases. Our biological analysis confirmed that N-GlcNAc₂-modified proteins were localized with BiP proteins in the ER. G2/M arrest was caused by glucose deprivation in T24 cells. Moreover, the knockdown of unfolded protein response genes induced the expressional recovery of mitotic kinases under glucose deprivation. Taken together, our results suggest N-GlcNAc₂-modified proteins produced under glucose deprivation caused unfolded protein response in the ER, and that this response induced G2/M arrest.

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.

A novel N-terminal domain may dictate the glucose response of Mondo proteins

Glucose is a fundamental energy source for both prokaryotes and eukaryotes. The balance between glucose utilization and storage is integral for proper energy homeostasis, and defects are associated with several diseases, e.g. type II diabetes. In vertebrates, the transcription factor ChREBP is a major component in glucose metabolism, while its ortholog MondoA is involved in glucose uptake. Both MondoA and ChREBP contain five Mondo conserved regions (MCRI-V) that affect their cellular localization and transactivation ability. While phosphorylation has been shown to affect ChREBP function, the mechanisms controlling glucose response of both ChREBP and MondoA remain elusive. By incorporating sequence analysis techniques, structure predictions, and functional annotations, we synthesized data surrounding Mondo family proteins into a cohesive, accurate, and general model involving the MCRs and two additional domains that determine ChREBP and MondoA glucose response. Paramount, we identified a conserved motif within the transactivation region of Mondo family proteins and propose that this motif interacts with the phosphorylated form of glucose. In addition, we discovered a putative nuclear receptor box in non-vertebrate Mondo and vertebrate ChREBP sequences that reveals a potentially novel interaction with nuclear receptors. These interactions are likely involved in altering ChREBP and MondoA conformation to form an active complex and induce transcription of genes involved in glucose metabolism and lipogenesis.

Inactivation of glucosamine-6-phosphate synthase by N3-oxoacyl derivatives of L-2,3-diaminopropanoic acid

N(3)-Oxoacyl derivatives of L-2,3-diaminopropanoic acid 1-4, containing either an epoxide group or a conjugated double bond system, inactivate Saccharomyces cerevisiae glucosamine-6-phosphate (GlcN-6-P) synthase in a time- and concentration dependent manner. The results of kinetics studies on inactivation suggested a biphasic course, with formation of the enzyme-ligand complex preceding irreversible modification of the enzyme. The examined compounds differed markedly in their affinity to the enzyme active site. Inhibitors containing a phenyl ketone moiety bound much more strongly than their methyl ketone counterparts. The molecular mechanism of enzyme inactivation by phenyl ketone compounds 1 and 3 was elucidated by using a stepwise approach with 2D NMR, MS and UV-visible spectroscopy. A substituted thiazine derivative was identified as the final product of a model reaction between an epoxide compound, 1, and L-cysteine ethyl ester (CEE); and the respective cyclic product, found as a result of reaction between 1 and CGIF tetrapeptide, was identical to the N-terminal fragment of GlcN-6-P synthase. On the other hand, the reaction of a double-bond-containing compound, 3, with CEE, CGIF and GlcN-6-P synthase led to the formation of a C-S bond, without any further conversion or rearrangement. Molecular mechanisms of the reactions studied are proposed.

Hexosamine biosynthetic pathway mutations cause neuromuscular transmission defect

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.