Disorder-to-order active site capping regulates the rate-limiting step of the inositol pathway.

Myo-inositol-1-phosphate synthase (MIPS) catalyzes the NAD+-dependent isomerization of glucose-6-phosphate (G6P) into inositol-1-phosphate (IMP), controlling the rate-limiting step of the inositol pathway. Previous structural studies focused on the detailed molecular mechanism, neglecting large-scale conformational changes that drive the function of this 240 kDa homotetrameric complex. In this study, we identified the active, endogenous MIPS in cell extracts from the thermophilic fungus Thermochaetoides thermophila. By resolving the native structure at 2.48 Å (FSC = 0.143), we revealed a fully populated active site. Utilizing 3D variability analysis, we uncovered conformational states of MIPS, enabling us to directly visualize an order-to-disorder transition at its catalytic center. An acyclic intermediate of G6P occupied the active site in two out of the three conformational states, indicating a catalytic mechanism where electrostatic stabilization of high-energy intermediates plays a crucial role. Examination of all isomerases with known structures revealed similar fluctuations in secondary structure within their active sites. Based on these findings, we established a conformational selection model that governs substrate binding and eventually inositol availability. In particular, the ground state of MIPS demonstrates structural configurations regardless of substrate binding, a pattern observed across various isomerases. These findings contribute to the understanding of MIPS structure-based function, serving as a template for future studies targeting regulation and potential therapeutic applications.

Hexokinase 1 forms rings that regulate mitochondrial fission during energy stress.

Metabolic enzymes can adapt during energy stress, but the consequences of these adaptations remain understudied. Here, we discovered that hexokinase 1 (HK1), a key glycolytic enzyme, forms rings around mitochondria during energy stress. These HK1-rings constrict mitochondria at contact sites with the endoplasmic reticulum (ER) and mitochondrial dynamics protein (MiD51). HK1-rings prevent mitochondrial fission by displacing the dynamin-related protein 1 (Drp1) from mitochondrial fission factor (Mff) and mitochondrial fission 1 protein (Fis1). The disassembly of HK1-rings during energy restoration correlated with mitochondrial fission. Mechanistically, we identified that the lack of ATP and glucose-6-phosphate (G6P) promotes the formation of HK1-rings. Mutations that affect the formation of HK1-rings showed that HK1-rings rewire cellular metabolism toward increased TCA cycle activity. Our findings highlight that HK1 is an energy stress sensor that regulates the shape, connectivity, and metabolic activity of mitochondria. Thus, the formation of HK1-rings may affect mitochondrial function in energy-stress-related pathologies.

Peri active site catalysis of proline isomerisation is the molecular basis of allomorphy in ß-phosphoglucomutase.

Metabolic regulation occurs through precise control of enzyme activity. Allomorphy is a post-translational fine control mechanism where the catalytic rate is governed by a conformational switch that shifts the enzyme population between forms with different activities. ß-Phosphoglucomutase (ßPGM) uses allomorphy in the catalysis of isomerisation of ß-glucose 1-phosphate to glucose 6-phosphate via ß-glucose 1,6-bisphosphate. Herein, we describe structural and biophysical approaches to reveal its allomorphic regulatory mechanism. Binding of the full allomorphic activator ß-glucose 1,6-bisphosphate stimulates enzyme closure, progressing through NAC I and NAC III conformers. Prior to phosphoryl transfer, loops positioned on the cap and core domains are brought into close proximity, modulating the environment of a key proline residue. Hence accelerated isomerisation, likely via a twisted anti/C4-endo transition state, leads to the rapid predominance of active cis-P ßPGM. In contrast, binding of the partial allomorphic activator fructose 1,6-bisphosphate arrests ßPGM at a NAC I conformation and phosphoryl transfer to both cis-P ßPGM and trans-P ßPGM occurs slowly. Thus, allomorphy allows a rapid response to changes in food supply while not otherwise impacting substantially on levels of important metabolites.

Removal mechanisms and metabolic responses of Chlorella pyrenoidosa to dissolved organic phosphorus.

Microalgae-based biotechnology holds significant potential for addressing dual challenges of phosphorus removal and recovery from wastewater; however, the removal mechanism and metabolic adaptation of microalgae to dissolved organic phosphorus (DOP) are still unclear. This study investigated the removal mechanisms and metabolomic responses of the Chlorella pyrenoidosa to different DOP forms, including adenosine triphosphate (ATP), glucose-6-phosphate (G-6-P), and ß-glycerophosphate (ß-GP). The results showed C. pyrenoidosa could efficiently take up above 96% DOP through direct transport and post-hydrolysis pathways. The uptake of inorganic phosphorus (IP) followed pseudo first order kinetic model, while DOP followed pseudo second order kinetic model. Metabolite profiling revealed substantial alterations in central carbon metabolism depending on the DOP source. G-6-P upregulated glycolytic and TCA cycle intermediates, reflecting enhanced carbohydrates, amino acids and nucleotides biosynthesis. In contrast, ATP down-regulated carbohydrate and purine metabolism, inhibiting sustainable growth of microalgae. This study offers theoretical support for phosphorus-containing wastewater treatment using microalgae.

N-acetylglucosamine supplementation fails to bypass the critical acetylation of glucosamine-6-phosphate required for Toxoplasma gondii replication and invasion.

The cell surface of Toxoplasma gondii is rich in glycoconjugates which hold diverse and vital functions in the lytic cycle of this obligate intracellular parasite. Additionally, the cyst wall of bradyzoites, that shields the persistent form responsible for chronic infection from the immune system, is heavily glycosylated. Formation of glycoconjugates relies on activated sugar nucleotides, such as uridine diphosphate N-acetylglucosamine (UDP-GlcNAc). The glucosamine-phosphate-N-acetyltransferase (GNA1) generates N-acetylglucosamine-6-phosphate critical to produce UDP-GlcNAc. Here, we demonstrate that downregulation of T. gondii GNA1 results in a severe reduction of UDP-GlcNAc and a concomitant drop in glycosylphosphatidylinositols (GPIs), leading to impairment of the parasite's ability to invade and replicate in the host cell. Surprisingly, attempts to rescue this defect through exogenous GlcNAc supplementation fail to completely restore these vital functions. In depth metabolomic analyses elucidate diverse causes underlying the failed rescue: utilization of GlcNAc is inefficient under glucose-replete conditions and fails to restore UDP-GlcNAc levels in GNA1-depleted parasites. In contrast, GlcNAc-supplementation under glucose-deplete conditions fully restores UDP-GlcNAc levels but fails to rescue the defects associated with GNA1 depletion. Our results underscore the importance of glucosamine-6-phosphate acetylation in governing T. gondii replication and invasion and highlight the potential of the evolutionary divergent GNA1 in Apicomplexa as a target for the development of much-needed new therapeutic strategies.

Pyruvate kinase 2 from Synechocystis sp. PCC 6803 increased substrate affinity via glucose-6-phosphate and ribose-5-phosphate for phosphoenolpyruvate consumption.

Pyruvate kinase (Pyk, EC 2.7.1.40) is a glycolytic enzyme that generates pyruvate and adenosine triphosphate (ATP) from phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP), respectively. Pyk couples pyruvate and tricarboxylic acid metabolisms. Synechocystis sp. PCC 6803 possesses two pyk genes (encoded pyk1, sll0587 and pyk2, sll1275). A previous study suggested that pyk2 and not pyk1 is essential for cell viability; however, its biochemical analysis is yet to be performed. Herein, we biochemically analyzed Synechocystis Pyk2 (hereafter, SyPyk2). The optimum pH and temperature of SyPyk2 were 7.0 and 55 °C, respectively, and the Km values for PEP and ADP under optimal conditions were 1.5 and 0.053 mM, respectively. SyPyk2 is activated in the presence of glucose-6-phosphate (G6P) and ribose-5-phosphate (R5P); however, it remains unaltered in the presence of adenosine monophosphate (AMP) or fructose-1,6-bisphosphate. These results indicate that SyPyk2 is classified as PykA type rather than PykF, stimulated by sugar monophosphates, such as G6P and R5P, but not by AMP. SyPyk2, considering substrate affinity and effectors, can play pivotal roles in sugar catabolism under nonphotosynthetic conditions.

Widely-targeted metabolomics and transcriptomics identify metabolites associated with flowering regulation of Choy Sum.

Choy Sum, a stalk vegetable highly valued in East and Southeast Asia, is characterized by its rich flavor and nutritional profile. Metabolite accumulation is a key factor in Choy Sum stalk development; however, no research has focused on metabolic changes during the development of Choy Sum, especially in shoot tip metabolites, and their effects on growth and flowering. Therefore, in the present study, we used a widely targeted metabolomic approach to analyze metabolites in Choy Sum stalks at the seedling (S1), bolting (S3), and flowering (S5) stages. In total, we identified 493 metabolites in 31 chemical categories across all three developmental stages. We found that the levels of most carbohydrates and amino acids increased during stalk development and peaked at S5. Moreover, the accumulation of amino acids and their metabolites was closely related to G6P, whereas the expression of flowering genes was closely related to the content of T6P, which may promote flowering by upregulating the expressions of BcSOC1, BcAP1, and BcSPL5. The results of this study contribute to our understanding of the relationship between the accumulation of stem tip substances during development and flowering and of the regulatory mechanisms of stalk development in Choy Sum and other related species.

Structural insights in to the atypical type-I ABC Glucose-6-phosphate importer VCA0625-27 of Vibrio cholerae.

Sugar phosphates are potential sources of carbon and phosphate for bacteria. Despite that the process of internalization of Glucose-6-Phosphate (G6P) through plasma membrane remained elusive in several bacteria. VCA0625-27, made of periplasmic ligand binding protein (PLBP) VCA0625, an atypical monomeric permease VCA0626, and a cytosolic ATPase VCA0627, recently emerged as hexose-6-phosphate uptake system of Vibrio cholerae. Here we report high resolution crystal structure of VCA0625 in G6P bound state that largely resembles AfuA of Actinobacillus pleuropneumoniae. MD simulations on VCA0625 in apo and G6P bound states unraveled an 'open to close' and swinging bi-lobal motions, which are diminished upon G6P binding. Mutagenesis followed by biochemical assays on VCA0625 underscored that R34 works as gateway to bind G6P. Although VCA0627 binds ATP, it is ATPase deficient in the absence of VCA0625 and VCA0626, which is a signature phenomenon of type-I ABC importer. Further, modeling, docking and systematic sequence analysis allowed us to envisage the existence of similar atypical type-I G6P importer with fused monomeric permease in 27 other gram-negative bacteria.

The oxidative pentose phosphate pathway in photosynthesis: a tale of two shunts.

CO2 release in the light (RL) and its presumed source, oxidative pentose phosphate pathways, were found to be insensitive to CO2 concentration. The oxidative pentose phosphate pathways form glucose 6-phosphate (G6P) shunts that bypass the nonoxidative pentose phosphate reactions of the Calvin-Benson cycle. Using adenosine diphosphate glucose and uridine diphosphate glucose as proxies for labeling of G6P in the stroma and cytosol respectively, it was found that only the cytosolic shunt was active. Uridine diphosphate glucose, a proxy for cytosolic G6P, and 6-phosphogluconate (6PG) were significantly less labeled than Calvin-Benson cycle intermediates in the light. But ADP glucose, a proxy for stromal G6P, is labeled to the same degree as Calvin-Benson cycle intermediates and much greater than 6PG. A metabolically inert pool of sedoheptulose bisphosphate can slowly equilibrate keeping the label in sedoheptulose lower than in other stromal metabolites. Finally, phosphorylation of fructose 6-phosphate (F6P) in the cytosol can allow some unlabeled carbon in cytosolic F6P to dilute label in phosphenolpyruvate. The results clearly show that there is oxidative pentose phosphate pathway activity in the cytosol that provides a shunt around the nonoxidative pentose phosphate pathway reactions of the Calvin-Benson cycle and is not strongly CO2-sensitive.

Enabling High Activation of Glucose-6-Phosphate Dehydrogenase Activity Through Liquid Condensate Formation and Compression.

Droplet formation via liquid-liquid phase separation is thought to be involved in the regulation of various biological processes, including enzymatic reactions. We investigated a glycolytic enzymatic reaction, the conversion of glucose-6-phosphate to 6-phospho-D-glucono-1,5-lactone with concomitant reduction of NADP+ to NADPH both in the absence and presence of dynamically controlled liquid droplet formation. Here, the nucleotide serves as substrate as well as the scaffold required for the formation of liquid droplets. To further expand the process parameter space, temperature and pressure dependent measurements were performed. Incorporation of the reactants in the liquid droplet phase led to a boost in enzymatic activity, which was most pronounced at medium-high pressures. The crowded environment of the droplet phase induced a marked increase of the affinity of the enzyme and substrate. An increase in turnover number in the droplet phase at high pressure contributed to a further strong increase in catalytic efficiency. Enzyme systems that are dynamically coupled to liquid condensate formation may be the key to deciphering many biochemical reactions. Expanding the process parameter space by adjusting temperature and pressure conditions can be a means to further increase the efficiency of industrial enzyme utilization and help uncover regulatory mechanisms adopted by extremophiles.

Hepatic ChREBP orchestrates intrahepatic carbohydrate metabolism to limit hepatic glucose 6-phosphate and glycogen accumulation in a mouse model for acute Glycogen Storage Disease type Ib.

OBJECTIVE: Carbohydrate Response Element Binding Protein (ChREBP) is a glucose 6-phosphate (G6P)-sensitive transcription factor that acts as a metabolic switch to maintain intracellular glucose and phosphate homeostasis. Hepatic ChREBP is well-known for its regulatory role in glycolysis, the pentose phosphate pathway, and de novo lipogenesis. The physiological role of ChREBP in hepatic glycogen metabolism and blood glucose regulation has not been assessed in detail, and ChREBP's contribution to carbohydrate flux adaptations in hepatic Glycogen Storage Disease type 1 (GSD I) requires further investigation. METHODS: The current study aimed to investigate the role of ChREBP as a regulator of glycogen metabolism in response to hepatic G6P accumulation, using a model for acute hepatic GSD type Ib. The immediate biochemical and regulatory responses to hepatic G6P accumulation were evaluated upon G6P transporter inhibition by the chlorogenic acid S4048 in mice that were either treated with a short hairpin RNA (shRNA) directed against ChREBP (shChREBP) or a scrambled shRNA (shSCR). Complementary stable isotope experiments were performed to quantify hepatic carbohydrate fluxes in vivo. RESULTS: ShChREBP treatment normalized the S4048-mediated induction of hepatic ChREBP target genes to levels observed in vehicle- and shSCR-treated controls. In parallel, hepatic shChREBP treatment in S4048-infused mice resulted in a more pronounced accumulation of hepatic glycogen and further reduction of blood glucose levels compared to shSCR treatment. Hepatic ChREBP knockdown modestly increased glucokinase (GCK) flux in S4048-treated mice while it enhanced UDP-glucose turnover as well as glycogen synthase and phosphorylase fluxes. Hepatic GCK mRNA and protein levels were induced by shChREBP treatment in both vehicle- and S4048-treated mice, while glycogen synthase 2 (GYS2) and glycogen phosphorylase (PYGL) mRNA and protein levels were reduced. Finally, knockdown of hepatic ChREBP expression reduced starch domain binding protein 1 (STBD1) mRNA and protein levels while it inhibited acid alpha-glucosidase (GAA) activity, suggesting reduced capacity for lysosomal glycogen breakdown. CONCLUSIONS: Our data show that ChREBP activation controls hepatic glycogen and blood glucose levels in acute hepatic GSD Ib through concomitant regulation of glucose phosphorylation, glycogenesis, and glycogenolysis. ChREBP-mediated control of GCK enzyme levels aligns with corresponding adaptations in GCK flux. In contrast, ChREBP activation in response to acute hepatic GSD Ib exerts opposite effects on GYS2/PYGL enzyme levels and their corresponding fluxes, indicating that GYS2/PYGL expression levels are not limiting to their respective fluxes under these conditions.

Discovery and biochemical characterization of two hexokinases from Crassostrea gigas.

Hexokinases (HKs) play a vital role in glucose metabolism, which controls the first committed step catalyzing the production of glucose-6-phosphate from glucose. Two HKs (CGIHK1 and CGIHK2) from the Pacific oyster Crassostrea giga were cloned and characterized. CGIHK1 and CGIHK2 were recombinantly expressed in Escherichia coli and successfully purified by the Ni-NTA column. The optimum pH of the two enzymes was pH 8.0 and 8.5, respectively. The optimum temperature of the two enzymes was 42 °C and 50 °C, respectively. Both enzymes showed a clear requirement for divalent magnesium and were strongly inhibited by SDS. CGIHK1 exhibited highly strict substrate specificity to glucose, while CGIHK2 could also catalyze other 11 monosaccharide substrates. This is the first report on the in vitro biosynthesis of glucose-6-phosphate by the hexokinases from Crassostrea gigas. The facile expression and purification procedures combined with different substrate specificities make CGIHK1 and CGIHK2 candidates for the biosynthesis of glucose-6-phosphate and other sugar-phosphates.

Synthesis of a novel fluorinated phosphonyl C-glycoside, (3-deoxy-3-fluoro-ß-d-glucopyranosyl)methylphosphonate, a potential inhibitor of ß-phosphoglucomutase.

ß-phosphoglucomutase (ßPGM) catalyzes the conversion of ß-glucose 1-phosphate (ßG1P) to glucose-6-phosphate (G6P), a universal source of cellular energy, in a two-step process. Transition state analogue (TSA) complexes formed from substrate analogues and a metal fluoride (MgF3- and AlF4-) enable analysis of each of these enzymatic steps independently. Novel substrate analogues incorporating fluorine offer opportunities to interrogate the enzyme mechanism using 19F NMR spectroscopy. Herein, the synthesis of a novel fluorinated phosphonyl C-glycoside (3-deoxy-3-fluoro-ß-d-glucopyranosyl)methylphosphonate (1), in 12 steps (0.85 % overall yield) is disclosed. A four-stage synthetic strategy was employed, involving: 1) fluorine addition to the monosaccharide, 2) selective anomeric deprotection, 3) phosphonylation of the anomeric centre, and 4) global deprotection. Analysis of ßPGM and 1 will be reported in due course.

Phosphonate and Thiasugar Analogues of Glucosamine-6-phosphate: Activation of the glmS Riboswitch and Antibiotic Activity.

The glmS riboswitch is a motif found in 5'-untranslated regions of bacterial mRNA that controls the synthesis of glucosamine-6-phosphate (GlcN6P), an essential building block for the bacterial cell wall, by a feedback mechanism. Activation of the glmS riboswitch by GlcN6P mimics interferes with the ability of bacteria to synthesize its cell wall. Accordingly, GlcN6P mimics acting as glmS activators are promising candidates for future antibiotic drugs that may overcome emerging bacterial resistance against established antibiotics. We describe the synthesis of a series of phosphonate mimics of GlcN6P as well as the thiasugar analogue of GlcN6P. The phosphonate mimics differ in their pKa value to answer the question of whether derivatives with a pKa matching that of GlcN6P would be efficient glmS activators. We found that all derivatives activate the riboswitch, however, less efficiently than GlcN6P. This observation can be explained by the missing hydrogen bonds in the case of phosphonates and is valuable information for the design of future GlcN6P mimics. The thiasugar analogue of GlcN6P on the other hand turned out to be a glmS riboswitch activator with the same activity as the natural metabolite GlcN6P. The nonphosphorylated thiasugar displayed antimicrobial activity against certain bacilli. Therefore, the compound is a promising lead structure for the development of future antibiotics with a potentially novel mode of action.

HEXOKINASE1 and glucose-6-phosphate fuel plant growth and development.

As photoautotrophic organisms, plants produce an incredible spectrum of pigments, anti-herbivory compounds, structural materials and energic intermediates. These biosynthetic routes help plants grow, reproduce and mitigate stress. HEXOKINASE1 (HXK1), a metabolic enzyme and glucose sensor, catalyzes the phosphorylation of hexoses, a key introductory step for many of these pathways. However, previous studies have largely focused on the glucose sensing and signaling functions of HXK1, and the importance of the enzyme's catalytic function is only recently being connected to plant development. In this brief Spotlight, we describe the developmental significance of plant HXK1 and its role in plant metabolic pathways, specifically in glucose-6-phosphate production. Furthermore, we describe the emerging connections between metabolism and development and suggest that HXK1 signaling and catalytic activity regulate discrete areas of plant development.

G6PC1 and G6PC2 influence G6P flux but not HSD11B1 activity.

In the endoplasmic reticulum (ER) lumen, glucose-6-phosphatase catalytic subunit 1 and 2 (G6PC1; G6PC2) hydrolyze glucose-6-phosphate (G6P) to glucose and inorganic phosphate whereas hexose-6-phosphate dehydrogenase (H6PD) hydrolyzes G6P to 6-phosphogluconate (6PG) in a reaction that generates NADPH. 11ß-hydroxysteroid dehydrogenase type 1 (HSD11B1) utilizes this NADPH to convert inactive cortisone to cortisol. HSD11B1 inhibitors improve insulin sensitivity whereas G6PC inhibitors are predicted to lower fasting blood glucose (FBG). This study investigated whether G6PC1 and G6PC2 influence G6P flux through H6PD and vice versa. Using a novel transcriptional assay that utilizes separate fusion genes to quantitate glucocorticoid and glucose signaling, we show that overexpression of H6PD and HSD11B1 in the islet-derived 832/13 cell line activated glucocorticoid-stimulated fusion gene expression. Overexpression of HSD11B1 blunted glucose-stimulated fusion gene expression independently of altered G6P flux. While overexpression of G6PC1 and G6PC2 blunted glucose-stimulated fusion gene expression, it had minimal effect on glucocorticoid-stimulated fusion gene expression. In the liver-derived HepG2 cell line, overexpression of H6PD and HSD11B1 activated glucocorticoid-stimulated fusion gene expression but overexpression of G6PC1 and G6PC2 had no effect. In rodents, HSD11B1 converts 11-dehydrocorticosterone (11-DHC) to corticosterone. Studies in wild-type and G6pc2 knockout mice treated with 11-DHC for 5 weeks reveal metabolic changes unaffected by the absence of G6PC2. These data suggest that HSD11B1 activity is not significantly affected by the presence or absence of G6PC1 or G6PC2. As such, G6PC1 and G6PC2 inhibitors are predicted to have beneficial effects by reducing FBG without causing a deleterious increase in glucocorticoid signaling.

Acute stress induces long-term metabolic, functional, and structural remodeling of the heart.

Takotsubo cardiomyopathy is a stress-induced cardiovascular disease with symptoms comparable to those of an acute coronary syndrome but without coronary obstruction. Takotsubo was initially considered spontaneously reversible, but epidemiological studies revealed significant long-term morbidity and mortality, the reason for which is unknown. Here, we show in a female rodent model that a single pharmacological challenge creates a stress-induced cardiomyopathy similar to Takotsubo. The acute response involves changes in blood and tissue biomarkers and in cardiac in vivo imaging acquired with ultrasound, magnetic resonance and positron emission tomography. Longitudinal follow up using in vivo imaging, histochemistry, protein and proteomics analyses evidences a continued metabolic reprogramming of the heart towards metabolic malfunction, eventually leading to irreversible damage in cardiac function and structure. The results combat the supposed reversibility of Takotsubo, point to dysregulation of glucose metabolic pathways as a main cause of long-term cardiac disease and support early therapeutic management of Takotsubo.

Phosphonate and α-fluorophosphonate analogs of d-glucose 6-phosphate as active-site probes of 1l-myo-inositol 1-phosphate synthase.

Phosphate ester analogs in which the bridging oxygen is replaced with a methylene or fluoromethylene group are well known non-hydrolyzable mimics of use as inhibitors and substrate analogs for reactions involving phosphate esters. Properties of the replaced oxygen are often best mimicked by a mono-fluoromethylene group, but such groups are challenging to synthesize and can exist as two stereoisomers. Here, we describe the protocol for our method of synthesizing the α-fluoromethylene analogs of d-glucose 6-phosphate (G6P), as well as the methylene and difluoromethylene analogs, and their application in the study of 1l-myo-inositol-1-phosphate synthase (mIPS). mIPS catalyzes the synthesis of 1l-myo-inositol 1-phosphate (mI1P) from G6P, in an NAD-dependent aldol cyclization. Its key role in myo-inositol metabolism makes it a putative target for the treatment of several health disorders. The design of these inhibitors allowed for the possibility of substrate-like behavior, reversible inhibition, or mechanism-based inactivation. In this chapter, the synthesis of these compounds, expression and purification of recombinant hexahistidine-tagged mIPS, the mIPS kinetic assay and methods for determining the behavior of the phosphate analogs in the presence of mIPS, and a docking approach to rationalizing the observed behavior are described.

Dioxygen and glucose force motion of the electron-transfer switch in the iron(III) flavohemoglobin-type nitric oxide dioxygenase.

Kinetic and structural investigations of the flavohemoglobin-type NO dioxygenase have suggested critical roles for transient Fe(III)O2 complex formation and O2-forced movements affecting hydride transfer to the FAD cofactor and electron-transfer to the Fe(III)O2 complex. Stark-effect theory together with structural models and dipole and internal electrostatic field determinations provided a semi-quantitative spectroscopic method for investigating the proposed Fe(III)O2 complex and O2-forced movements. Deoxygenation of the enzyme causes Stark effects on the ferric heme Soret and charge-transfer bands revealing the Fe(III)O2 complex. Deoxygenation also elicits Stark effects on the FAD that expose forces and motions that create a more restricted NADH access to FAD for hydride transfer and switch electron-transfer off. Glucose also forces the enzyme toward an off state. Amino acid substitutions at the B10, E7, E11, G8, D5, and F7 positions influence the Stark effects of O2 on resting heme spin states and FAD consistent with the proposed roles of the side chains in the enzyme mechanism. Deoxygenation of ferric myoglobin and hemoglobin A also induces Stark effects on the hemes suggesting a common 'oxy-met' state. The ferric myoglobin and hemoglobin heme spectra are also glucose-responsive. A conserved glucose or glucose-6-phosphate binding site is found bridging the BC-corner and G-helix in flavohemoglobin and myoglobin suggesting novel allosteric effector roles for glucose or glucose-6-phosphate in the NO dioxygenase and O2 storage functions. The results support the proposed roles of a ferric O2 intermediate and protein motions in regulating electron-transfer during NO dioxygenase turnover.

Alterations in Primary Carbon Metabolism in Cucumber Infected with Pseudomonas syringae pv lachrymans: Local and Systemic Responses.

The reconfiguration of the primary metabolism is essential in plant-pathogen interactions. We compared the local metabolic responses of cucumber leaves inoculated with Pseudomonas syringae pv lachrymans (Psl) with those in non-inoculated systemic leaves, by examining the changes in the nicotinamide adenine dinucleotides pools, the concentration of soluble carbohydrates and activities/gene expression of carbohydrate metabolism-related enzymes, the expression of photosynthesis-related genes, and the tricarboxylic acid cycle-linked metabolite contents and enzyme activities. In the infected leaves, Psl induced a metabolic signature with an altered [NAD(P)H]/[NAD(P)+] ratio; decreased glucose and sucrose contents, along with a changed invertase gene expression; and increased glucose turnover and accumulation of raffinose, trehalose, and myo-inositol. The accumulation of oxaloacetic and malic acids, enhanced activities, and gene expression of fumarase and l-malate dehydrogenase, as well as the increased respiration rate in the infected leaves, indicated that Psl induced the tricarboxylic acid cycle. The changes in gene expression of ribulose-l,5-bis-phosphate carboxylase/oxygenase large unit, phosphoenolpyruvate carboxylase and chloroplast glyceraldehyde-3-phosphate dehydrogenase were compatible with a net photosynthesis decline described earlier. Psl triggered metabolic changes common to the infected and non-infected leaves, the dynamics of which differed quantitatively (e.g., malic acid content and metabolism, glucose-6-phosphate accumulation, and glucose-6-phosphate dehydrogenase activity) and those specifically related to the local or systemic response (e.g., changes in the sugar content and turnover). Therefore, metabolic changes in the systemic leaves may be part of the global effects of local infection on the whole-plant metabolism and also represent a specific acclimation response contributing to balancing growth and defense.