Allosteric regulatory properties of muscle phosphofructokinase

Site-specific crosslinking reveals Phosphofructokinase-L inhibition drives self-assembly and attenuation of protein interactions

Phosphofructokinase is the central enzyme in glycolysis and constitutes a highly regulated step. The liver isoform (PFKL) compartmentalizes during activation and inhibition in vitro and in vivo, respectively. Compartmentalized PFKL is hypothesized to modulate metabolic flux consistent with its central role as the rate limiting step in glycolysis. PFKL tetramers self-assemble at two interfaces in the monomer (interface 1 and 2), yet how these interfaces contribute to PFKL compartmentalization and drive protein interactions remains unclear. Here, we used site-specific incorporation of noncanonical photocrosslinking amino acids to identify PFKL interactors at interface 1, 2, and the active site. Tandem mass tag-based quantitative interactomics reveals interface 2 as a hotspot for PFKL interactions, particularly with cytoskeletal, glycolytic, and carbohydrate derivative metabolic proteins. Furthermore, PFKL compartmentalization into puncta was observed in human cells using citrate inhibition. Puncta formation attenuated crosslinked protein-protein interactions with the cytoskeleton at interface 2. This result suggests that PFKL compartmentalization sequesters interface 2, but not interface 1, and may modulate associated protein assemblies with the cytoskeleton.

Site-Specific Crosslinking Reveals Phosphofructokinase-L Inhibition Drives Self-Assembly and Attenuation of Protein Interactions

Phosphofructokinase is the central enzyme in glycolysis and constitutes a highly regulated step. The liver isoform (PFKL) compartmentalizes during activation and inhibition in vitro and in vivo respectively. Compartmentalized PFKL is hypothesized to modulate metabolic flux consistent with its central role as the rate limiting step in glycolysis. PFKL tetramers self-assemble at two interfaces in the monomer (interface 1 and 2), yet how these interfaces contribute to PFKL compartmentalization and drive protein interactions remains unclear. Here, we used site-specific incorporation of noncanonical photocrosslinking amino acids to identify PFKL interactors at interface 1, 2, and the active site. Tandem mass tag-based quantitative interactomics reveals interface 2 as a hotspot for PFKL interactions, particularly with cytoskeletal, glycolytic, and carbohydrate derivative metabolic proteins. Furthermore, PFKL compartmentalization into puncta was observed in human cells using citrate inhibition. Puncta formation attenuated crosslinked protein-protein interactions with the cytoskeleton at interface 2. This result suggests that PFKL compartmentalization sequesters interface 2, but not interface 1, and may modulate associated protein assemblies with the cytoskeleton.

Extreme Physiology Extreme Tolerance to Hypoxia, Hypercapnia, and Pain in the Naked Mole-Rat

Challenging environmental conditions can drive the evolution of extreme physiological traits. The naked mole-rat has evolved to survive and thrive in a low oxygen, high carbon dioxide environment that would be deadly to humans and most other mammals. The naked mole-rat's lifestyle is unusual in that this species combines subterranean living and living in large, social groups of up to 300 + individuals. Many respiring animals in a closed environment can lead to depletion of oxygen (hypoxia) and accumulation of carbon dioxide (hypercapnia). Naked mole-rats display a variety of physiological traits that negate the adverse effects of living in this atmosphere. For hypoxia tolerance, naked mole-rats have a low resting metabolism, high affinity hemoglobin, intrinsic brain tolerance, the ability to use fructose for anaerobic glycolysis, and the ability to enter a low energy, suspended animation-like state. For hypercapnia tolerance, these animals have a mutation in a voltage gated sodium channel that effectively eliminates neuronal responses to tissue acidosis. In other mammals, acidosis from exposure to high concentrations of carbon dioxide induces pain and pulmonary edema. Understanding these mechanisms of extreme physiology is not only inherently interesting, but it may lead to biomedical breakthroughs in research on heart attacks, strokes, and pain pathologies.

Metabolites as signalling molecules

Traditional views of cellular metabolism imply that it is passively adapted to meet the demands of the cell. It is becoming increasingly clear, however, that metabolites do more than simply supply the substrates for biological processes; they also provide critical signals, either through effects on metabolic pathways or via modulation of other regulatory proteins. Recent investigation has also uncovered novel roles for several metabolites that expand their signalling influence to processes outside metabolism, including nutrient sensing and storage, embryonic development, cell survival and differentiation, and immune activation and cytokine secretion. Together, these studies suggest that, in contrast to the prevailing notion, the biochemistry of a cell is frequently governed by its underlying metabolism rather than vice versa. This important shift in perspective places common metabolites as key regulators of cell phenotype and behaviour. Yet the signalling metabolites, and the cognate targets and transducers through which they signal, are only beginning to be uncovered. In this Review, we discuss the emerging links between metabolism and cellular behaviour. We hope this will inspire further dissection of the mechanisms through which metabolic pathways and intermediates modulate cell function and will suggest possible drug targets for diseases linked to metabolic deregulation.

The Na+/K+ pump dominates control of glycolysis in hippocampal dentate granule cells

Cellular ATP that is consumed to perform energetically expensive tasks must be replenished by new ATP through the activation of metabolism. Neuronal stimulation, an energetically demanding process, transiently activates aerobic glycolysis, but the precise mechanism underlying this glycolysis activation has not been determined. We previously showed that neuronal glycolysis is correlated with Ca2+ influx, but is not activated by feedforward Ca2+ signaling (Díaz-García et al., 2021a). Since ATP-powered Na+ and Ca2+ pumping activities are increased following stimulation to restore ion gradients and are estimated to consume most neuronal ATP, we aimed to determine if they are coupled to neuronal glycolysis activation. By using two-photon imaging of fluorescent biosensors and dyes in dentate granule cell somas of acute mouse hippocampal slices, we observed that production of cytoplasmic NADH, a byproduct of glycolysis, is strongly coupled to changes in intracellular Na+, while intracellular Ca2+ could only increase NADH production if both forward Na+/Ca2+ exchange and Na+/K+ pump activity were intact. Additionally, antidromic stimulation-induced intracellular [Na+] increases were reduced >50% by blocking Ca2+ entry. These results indicate that neuronal glycolysis activation is predominantly a response to an increase in activity of the Na+/K+ pump, which is strongly potentiated by Na+ influx through the Na+/Ca2+ exchanger during extrusion of Ca2+ following stimulation.

Ovine conceptus homogenates metabolize fructose for metabolic support during the peri-implantation period of pregnancy

Roles of fructose in elongating ovine conceptuses are poorly understood, despite it being the major hexose sugar in fetal fluids and plasma throughout gestation. Therefore, we determined if elongating ovine conceptuses utilize fructose via metabolic pathways for survival and development. Immunohistochemical analyses revealed that trophectoderm and extra-embryonic endoderm express ketohexokinase and aldolase B during the peri-implantation period of pregnancy for conversion of fructose into fructose-1-phosphate for entry into glycolysis and related metabolic pathways. Conceptus homogenates were cultured with 14C-labeled glucose and/or fructose under oxygenated and hypoxic conditions to assess contributions of glucose and fructose to the pentose cycle (PC), tricarboxylic acid cycle, glycoproteins, and lipid synthesis. Results indicated that both glucose and fructose contributed carbons to each of these pathways, except for lipid synthesis, and metabolized to pyruvate and lactate, with lactate being the primary product of glycolysis under oxygenated and hypoxic conditions. We also found that: 1) conceptuses preferentially oxidized glucose over fructose (P < 0.05); 2) incorporation of fructose and glucose at 4 mM each into the PC by Day 17 conceptus homogenates was similar in the presence or absence of glucose, but incorporation of glucose into the PC was enhanced by the presence of fructose (P < 0.05); 3) incorporation of fructose into the PC in the absence of glucose was greater under oxygenated conditions (P < 0.01); and 4) incorporation of glucose into the PC under oxygenated conditions was greater in the presence of fructose (P = 0.05). These results indicate that fructose is an important metabolic substrate for ovine conceptuses.

Molecular mechanism of glycolytic flux control intrinsic to human phosphoglycerate kinase

Glycolysis plays a fundamental role in energy production and metabolic homeostasis. The intracellular [adenosine triphosphate]/[adenosine diphosphate] ([ATP]/[ADP]) ratio controls glycolytic flux; however, the regulatory mechanism underlying reactions catalyzed by individual glycolytic enzymes enabling flux adaptation remains incompletely understood. Phosphoglycerate kinase (PGK) catalyzes the reversible phosphotransfer reaction, which directly produces ATP in a near-equilibrium step of glycolysis. Despite extensive studies on the transcriptional regulation of PGK expression, the mechanism in response to changes in the [ATP]/[ADP] ratio remains obscure. Here, we report a protein-level regulation of human PGK (hPGK) by utilizing the switching ligand-binding cooperativities between adenine nucleotides and 3-phosphoglycerate (3PG). This was revealed by nuclear magnetic resonance (NMR) spectroscopy at physiological salt concentrations. MgADP and 3PG bind to hPGK with negative cooperativity, whereas MgAMPPNP (a nonhydrolyzable ATP analog) and 3PG bind to hPGK with positive cooperativity. These opposite cooperativities enable a shift between different ligand-bound states depending on the intracellular [ATP]/[ADP] ratio. Based on these findings, we present an atomic-scale description of the reaction scheme for hPGK under physiological conditions. Our results indicate that hPGK intrinsically modulates its function via ligand-binding cooperativities that are finely tuned to respond to changes in the [ATP]/[ADP] ratio. The alteration of ligand-binding cooperativities could be one of the self-regulatory mechanisms for enzymes in bidirectional pathways, which enables rapid adaptation to changes in the intracellular environment.

Mitochondrial Efflux of Citrate and Isocitrate Is Fully Dispensable for Glucose-Stimulated Insulin Secretion and Pancreatic Islet β-Cell Function

The defining feature of pancreatic islet β-cell function is the precise coordination of changes in blood glucose levels with insulin secretion to regulate systemic glucose homeostasis. While ATP has long been heralded as a critical metabolic coupling factor to trigger insulin release, glucose-derived metabolites have been suggested to further amplify fuel-stimulated insulin secretion. The mitochondrial export of citrate and isocitrate through the citrate-isocitrate carrier (CIC) has been suggested to initiate a key pathway that amplifies glucose-stimulated insulin secretion, though the physiological significance of β-cell CIC-to-glucose homeostasis has not been established. Here, we generated constitutive and adult CIC β-cell knockout (KO) mice and demonstrate that these animals have normal glucose tolerance, similar responses to diet-induced obesity, and identical insulin secretion responses to various fuel secretagogues. Glucose-stimulated NADPH production was impaired in β-cell CIC KO islets, whereas glutathione reduction was retained. Furthermore, suppression of the downstream enzyme cytosolic isocitrate dehydrogenase (Idh1) inhibited insulin secretion in wild-type islets but failed to impact β-cell function in β-cell CIC KO islets. Our data demonstrate that the mitochondrial CIC is not required for glucose-stimulated insulin secretion and that additional complexities exist for the role of Idh1 and NADPH in the regulation of β-cell function.

Targeting immune cell metabolism in kidney diseases

Insights into the relationship between immunometabolism and inflammation have enabled the targeting of several immunity-mediated inflammatory processes that underlie infectious diseases and cancer or drive transplant rejection, but this field remains largely unexplored in kidney diseases. The kidneys comprise heterogeneous cell populations, contain distinct microenvironments such as areas of hypoxia and hypersalinity, and are responsible for a functional triad of filtration, reabsorption and secretion. These distinctive features create myriad potential metabolic therapeutic targets in the kidney. Immune cells have crucial roles in the maintenance of kidney homeostasis and in the response to kidney injury, and their function is intricately connected to their metabolic properties. Changes in nutrient availability and biomolecules, such as cytokines, growth factors and hormones, initiate cellular signalling events that involve energy-sensing molecules and other metabolism-related proteins to coordinate immune cell differentiation, activation and function. Disruption of homeostasis promptly triggers the metabolic reorganization of kidney immune and non-immune cells, which can promote inflammation and tissue damage. The metabolic differences between kidney and immune cells offer an opportunity to specifically target immunometabolism in the kidney.

A Sweet Story of Metabolic Innovation in the Naked Mole-Rat

The naked mole-rat's (Heterocephalus glaber) social and subterranean lifestyle imposes several evolutionary pressures which have shaped its physiology. One example is low oxygen availability in a crowded burrow system which the naked mole-rat has adapted to via several mechanisms. Here we describe a metabolic rewiring which enables the naked mole-rat to switch substrates in glycolysis from glucose to fructose thereby circumventing feedback inhibition at phosphofructokinase (PFK1) to allow unrestrained glycolytic flux and ATP supply under hypoxia. Preferential shift to fructose metabolism occurs in other species and biological systems as a means to provide fuel, water or like in the naked mole-rat, protection in a low oxygen environment. We review fructose metabolism through an ecological lens and suggest that the metabolic adaptation to utilize fructose in the naked mole-rat may have evolved to simultaneously combat multiple challenges posed by its hostile environment.

Probing carbohydrate metabolism using hyperpolarized 13 C-labeled molecules

Glycolysis is a fundamental metabolic process in all organisms. Anomalies in glucose metabolism are linked to various pathological conditions. In particular, elevated aerobic glycolysis is a characteristic feature of rapidly growing cells. Glycolysis and the closely related pentose phosphate pathway can be monitored in real time by hyperpolarized 13 C-labeled metabolic substrates such as 13 C-enriched, deuterated D-glucose derivatives, [2-13 C]-D-fructose, [2-13 C] dihydroxyacetone, [1-13 C]-D-glycerate, [1-13 C]-D-glucono-δ-lactone and [1-13 C] pyruvate in healthy and diseased tissues. Elevated glycolysis in tumors (the Warburg effect) was also successfully imaged using hyperpolarized [U-13 C6 , U-2 H7 ]-D-glucose, while the size of the preexisting lactate pool can be measured by 13 C MRS and/or MRI with hyperpolarized [1-13 C]pyruvate. This review summarizes the application of various hyperpolarized 13 C-labeled metabolites to the real-time monitoring of glycolysis and related metabolic processes in normal and diseased tissues.

High-fructose corn syrup enhances intestinal tumor growth in mice

Excessive consumption of beverages sweetened with high-fructose corn syrup (HFCS) is associated with obesity and with an increased risk of colorectal cancer. Whether HFCS contributes directly to tumorigenesis is unclear. We investigated the effects of daily oral administration of HFCS in adenomatous polyposis coli (APC) mutant mice, which are predisposed to develop intestinal tumors. The HFCS-treated mice showed a substantial increase in tumor size and tumor grade in the absence of obesity and metabolic syndrome. HFCS increased the concentrations of fructose and glucose in the intestinal lumen and serum, respectively, and the tumors transported both sugars. Within the tumors, fructose was converted to fructose-1-phosphate, leading to activation of glycolysis and increased synthesis of fatty acids that support tumor growth. These mouse studies support the hypothesis that the combination of dietary glucose and fructose, even at a moderate dose, can enhance tumorigenesis.

Inter-Active Site Communication Mediated by the Dimer Interface β-Sheet in the Half-the-Sites Enzyme, Thymidylate Synthase

Thymidylate synthase (TS) is a dimeric enzyme conserved in all life forms that exhibits the allosteric feature of half-the-sites activity. Neither the reason for nor the mechanism of this phenomenon is understood. We used a combined nuclear magnetic resonance (NMR) and molecular dynamics approach to study a stable intermediate preceding hydride transfer, which is the rate-limiting and half-the-sites step. In NMR titrations with ligands leading to this intermediate, we measured chemical shifts of the apoenzyme (lig₀), the saturated holoenzyme (lig₂), and the typically elusive singly bound (lig₁) states. Approximately 40 amides showed quartet patterns providing direct NMR evidence of coupling between the active site and probes >30 Å away in the distal subunit. Quartet peak patterns have symmetrical character, indicating reciprocity in communicating the first and second binding events to the distal protomer. Quartets include key catalytic residues and map to the dimer interface β-sheet, which also represents the shortest path between the two active sites. Simulations corroborate the coupling observed in solution in that there is excellent overlap between quartet residues and main-chain atoms having intersubunit cross-correlated motions. Simulations identify five hot spot residues, three of which lie at the kink in the unique β-bulge abutting the active sites on either end of the sheet. Interstrand cross-correlated motions become more organized and pronounced as the enzyme progresses from lig₀ to lig₁ and ultimately lig₂. Coupling in the apparently symmetrical complex has implications for half-the-sites reactivity and potentially resolves the paradox of inequivalent TS active sites despite the vast majority of X-ray structures appearing to be symmetrical.

Enhancement of ATP production ameliorates motor and cognitive impairments in a mouse model of MPTP-induced Parkinson's disease

Approximately 30-40% of patients with Parkinson's disease (PD) exhibit cognitive impairments. However, there are currently no clinically effective drugs for the treatment of cognitive impairment in patients with PD. Previous studies have suggested that mitochondrial dysfunction such as decreased adenosine triphosphate (ATP) production triggers dopaminergic neurodegeneration in patients with PD and that mitochondria represent a potential target for the development of novel treatments for preventing PD. Therefore, in the present study, we investigated the cognition-enhancing effects of ethyl pyruvate (EP) and 1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride (SA4503) in mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism. PD model mice were generated via treatment with MPTP (25 mg/kg, i.p.) once a day for 5 consecutive days. Twenty-four hours after the final injection of MPTP, mice were intraperitoneally injected with EP (25, 50, 100 mg/kg) or SA4503 (1 mg/kg) once a day for 4 weeks. Chronic administration of EP (100 mg/kg i.p.) or SA4503 (1 mg/kg, i.p.) improved both motor deficits and cognitive impairments in MPTP-treated mice. Furthermore, treatment with EP or SA4503 attenuated decreases in the levels of ATP and tyrosine hydroxylase (TH) in the substantia nigra pars compacta (SNpc)/ventral tegmental area (VTA), striatum, and hippocampal CA1 region. Administration of EP or SA4503 protected the dopaminergic neurons from MPTP-induce toxicity and restored the dopamine levels in the striatum. Elevated 4-hydroxy-2-nonenal- (4-HNE-) and nitrotyrosine-reactive protein levels induced by MPTP-treatment were suppressed by EP or SA4503 treatment in the SNpc-VTA, striatum, and hippocampal CA1 region. These observations suggest that EP and SA4503 attenuate cognitive impairments and motor dysfunction in mice with MPTP-induced PD.

The F1Fo-ATP Synthase β Subunit Is Required for Candida albicans Pathogenicity Due to Its Role in Carbon Flexibility

Previous work has explored link between mitochondrial biology and fungal pathogenicity in F₁F_o-ATP synthase in Candida albicans. In this work we have detailed the more specific roles of the F₁F_o-ATP synthase β subunit, a key protein subunit of F₁F_o-ATP synthase. The ability to assimilate alternative carbons in glucose-limited host niches is known to be a critical factor for infection caused by opportunistic pathogens including C. albicans. The function of the F₁F_o-ATP synthase β subunit was characterized through the construction of an ATP2 gene null mutant (atp2Δ/Δ) and the gene-reconstituted strain (atp2Δ/ATP2) in order to understand the link between carbon metabolism and C. albicans pathogenesis. Cell growth, viability, cellular ATP content, mitochondrial membrane potential (ΔΨm), and intracellular ROS were compared between null mutant and control strain. Results showed that growth of the atp2Δ/Δ mutant in synthetic medium was slower than in complex medium. However, the synthetic medium delayed the onset of reduced cell viability and kept cellular ATP content from becoming fully depleted. Consistent with these observations, we identified transcriptional changes in metabolic response that activated other ATP-generating pathways, thereby improving cell viability during the initial phase. Unlike glucose effects, the atp2Δ/Δ mutant exhibited an immediate and sharp reduction in cell viability on non-fermentable carbon sources, consistent with an immediate depletion of cellular ATP content. Along with a reduced viability in non-fermentable carbon sources, the atp2Δ/Δ mutant displayed avirulence in a murine model of disseminated candidiasis as well as lower fungal loads in mouse organs. Regardless of the medium, however, a decrease in mitochondrial membrane potential (ΔΨm) was found in the atp2Δ/Δ mutant but ROS levels remained in the normal range. These results suggest that the F₁F_o-ATP synthase β subunit is required for C. albicans pathogenicity and operates by affecting metabolic flexibility in carbon consumption.

Turning the respiratory flexibility of Mycobacterium tuberculosis against itself

The Mycobacterium tuberculosis (Mtb) electron transport chain (ETC) has received significant attention as a drug target, however its vulnerability may be affected by its flexibility in response to disruption. Here we determine the effect of the ETC inhibitors bedaquiline, Q203 and clofazimine on the Mtb ETC, and the value of the ETC as a drug target, by measuring Mtb's respiration using extracellular flux technology. We find that Mtb's ETC rapidly reroutes around inhibition by these drugs and increases total respiration to maintain ATP levels. Rerouting is possible because Mtb rapidly switches between terminal oxidases, and, unlike eukaryotes, is not susceptible to back pressure. Increased ETC activity potentiates clofazimine's production of reactive oxygen species, causing rapid killing in vitro and in a macrophage model. Our results indicate that combination therapy targeting the ETC can be exploited to enhance killing of Mtb.

Stimulation of acetoin production in metabolically engineered Lactococcus lactis by increasing ATP demand

Having a sufficient supply of energy, usually in the form of ATP, is essential for all living organisms. In this study, however, we demonstrate that it can be beneficial to reduce ATP availability when the objective is microbial production. By introducing the ATP hydrolyzing F₁-ATPase into a Lactococcus lactis strain engineered into producing acetoin, we show that production titer and yield both can be increased. At high F₁-ATPase expression level, the acetoin production yield could be increased by 10 %; however, because of the negative effect that the F₁-ATPase had on biomass yield and growth, this increase was at the cost of volumetric productivity. By lowering the expression level of the F₁-ATPase, both the volumetric productivity and the final yield could be increased by 5 % compared to the reference strain not overexpressing the F₁-ATPase, and in batch fermentation, it was possible to convert 176 mM (32 g/L) of glucose into 146.5 mM (12.9 g/L) acetoin with a yield of 83 % of the theoretical maximum. To further demonstrate the potential of the cell factory developed, we complemented it with the lactose plasmid pLP712, which allowed for growth and acetoin production from a dairy waste stream, deproteinized whey. Using this cheap and renewable feedstock, efficient acetoin production with a titer of 157 mM (14 g/L) acetoin was accomplished.

The space of enzyme regulation in HeLa cells can be inferred from its intracellular metabolome

During the transition from a healthy state to a cancerous one, cells alter their metabolism to increase proliferation. The underlying metabolic alterations may be caused by a variety of different regulatory events on the transcriptional or post-transcriptional level whose identification contributes to the rational design of therapeutic targets. We present a mechanistic strategy capable of inferring enzymatic regulation from intracellular metabolome measurements that is independent of the actual mechanism of regulation. Here, enzyme activities are expressed by the space of all feasible kinetic constants (k-cone) such that the alteration between two phenotypes is given by their corresponding kinetic spaces. Deriving an expression for the transformation of the healthy to the cancer k-cone we identified putative regulated enzymes between the HeLa and HaCaT cell lines. We show that only a few enzymatic activities change between those two cell lines and that this regulation does not depend on gene transcription but is instead post-transcriptional. Here, we identify phosphofructokinase as the major driver of proliferation in HeLa cells and suggest an optional regulatory program, associated with oxidative stress, that affects the activity of the pentose phosphate pathway.

Targeting the sugar metabolism of tumors with a first-in-class 6-phosphofructo-2-kinase (PFKFB4) inhibitor

Human tumors exhibit increased glucose uptake and metabolism as a result of high demand for ATP and anabolic substrates and this metabolotype is a negative prognostic indicator for survival. Recent studies have demonstrated that cancer cells from several tissue origins and genetic backgrounds require the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4), a regulatory enzyme that synthesizes an allosteric activator of glycolysis, fructose-2,6-bisphosphate. We report the discovery of a first-in-class PFKFB4 inhibitor, 5-(n-(8-methoxy-4-quinolyl)amino)pentyl nitrate (5MPN), using structure-based virtual computational screening. We find that 5MPN is a selective inhibitor of PFKFB4 that suppresses the glycolysis and proliferation of multiple human cancer cell lines but not non-transformed epithelial cells in vitro. Importantly, 5MPN has high oral bioavailability and per os administration of a non-toxic dose of 5MPN suppresses the glucose metabolism and growth of tumors in mice.

Allosteric regulation in phosphofructokinase from the extreme thermophile Thermus thermophilus

An investigation into the kinetics and regulatory properties of the type-1 phosphofructokinase (PFK) from the extreme thermophile Thermus thermophilus (TtPFK) reveals an enzyme that is inhibited by PEP and activated by ADP by modifying the affinity exhibited for the substrate fructose 6-phosphate (Fru-6-P) in a manner analogous to other prokaryotic PFKs. However, TtPFK binds both of these allosteric ligands significantly more tightly than other bacterial PFKs while effecting a substantially more modest extent of inhibition or activation at 25 °C, reinforcing the principle that binding affinity and effectiveness can be both independent and uncorrelated to one another. These properties have allowed us to establish rigorously that PEP only inhibits by antagonizing the binding of Fru-6-P and not by influencing turnover, a conclusion that requires kcat to be determined under conditions in which both inhibitor and substrate are saturating simultaneously. In addition, the temperature dependence of the allosteric effects on Fru-6-P binding indicate that the coupling free energies are entropy-dominated, as observed previously for PFK from Bacillus stearothermophilus but not for PFK from Escherichia coli , supporting the hypothesis that entropy-dominated allosteric effects may be a characteristic of enzymes derived from thermostable organisms. For such enzymes, the root cause of the allosteric effect may not be easily discerned from static structural information such as that obtained from X-ray crystallography.

Aripiprazole increases NADPH level in PC12 cells: the role of NADPH oxidase

In aripiprazole-treated PC12 cells, we previously showed that the mitochondrial membrane potential (Δψm) was rather increased in spite of lowered cytochrome c oxidase activity. To address these inconsistent results, we focused the NADPH generation by glucose-6-phosphate dehydrogenase (G6PD), a rate-limiting enzyme of the pentose phosphate pathway (PPP), to titrate reactive oxygen species (ROS) that results in the Δψm maintenance. G6PD may be also involved in another inconsistent result of lowered intracellular lactate level in aripiprazole-treated PC12 cells, because PPP competes glucose-6-phosphate with the glycolytic pathway, resulting in the downregulation of glycolysis. Therefore, we assayed intracellular amounts of NADPH, ROS, and the activities of the enzymes generating or consuming NADPH (G6PD, NADP(+)-dependent isocitrate dehydrogenase, NADP(+)-dependent malic enzyme, glutathione reductase, and NADPH oxidase [NOX]) and estimated glycolysis in 50 μM aripiprazole-, clozapine-, and haloperidol-treated PC12 cells. NADPH levels were enhanced only in aripiprazole-treated ones. Only haloperidol increased ROS. However, the enzyme activities did not show significant changes toward enhancing NADPH level except for the aripiprazole-induced decrease in NOX activity. Thus, the lowered NOX activity could have contributed to the aripiprazole-induced increase in the NADPH level by lowering ROS generation, resulting in maintained Δψm. Although the aforementioned assumption was invalid, the ratio of fructose-1,6-bisphosphate to fructose-6-phosphate was decreased by all antipsychotics examined. Pyruvate kinase activity was enhanced only by aripiprazole. In summary, these observations indicate that aripiprazole possibly possesses the pharmacological superiority to clozapine and haloperidol in the ROS generation and the adjustment of glycolytic pathway.

Structure and allosteric regulation of eukaryotic 6-phosphofructokinases

Although the crystal structures of prokaryotic 6-phosphofructokinase, a key enzyme of glycolysis, have been available for almost 25 years now, structural information about the more complex and highly regulated eukaryotic enzymes is still lacking until now. This review provides an overview of the current knowledge of eukaryotic 6-phosphofructokinase based on recent crystal structures, kinetic analyses and site-directed mutagenesis data with special focus on the molecular architecture and the structural basis of allosteric regulation.

Regulation of yeast central metabolism by enzyme phosphorylation

As a frequent post-translational modification, protein phosphorylation regulates many cellular processes. Although several hundred phosphorylation sites have been mapped to metabolic enzymes in Saccharomyces cerevisiae, functionality was demonstrated for few of them. Here, we describe a novel approach to identify in vivo functionality of enzyme phosphorylation by combining flux analysis with proteomics and phosphoproteomics. Focusing on the network of 204 enzymes that constitute the yeast central carbon and amino-acid metabolism, we combined protein and phosphoprotein levels to identify 35 enzymes that change their degree of phosphorylation during growth under five conditions. Correlations between previously determined intracellular fluxes and phosphoprotein abundances provided first functional evidence for five novel phosphoregulated enzymes in this network, adding to nine known phosphoenzymes. For the pyruvate dehydrogenase complex E1 α subunit Pda1 and the newly identified phosphoregulated glycerol-3-phosphate dehydrogenase Gpd1 and phosphofructose-1-kinase complex β subunit Pfk2, we then validated functionality of specific phosphosites through absolute peptide quantification by targeted mass spectrometry, metabolomics and physiological flux analysis in mutants with genetically removed phosphosites. These results demonstrate the role of phosphorylation in controlling the metabolic flux realised by these three enzymes.

Evidence of 5-HT components in human sperm: implications for protein tyrosine phosphorylation and the physiology of motility

Serotonin (5-hydroxytryptamine; C(10)H(12)N(2)O (5-HT)) is produced in the CNS and in some cells of peripheral tissues. In the mammalian male reproductive system, both 5-HT and tryptophan hydroxylase (TPH) have been described in Leydig cells of the testis and in principal cells of the caput epididymis. In capacitated hamster sperm, it has been shown that 5-HT promotes the acrosomal reaction. The aim of this work was to explore the existence of components of the serotoninergic system and their relevance in human sperm physiology. We used both immunocytochemistry and western blot to detect serotoninergic markers such as 5-HT, TPH1, MAO(A), 5-HT(1B), 5-HT(3), and 5HT(T); HPLC for TPH enzymatic activity; Computer Assisted Semen Analysis assays to measure sperm motility parameters and pharmacological approaches to show the effect of 5-HT in sperm motility and tyrosine phosphorylation was assessed by western blot. We found the presence of serotoninergic markers (5-HT, TPH1, MAO(A), 5-HT(1B), 5-HT(2A), 5-HT(3), 5-HT(T), and TPH enzymatic activity) in human sperm. In addition, we observed a significant increase in tyrosine phosphorylation and changes in sperm motility after 5-HT treatment. In conclusion, our data demonstrate the existence of components of a serotoninergic system in human sperm and support the notion for a functional role of 5-HT in mammalian sperm physiology, which can be modulated pharmacologically.

Herpes simplex type 1 activates glycolysis through engagement of the enzyme 6-phosphofructo-1-kinase (PFK-1)

Viruses such as HIV, HCV, Mayaro and HCMV affect cellular metabolic pathways, including glycolysis. Although some studies have suggested that the inhibition of glycolysis affects HSV-1 replication and that HSV-1-infected eyes have increased lactate production, the mechanisms by which HSV-1 induces glycolysis have never been investigated in detail. In this study, we observed an increase in glucose uptake, lactate efflux and ATP content in HSV-1-infected cells. HSV-1 triggered a MOI-dependent increase in the activity of phosphofructokinase-1 (PFK-1), a key rate-limiting enzyme of the glycolytic pathway. After HSV-1 infection, we observed increased PFK-1 expression, which increased PFK-1 total activity, and the phosphorylation of this enzyme at serine residues. HSV-1-induced glycolysis was associated with increased ATP content, and these events were critical for viral replication. In summary, our results suggest that HSV-1 triggers glycolysis through a different mechanism than other herpesviruses, such as HCMV. Thus, this study contributes to a better understanding of HSV-1 pathogenesis and provides insights into novel targets for antiviral therapy.

HIGHLIGHTS

►HSV-1 activates glycolysis by PFK-1 activation. ►In HSV-1-infected cells PFK-1 synthesis is up-regulated and phosphorylated at serine residues. ►PFK-1 knockdown impairs HSV-1 replication. ►HSV-1-mediated glycolysis activation increases ATP content.

The kinetics and regulation of phosphofructokinase from Teladorsagia circumcincta

Phosphofructokinase (PFK-1) activity was examined in L(3) and adult Teladorsagia circumcincta, both of which exhibit oxygen consumption. Although activities were higher in the adult stage, the kinetic properties of the enzyme were similar in both life cycle stages. T. circumcincta PFK-1 was subject to allosteric inhibition by high ATP concentration, which increased both the Hill coefficient (from 1.4±0.2 to 1.7±0.2 in L(3)s and 2.0±0.3 to 2.4±0.4 in adults) and the K(½) for fructose 6 phosphate (from 0.35±0.02 to 0.75±0.05mM in L(3)s and 0.40±0.03 to 0.65±0.05mM in adults). The inhibitory effects of high ATP concentration could be reversed by fructose 2,6 bisphosphate and AMP, but glucose 1,6 bisphosphate had no effect on activity. Similarly, phosphoenolpyruvate had no effect on activity, while citrate, isocitrate and malate exerted mild inhibitory effects, but only at concentrations exceeding 2mM. The observed kinetic properties for T. circumcincta PFK-1 were very similar to those reported for purified Ascaris suum PFK-1, though slight differences in sensitivity to ATP concentration suggests there may be subtle variations at the active site. These results are consistent with the conservation of properties of PFK-1 amongst nematode species, despite between species variation in the ability to utilise oxygen.

Structure of the apo form of Bacillus stearothermophilus phosphofructokinase

The crystal structure of the unliganded form of Bacillus stearothermophilus phosphofructokinase (BsPFK) was determined using molecular replacement to 2.8 Å resolution (Protein Data Bank entry 3U39 ). The apo BsPFK structure serves as the basis for the interpretation of any structural changes seen in the binary or ternary complexes. When the apo BsPFK structure is compared with the previously published liganded structures of BsPFK, the structural impact that the binding of the ligands produces is revealed. This comparison shows that the apo form of BsPFK resembles the substrate-bound form of BsPFK, a finding that differs from previous predictions.

Integrative metabolome and transcriptome profiling reveals discordant energetic stress between mouse strains with differential sensitivity to acrolein-induced acute lung injury

SCOPE

This investigation sought to better understand the metabolic role of the lung and to generate insights into the pathogenesis of acrolein-induced acute lung injury. A respiratory irritant, acrolein is generated by overheating cooking oils or by domestic cooking using biomass fuels, and is in environmental tobacco smoke, a health hazard in the restaurant workplace.

METHODS AND RESULTS

Using SM/J (sensitive) and 129X1/SvJ (resistant) inbred mouse strains, the lung metabolome was integrated with the transcriptome profile before and after acrolein exposure. A total of 280 small molecules were identified and mean values (log 2 >0.58 or <-0.58, p<0.05) were considered different for between-strain comparisons or within-strain responses to acrolein treatment. At baseline, 24 small molecules increased and 33 small molecules decreased in the SM/J mouse lung as compared to 129X1/SvJ mouse lung. Notable among the increased compounds was malonylcarnitine. Following acrolein exposure, several molecules indicative of glycolysis and branched chain amino acid metabolism increased similarly in both strains, whereas SM/J mice were less effective in generating metabolites related to fatty acid β-oxidation.

CONCLUSION

These findings suggest management of energetic stress varies between these strains, and that the ability to evoke auxiliary energy generating pathways rapidly and effectively may be critical in enhancing survival during acute lung injury in mice.

Divergent effects of human cytomegalovirus and herpes simplex virus-1 on cellular metabolism

Viruses rely on the metabolic network of the host cell to provide energy and macromolecular precursors to fuel viral replication. Here we used mass spectrometry to examine the impact of two related herpesviruses, human cytomegalovirus (HCMV) and herpes simplex virus type-1 (HSV-1), on the metabolism of fibroblast and epithelial host cells. Each virus triggered strong metabolic changes that were conserved across different host cell types. The metabolic effects of the two viruses were, however, largely distinct. HCMV but not HSV-1 increased glycolytic flux. HCMV profoundly increased TCA compound levels and flow of two carbon units required for TCA cycle turning and fatty acid synthesis. HSV-1 increased anapleurotic influx to the TCA cycle through pyruvate carboxylase, feeding pyrimidine biosynthesis. Thus, these two related herpesviruses drive diverse host cells to execute distinct, virus-specific metabolic programs. Current drugs target nucleotide metabolism for treatment of both viruses. Although our results confirm that this is a robust target for HSV-1, therapeutic interventions at other points in metabolism might prove more effective for treatment of HCMV.

Enveloped viruses draw on cellular machinery and materials to generate copies of their genome, structural proteins, and membrane. These biosynthetic processes use the host metabolic network to provide energy and small-molecule precursors. We have investigated how two important enveloped viruses, human cytomegalovirus and herpes simplex virus-1, alter host metabolism to provide materials for viral replication. We show that rather than passively relying on basal host cell metabolic activity, both viruses actively redirect host cell metabolism, implementing divergent metabolic programs that are robust to host cell type and virus strain. Human cytomegalovirus enhances lipid biosynthesis, while herpes simplex-1 gears central carbon metabolism toward the synthesis of pyrimidine nucleotides. Consistent with these changes, human cytomegalovirus is more sensitive to inhibition of fatty acid synthesis and herpes simplex virus-1 to inhibition of central metabolic reactions leading towards pyrimidine synthesis. As these two closely related viruses have divergent metabolic strategies, and since the metabolic perturbations point to potential drug targets, an important priority is defining the metabolic programs of other viruses.

Reversible high affinity inhibition of phosphofructokinase-1 by acyl-CoA: a mechanism integrating glycolytic flux with lipid metabolism

The enzyme phosphofructokinase-1 (PFK-1) catalyzes the first committed step of glycolysis and is regulated by a complex array of allosteric effectors that integrate glycolytic flux with cellular bioenergetics. Here, we demonstrate the direct, potent, and reversible inhibition of purified rabbit muscle PFK-1 by low micromolar concentrations of long chain fatty acyl-CoAs (apparent Ki∼1 μM). In sharp contrast, short chain acyl-CoAs, palmitoylcarnitine, and palmitic acid in the presence of CoASH were without effect. Remarkably, MgAMP and MgADP but not MgATP protected PFK-1 against inhibition by palmitoyl-CoA indicating that acyl-CoAs regulate PFK-1 activity in concert with cellular high energy phosphate status. Furthermore, incubation of PFK-1 with [1-(14)C]palmitoyl-CoA resulted in robust acylation of the enzyme that was reversible by incubation with acyl-protein thioesterase-1 (APT1). Importantly, APT1 reversed palmitoyl-CoA-mediated inhibition of PFK-1 activity. Mass spectrometric analyses of palmitoylated PFK-1 revealed four sites of acylation, including Cys-114, Cys-170, Cys-351, and Cys-577. PFK-1 in both skeletal muscle extracts and in purified form was inhibited by S-hexadecyl-CoA, a nonhydrolyzable palmitoyl-CoA analog, demonstrating that covalent acylation of PFK-1 was not required for inhibition. Tryptic footprinting suggested that S-hexadecyl-CoA induced a conformational change in PFK-1. Both palmitoyl-CoA and S-hexadecyl-CoA increased the association of PFK-1 with Ca2+/calmodulin, which attenuated the binding of palmitoylated PFK-1 to membrane vesicles. Collectively, these results demonstrate that fatty acyl-CoA modulates phosphofructokinase activity through both covalent and noncovalent interactions to regulate glycolytic flux and enzyme membrane localization via the branch point metabolic node that mediates lipid flux through anabolic and catabolic pathways.

The crystal structures of eukaryotic phosphofructokinases from baker's yeast and rabbit skeletal muscle

Phosphofructokinase 1 (PFK) is a multisubunit allosteric enzyme that catalyzes the principal regulatory step in glycolysis-the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate by ATP. The activity of eukaryotic PFK is modulated by a number of effectors in response to the cell's needs for energy and building blocks for biosynthesis. The crystal structures of eukaryotic PFKs-from Saccharomyces cerevisiae and rabbit skeletal muscle-demonstrate how successive gene duplications and fusion are reflected in the protein structure and how they allowed the evolution of new functionalities. The basic framework inherited from prokaryotes is conserved, and additional levels of structural and functional complexity have evolved around it. Analysis of protein-ligand complexes has shown how PFK is activated by fructose 2,6-bisphosphate (a powerful PFK effector found only in eukaryotes) and reveals a novel nucleotide binding site. Crystallographic results have been used as the basis for structure-based effector design.

Evolution of allosteric citrate binding sites on 6-phosphofructo-1-kinase

As an important part of metabolism, metabolic flux through the glycolytic pathway is tightly regulated. The most complex control is exerted on 6-phosphofructo-1-kinase (PFK1) level; this control overrules the regulatory role of other allosteric enzymes. Among other effectors, citrate has been reported to play a vital role in the suppression of this enzyme's activity. In eukaryotes, amino acid residues forming the allosteric binding site for citrate are found both on the N- and the C-terminal region of the enzyme. These site has evolved from the phosphoenolpyruvate/ADP binding site of bacterial PFK1 due to the processes of duplication and tandem fusion of prokaryotic ancestor gene followed by the divergence of the catalytic and effector binding sites. Stricter inhibition of the PFK1 enzyme was needed during the evolution of multi-cellular organisms, and the most stringent control of PFK1 by citrate occurs in vertebrates. By substituting a single amino acid (K557R or K617A) as a component of the allosteric binding site in the C-terminal region of human muscle type PFK-M with a residue found in the corresponding site of a fungal enzyme, the inhibitory effect of citrate was attenuated. Moreover, the proteins carrying these single mutations enabled growth of E. coli transformants encoding mutated human PFK-M in a glucose-containing medium that did not support the growth of E. coli transformed with native human PFK-M. Substitution of another residue at the citrate-binding site (D591V) of human PFK-M resulted in the complete loss of activity. Detailed analyses revealed that the mutated PFK-M subunits formed dimers but were unable to associate into the active tetrameric holoenzyme. These results suggest that stricter control over glycolytic flux developed in metazoans, whose somatic cells are largely characterized by slow proliferation.

Enhanced D-ribose biosynthesis in batch culture of a transketolase-deficient Bacillus subtilis strain by citrate

In this study, the effects of citrate addition on D-ribose production were investigated in batch culture of a transketolase-deficient strain, Bacillus subtilis EC2, in shake flasks and bioreactors. Batch cultures in shake flasks and a 5-l reactor indicated that supplementation with 0.2-0.5 g l(-1) of citrate enhanced D: -ribose production. When B. subtilis EC2 was cultivated in a 15-l reactor in a complex medium, the D: -ribose concentration was 70.9 g l(-1) with a ribose yield of 0.497 mol mol(-1). When this strain was grown in the same medium supplemented with 0.3 g l(-1) of citrate, 83.4 g l(-1) of D-ribose were obtained, and the ribose yield was increased to 0.587 mol mol(-1). Addition of citrate reduced the activities of pyruvate kinase and phosphofructokinase, while it increased those of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Metabolic flux distribution in the stationary phase indicated that citrate addition resulted in increased fluxes in the pentose phosphate pathway and TCA cycle, and decreased fluxes in the glycolysis and acetate pathways.

Subunit interactions and composition of the fructose 6-phosphate catalytic site and the fructose 2,6-bisphosphate allosteric site of mammalian phosphofructokinase

Mammalian phosphofructokinase originated by duplication, fusion, and divergence of a primitive prokaryotic gene, with the duplicated fructose 6-phosphate catalytic site in the C-terminal half becoming an allosteric site for the activator fructose 2,6-bisphosphate. It has been suggested that both sites are shared across the interface between subunits aligned in an antiparallel orientation, the N-terminal half of one subunit facing the C-terminal half of the other. The composition of these binding sites and the way in which subunits interact to form the dimer within the tetrameric enzyme have been reexamined by systematic point mutations to alanine of key amino acid residues of human muscle phosphofructokinase. We found that residues His-199, His-298, Arg-201, and Arg-292 contribute to the catalytic site and not to the allosteric site, because their mutation decreased the affinity for fructose 6-phosphate without affecting the activation by fructose 2,6-bisphosphate or its binding affinity. In contrast, residues Arg-566, Arg-655, and His-661 were critical components of the fructose bisphosphate allosteric site, because their mutation strongly reduced the action and affinity of the activator, with no alteration of substrate binding to the active site. Our results suggest that mammalian phosphofructokinase subunits associate with the N-terminal halves facing each other to form the two catalytic sites/dimer and the C-terminal halves forming the allosteric sites. Additionally, mutation of certain residues eliminated activation by fructose 1,6-bisphosphate, but not its binding, with little effect on activation by fructose 2,6-bisphosphate, indicating a divergence in the signal transduction route despite their binding to the same site.

Purification and kinetic characterization of 6-phosphofructo-1-kinase from the liver of gilthead sea bream (Sparus aurata)

6-phosphofructo-1-kinase (PFK) was purified to homogeneity from liver of gilthead sea bream (Sparus aurata) and kinetic properties of the enzyme were determined. The native enzyme had an apparent molecular mass of 510 kDa and was composed of 86 kDa subunits, suggesting homohexameric structure. At pH 7, S. aurata liver PFK (PFKL) showed sigmoidal kinetics for fructose-6-phosphate (fru-6-P) and hyperbolic kinetics for ATP. Fructose-2,6-bisphosphate (fru-2,6-P2) converted saturation curves for fru-6-P to hyperbolic and activated PFKL synergistically with AMP. Fru-2,6-P2 counteracted the inhibition caused by ATP, ADP and citrate. Compared to the S. aurata muscle isozyme, PFKL had lower affinity for fru-6-P, higher cooperativity, hyperbolic kinetics in relation to ATP, increased susceptibility to inhibition by ATP, and was less affected by AMP, ADP and inhibition by 3-phosphoglycerate, phosphoenolpyruvate, 6-phosphogluconate or phosphocreatine. The effect of starvation-refeeding on PFKL expression was studied at the levels of enzyme activity and protein content in the liver of S. aurata. Our findings indicate that short-term recovery of PFKL activity after refeeding previously starved fish, may result from allosteric regulation by fru-2,6-P2, whereas combination of activation by fru-2,6-P2 and increase in protein content may determine the long-term recovery of the enzyme activity.

Serotonin stimulates mouse skeletal muscle 6-phosphofructo-1-kinase through tyrosine-phosphorylation of the enzyme altering its intracellular localization

Serotonin (5-HT) is a hormone implicated in the regulation of many physiological and pathological events. One of its most intriguing properties is the ability to up-regulate mitosis. Moreover, it has been shown that 5-HT stimulate glucose uptake on skeletal muscle, suggesting that 5-HT may regulate glucose metabolism of peripheric tissues. Here we demonstrate that 5-HT stimulates skeletal muscle 6-phosphofructo-1-kinase (PFK) activity in a dose-response manner, through 5-HT(2A) receptor subtype. Maximal activation of the enzyme (2.5-fold compared to control) is achieved in the presence of 25pM 5-HT, increasing both PFK maximal velocity and affinity for the substrate fructose-6-phosphate. These effects occur due to tyrosine phosphorylation of the enzyme that is 2-fold enhanced upon 5-HT stimulation of skeletal muscles preparation. Once 5-HT-induced tyrosine phosphorylation of PFK is prevented by genistein, a tyrosine kinase inhibitor, the hormone stimulatory effect on PFK is abrogated. Wortmannin, a phosphatidylinositol-3-kinase (PI3K) inhibitor, does not interfere on 5-HT-induced stimulation of PFK, supporting that the observed effects are independent on insulin signaling pathway. Furthermore, 5-HT promotes the association of PFK to the muscle f-actin, suggesting that the hormone alters PFK intracellular distribution, favoring its association to the cytoskeleton. Altogether, our results support evidences that 5-HT augments skeletal muscle glucose consumption through stimulation of glycolysis key regulatory enzyme, PFK, throughout tyrosine phosphorylation and intracellular redistribution of the enzyme.

Regulatory properties of 6-phosphofructokinase and control of glycolysis in boar spermatozoa

Glycolysis is crucial for sperm functions (motility and fertilization), but how this pathway is regulated in spermatozoa is not clear. This prompted to study the location and the regulatory properties of 6-phosphofructokinase (PFK, EC 2.7.1.11), the most important element for control of glycolytic flux. Unlike some other glycolytic enzymes, PFK showed no tight binding to sperm structures. It could readily be extracted from ejaculated boar spermatozoa by sonication and was then chromatographically purified. At physiological pH, the enzyme was allosterically inhibited by near-physiological concentrations of its co-substrate ATP, which induced co-operativity, i.e. reduced the affinity for the substrate fructose 6-phosphate. Inhibition by ATP was reinforced by citrate and H+. Above pH 8, PFK lost all its regulatory properties and showed maximum activity. However, in the physiological pH range, PFK activity was very sensitive to small changes in effectors. At near-physiological substrate concentrations, PFK activity requires activators (de-inhibitors) of which the combination of AMP and fructose 2,6-bisphosphate (F2,6P2) was most efficient as a result of synergistic effects. The kinetics of PFK suggest AMP, F2,6P2, H+, and citrate as allosteric effectors controlling PFK activity in boar spermatozoa. Using immunogold labeling, PFK was localized in the mid-piece and principal piece of the flagellum as well as in the acrosomal area at the top of the head and in the cytoplasmic droplets released from the mid-piece after ejaculation.

Chimeric phosphofructokinases involving exchange of the N- and C-terminal halves of mammalian isozymes: implications for ligand binding sites

Two phosphofructokinase (PFK) chimeras were constructed by exchanging the N- and C-terminal halves of the mammalian M- and C-type isozymes, to investigate the contribution of each terminus to the catalytic site and the fructose-2,6-P(2)/fructose-1,6-P(2) allosteric site. The homogeneously-purified chimeric enzymes organized into tetramers, and exhibited kinetic properties for fructose-6-P and MgATP similar to those of the native enzyme that furnished the N-terminal domain in each case, whereas their fructose-2,6-P(2) activatory characteristics coincided with those of the isozyme that provided the C-terminal half. This reflected the role of each domain in the formation of the corresponding binding site. Grafting the N-terminus of PFK-M onto the C-terminus of the fructose-1,6-P(2) insensitive PFK-C restored transduction of this signal to the catalytic site, which significance is also discussed.

Microarray analysis of gene expression in the supraoptic nucleus of normoosmotic and hypoosmotic rats

1. Hypoosmolality produces a dramatic inhibition of vasopressin (VP) and oxytocin (OT) gene expression in the supraoptic nucleus (SON). This study examines the effect of sustained hypoosmolality on global gene expression in the OT and VP magnocellular neurons (MCNs) of the hypothalamo-neurohypophysial system (HNS), in order to detect novel genes in this system that might be involved in osmoregulation in the MCNs. 2. For this purpose, we used Affymetrix oligonucleotide arrays to analyze the expression of specific genes in laser microdissected rat SONs, and their changes in expression during chronic hypoosmolality. We identified over 40 genes that had three-fold or more greater expression in the SON versus total hypothalamus, and that also changed more than two fold in expression as a result of the chronic hypoosmolar treatment. These genes contained both novel as well as genes previously known to be present in the SON. All of the raw data for the genes that are expressed in the SON and altered by hypoosmolality can be found on the following NINDS website URL address: http://data.ninds.nih.gov/Gainer/Publications.

The dimorphic yeast Yarrowia lipolytica possesses an atypical phosphofructokinase: characterization of the enzyme and its encoding gene

The phosphofructokinase from the non-conventional yeastYarrowia lipolytica(YlPfk) was purified to homogeneity, and its encoding gene isolated. YlPfk is an octamer of 869 kDa composed of a single type of subunit, and shows atypical kinetic characteristics. It did not exhibit cooperative kinetics for fructose 6-phosphate (Hill coefficient,h1·1;S0·552 μM), it was inhibited moderately by MgATP (Ki3·5 mM), and it was strongly inhibited by phosphoenolpyruvate (Ki61 μM). Fructose 2,6-bisphosphate did not activate the enzyme, and AMP and ADP were also without effect. The geneYlPFK1has no introns, and encodes a putative protein of 953 aa, with a molecular mass consistent with the subunit size found after purification. Disruption of the gene abolished growth in glucose and Pfk activity, while reintroduction of the gene restored both properties. This indicates thatY. lipolyticahas only one gene encoding Pfk, and supports the finding that the enzyme consists of identical subunits. Glucose did not interfere with growth of theYlpfk1disruptant in permissive carbon sources. The unusual kinetic characteristics of YlPfk, and the intracellular concentrations of glycolytic intermediates during growth in glucose, suggest that YlPfk may play an important role in the regulation of glucose metabolism inY. lipolytica, different from the role played by the enzyme inSaccharomyces cerevisiae.

Oocyte control of metabolic cooperativity between oocytes and companion granulosa cells: energy metabolism

Intercellular communication between oocytes and granulosa cells is essential for normal follicular differentiation and oocyte development. Subtraction hybridization was used to identify genes more highly expressed in cumulus cells than in mural granulosa cells of mouse antral follicles. This screen identified six genes involved in glycolysis: Eno1, Pkm2, Tpi, Aldoa, Ldh1, and Pfkp. When oocytes were microsurgically removed from cumulus cell-oocyte complexes, the isolated cumulus cells exhibited decreased expression levels of genes encoding glycolytic enzymes, glycolysis and activity of the tricarboxylic acid (TCA) cycle. These decreases were prevented by culturing the cumulus cells with paracrine factors secreted by fully grown oocytes. Paracrine factors from fully grown oocytes exhibited greater ability than those from growing oocytes to promote expression of genes encoding glycolytic enzymes and glycolysis in the granulosa cells of preantral follicles. However, neither fully grown nor growing oocytes secreted paracrine factors affecting activity of the TCA cycle. These results indicate that oocytes regulate glycolysis and the TCA cycle in granulosa cells in a manner specific to the population of granulosa cells and to the stage of growth and development of the oocyte. Oocytes control glycolysis in granulosa cells by regulating expression levels of genes encoding glycolytic enzymes. Therefore, mouse oocytes control the intercellular metabolic cooperativity between cumulus cells and oocytes needed for energy production by granulosa cells and required for oocyte and follicular development.

Posttranslational modification of 6-phosphofructo-1-kinase in Aspergillus niger

Two different enzymes exhibiting 6-phosphofructo-1-kinase (PFK1) activity were isolated from the mycelium of Aspergillus niger: the native enzyme with a molecular mass of 85 kDa, which corresponded to the calculated molecular mass of the deduced amino acid sequence of the A. niger pfkA gene, and a shorter protein of approximately 49 kDa. A fragment of identical size also was obtained in vitro by the proteolytic digestion of the partially purified native PFK1 with proteinase K. When PFK1 activity was measured during the proteolytic degradation of the native protein, it was found to be lost after 1 h of incubation, but it was reestablished after induction of phosphorylation by adding the catalytic subunit of cyclic AMP-dependent protein kinase to the system. By determining kinetic parameters, different ratios of activities measured at ATP concentrations of 0.1 and 1 mM were detected with fragmented PFK1, as with the native enzyme. Fructose-2,6-biphosphate significantly increased the Vmax of the fragmented protein, while it had virtually no effect on the native protein. The native enzyme could be purified only from the early stages of growth on a minimal medium, while the 49-kDa fragment appeared later and was activated at the time of a sudden change in the growth rate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sequential purifications of PFK1 enzymes by affinity chromatography during the early stages of the fungal development suggested spontaneous posttranslational modification of the native PFK1 in A. niger cells, while from the kinetic parameters determined for both isolated forms it could be concluded that the fragmented enzyme might be more efficient under physiological conditions.

Effects of insulin and actin on phosphofructokinase activity and cellular distribution in skeletal muscle

In this work, we report evidences that the association of phosphofructokinase and F-actin can be affected by insulin stimulation in rabbit skeletal muscle homogenates and that this association can be a mechanism of phos-phofructokinase regulation. Through co-sedimentation techniques, we observed that on insulin-stimulated tissues, approximately 70% of phosphofructokinase activity is co-located in an actin-enriched fraction, against 28% in control. This phenomenon is accompanied by a 100% increase in specific phosphofructokinase activity in stimulated homogenates. Purified F-actin causes an increase of 230% in phosphofructokinase activity and alters its kinetic parameters. The presence of F-actin increases the affinity of phosphofructokinase for fructose 6-phosphate nevertheless, with no changes in maximum velocity (Vmax). Here we propose that the modulation of cellular distribution of phosphofructokinase may be one of the mechanisms of control of glycolytic flux in mammalian muscle by insulin.

Phosphofructokinase muscle-specific isoform requires caveolin-3 expression for plasma membrane recruitment and caveolar targeting: implications for the pathogenesis of caveolin-related muscle diseases

Previous co-immunoprecipitation studies have shown that endogenous PFK-M (phosphofructokinase, muscle-specific isoform) associates with caveolin (Cav)-3 under certain metabolic conditions. However, it remains unknown whether Cav-3 expression is required for the plasma membrane recruitment and caveolar targeting of PFK-M. Here, we demonstrate that recombinant expression of Cav-3 dramatically affects the subcellular localization of PFK-M, by targeting PFK-M to the plasma membrane, and by trans-locating PFK-M to caveolae-enriched membrane domains. In addition, we show that the membrane recruitment and caveolar targeting of PFK-M appears to be strictly dependent on the concentration of extracellular glucose. Interestingly, recombinant expression of PFK-M with three Cav-3 mutants [DeltaTFT (63 to 65), P104L, and R26Q], which harbor the same mutations as seen in the human patients with Cav-3-related muscle diseases, causes a substantial reduction in PFK-M expression levels, and impedes the membrane recruitment of PFK-M. Analysis of skeletal muscle tissue samples from Cav-3(-/-) mice directly demonstrates that Cav-3 expression regulates the phenotypic behavior of PFK-M. More specifically, in Cav-3-null mice, PFK-M is no longer targeted to the plasma membrane, and is excluded from caveolar membrane domains. As such, our current results may be important in understanding the pathogenesis of Cav-3-related muscle diseases, such as limb-girdle muscular dystrophy-1C, distal myopathy, and rippling muscle disease, that are caused by mutations within the human Cav-3 gene.

Identification of C-terminal motifs responsible for transmission of inhibition by ATP of mammalian phosphofructokinase, and their contribution to other allosteric effects

Systematic deletions and point mutations in the C-terminal extension of mammalian PFK (phosphofructokinase) led us to identify Leu-767 and Glu-768 of the M-type isoform (PFK-M) as the motifs responsible for the role of this region in inhibition by MgATP. These amino acids are the only residues of the C-terminus that are conserved in all mammalian isoforms, and were found to have a similar function in the C-type isoenzyme. Both residues in PFK-C and Leu-767 in PFK-M were also observed to be critical for inhibition by citrate, which is synergistic with that by MgATP. Binding studies utilizing titration of intrinsic protein fluorescence indicated that the C-terminal part of the enzyme participates in the signal transduction route from the MgATP inhibitory site to the catalytic site, but does not contribute to the binding of this inhibitor, whereas it is essential for the binding of citrate. Mutations of the identified structural motifs did not alter either the action of other allosteric effectors that also interact with MgATP, such as the inhibitor phosphoenolpyruvate and the strong activator fructose 2,6-bisphosphate, or the co-operative effect of fructose 6-phosphate. The latter data provide evidence that activation by fructose 2,6-bisphosphate and fructose 6-phosphate co-operativity are not linked to the same allosteric transition as that mediating inhibition by MgATP.