11 Search Results

In eukaryotes, Target of Rapamycin (TOR), a conserved protein sensor kinase, integrates a diverse set of environmental cues, including growth factor signals, energy availability, and nutritional status, to direct cell growth. In plants, TOR is activated by light and sugars and regulates a wide range of cellular processes, including protein synthesis and metabolism. Fatty acid synthesis is key to membrane biogenesis that in turn, is required for cell growth. To elucidate the primary regulatory role(s) of TOR in lipid metabolism, we followed fatty acid and lipid changes in plants with altered TOR protein levels or activity for short durations, usingmore » Nicotiana benthamiana leaves, Arabidopsis seedlings and Brassica napus cell suspension cultures. Transient expression of TOR significantly elevated the levels of total fatty acids in Nicotiana benthamiana leaves, while treatment of Arabidopsis seedlings with Torin 2, a TOR specific inhibitor, for one day, caused significant reductions in fatty acids and membrane lipids. Similarly, incubating oil-producing Brassica napus suspension culture cells with Torin 2 for eight hours led to significant decreases in the levels of TFA and TAG. Taken together the results from three independent systems presented here establishes that TOR positively regulates lipid synthesis in plants, consistent with its role in animals. Furthermore, RNA-seq analysis of Torin 2-treated Arabidopsis seedlings showed that TOR promotes the upregulation of a number of genes involved in de novo fatty acid synthesis while downregulating genes involved in lipid turnover, which we propose as a mechanistic explanation for its promotion of lipid synthesis and accumulation.« less

Na⁺ is the most abundant ion in living organisms and plays essential roles in regulating nutrient uptake, muscle contraction, and neurotransmission. The identification of Na⁺ in protein structures is crucial for gaining a deeper understanding of protein function in a physiological context. LeuT, a bacterial homolog of the neurotransmitter:sodium symporter family, uses the Na⁺ gradient to power the uptake of amino acids into cells and has been used as a paradigm for the study of Na⁺-dependent transport systems. We have devised a low-energy multi-crystal approach for characterizing low-Z (Z ≤ 20) anomalous scattering ions such as Na⁺, Mg²⁺, K⁺, andmore » Ca²⁺ by combining Bijvoet-difference Fourier syntheses for ion detection and f” refinements for ion speciation. Using the approach, we experimentally identify two Na⁺ bound near the central leucine binding site in LeuT. Using LeuT microcrystals, we also demonstrate that Na⁺ may be depleted to study conformational changes in the LeuT transport cycle.« less

Abstract Autophagy is a catabolic process in which cytoplasmic components are delivered to vacuoles or lysosomes for degradation and nutrient recycling. Autophagy-mediated degradation of membrane lipids provides a source of fatty acids for the synthesis of energy-rich, storage lipid esters such as triacylglycerol (TAG). In eukaryotes, storage lipids are packaged into dynamic subcellular organelles, lipid droplets. In times of energy scarcity, lipid droplets can be degraded via autophagy in a process termed lipophagy to release fatty acids for energy production via fatty acid β-oxidation. On the other hand, emerging evidence suggests that lipid droplets are required for the efficient executionmore » of autophagic processes. Here, we review recent advances in our understanding of metabolic interactions between autophagy and TAG storage, and discuss mechanisms of lipophagy. Free fatty acids are cytotoxic due to their detergent-like properties and their incorporation into lipid intermediates that are toxic at high levels. Thus, we also discuss how cells manage lipotoxic stresses during autophagy-mediated mobilization of fatty acids from lipid droplets and organellar membranes for energy generation.« less

Unlike for metal catalysts where selective chemisorption probes (H₂, O₂, CO and N₂O) have been successfully applied to count the number of active surface sites and allow determination of TOF values, the analogous situation for oxide catalysts has developed very slowly. The reason for the sluggish development of selective chemisorption probes for oxides is that the same chemisorption probes that work well with metals aren’t compatible with oxides since these chemical probes weakly adsorb on oxides. Methanol, however, is a very reactive molecule that has been found to readily chemisorb on oxides and allows for quantitative determination of the numbermore » of active surface sites (Ns). Here, in this mini-review, the determination of the number of active sites by methanol chemisorption is reviewed with examples for pure oxides (one-component oxides), supported oxides (submonolayer, monolayer and above monolayer) and bulk mixed oxides (surface enriched oxide present in submonolayer, monolayer and in excess of monolayer coverages).« less

Although butadiene is currently a by-product of naphtha cracking, interest in producing butadiene from bio-based ethanol has increased because of the lower environmental impact of the ethanol to butadiene reaction. Furthermore, this work explores a multifunctional catalyst system composed of silica-supported Ag and ZrO₂ used for the cascade reaction of ethanol to butadiene at 573 K. The Ag and ZrO₂ components were synthesized on separate support particles enabling characterization of each component without interference from the other. High selectivity to butadiene (65%) at high ethanol conversion (75%) was achieved with an appropriate ratio of Ag and ZrO₂ in the reactor.more » Silver catalyzed the initial dehydrogenation of ethanol to acetaldehyde while ZrO₂ catalyzed the C-C coupling and subsequent dehydration reactions. The silica-supported ZrO₂ exhibited superior selectivity relative to bulk ZrO₂ in the Ag-promoted ethanol to butadiene reaction. Results from Zr K-edge X-ray absorption spectroscopy and UV-Vis spectroscopy showed that ZrO₂ was highly dispersed on the silica support over a range of loadings. Infrared spectroscopy of adsorbed pyridine, CO, and CO₂, and kinetics of probe reactions 1-butene double bond isomerization, 2-propanol decomposition, and ethanol hydrogenation of acetone were used to compare the acid-base nature and chemical reactivity of silica-supported ZrO₂ to bulk ZrO₂.« less

Photosynthetically derived sugars provide carbon skeletons for metabolism and carbon signals that favor anabolism. The amount of sugar available for fatty acid (FA) and triacylglycerol (TAG) synthesis depends on sugar compartmentation, transport, and demands from competing pathways. We are exploring the influence of sugar partitioning between the vacuole and cytoplasm on FA synthesis in Arabidopsis by building on our previous finding that reduced leaf sugar export in the sucrose-proton symporter2 (suc2) mutant, in combination with impaired starch synthesis in the ADP-glucose pyrophosphorylase (adg1) mutant, accumulates higher sugar levels and increased total FA and TAG compared to the wild type parent.more » Here we sought to relocalize sugar from the vacuole to the cytoplasm to drive additional FA/TAG synthesis and growth. Arabidopsis suc2 adg1 was therefore crossed with tonoplast monosaccharide transporter mutants tmt1 and tmt2 and overexpression of the sucrose/proton cotransporter SUC4 in which tmt1 tmt2 impairs sugar transport to the vacuole from the cytoplasm and SUC4 overexpression enhances sugar transport in the reverse direction from the vacuole to the cytoplasm. A resulting homozygous suc2 adg1 tmt1 tmt2 SUC4 line was used to test the hypothesis that increased intracellular carbon supply in the form of sugars would increase both FA and TAG accumulation. The data shows that relative to suc2 adg1, suc2 adg1 tmt1 tmt2 SUC4 significantly increases leaf total FA content by 1.29-fold to 10.9% of dry weight and TAG by 2.4-fold to 2.88%, supporting the hypothesis that mobilizing vacuolar sugar is a valid strategy for increasing vegetative oil accumulation.« less

The local microenvironment at the electrode-electrolyte interface plays an important role in electrocatalytic performance. Herein, we investigate the effect of acid electrolyte anion identity on the oxygen reduction reaction (ORR) activity and selectivity of smooth Ag and Pd catalyst thin films. Cyclic voltammetry in perchloric, nitric, sulfuric, phosphoric, hydrochloric, and hydrobromic acid, at pH1, reveals that Ag ORR activity trends as follows: HClO₄>HNO₃>H₂SO₄>H₃PO₄>HCl>>HBr, while Pd ORR activity trends as: HClO₄>H₂SO₄>HNO₃>H₃PO₄>HCl>>HBr. Moreover, rotating-ring-disk-electrode selectivity measurements demonstrate enhanced 4e^- selectivity on both Ag and Pd, by up to 35% H₂O₂ and 10% H₂O₂ respectively, in HNO₃ compared to in HClO₄. Relating physics-basedmore » modeling and experimental results, we postulate that ORR performance depends greatly on anion-related phenomena in the double layer, for instance competitive adsorption and non-covalent interactions.« less

We previously demonstrated that exogenous trehalose 6-phosphate (T6P) treatment stabilized WRINKLED1 (WRI1), a master transcriptional regulator of fatty acid (FA) synthesis and increased total FA content in Brassica napus ( B. napus ) embryo suspension cell culture. Here, we explore Arabidopsis lines heterologously expressing the Escherichia coli T6P synthase (otsA) or T6P phosphatase (otsB) to refine our understanding regarding the role of T6P in regulating fatty acid synthesis both in seeds and vegetative tissues. Arabidopsis 35S: otsA transgenic seeds showed an increase of 13% in fatty acid content compared to those of wild type (WT), while seeds of 35: otsBmore » transgenic seeds showed a reduction of 12% in fatty acid content compared to WT. Expression of otsB significantly reduced the level of WRI1 and expression of its target genes in developing seeds. Like Arabidopsis seeds constitutively expressing otsA, transient expression of otsA in Nicotiana benthamiana leaves resulted in strongly elevated levels of T6P. This was accompanied by an increase of 29% in de novo fatty acid synthesis rate, a 2.3-fold increase in triacylglycerol (TAG) and a 20% increase in total fatty acid content relative to empty vector (EV) controls. Taken together, these data support the heterologous expression of otsA as an approach to increasing TAG accumulation in plant seeds and vegetative tissues.« less

In a combined kinetics and density functional theory (DFT) study, we have explored several factors that affect the selectivity of acid- and base-catalyzed self- and cross-aldol condensation of acetone (ACE) and cyclopentanone (CPO). These factors include competitive adsorption, molecular structure, and electron polarization of the two ketones on the catalyst surface. Here, kinetic analysis shows that on MgO, self-condensation of both ACE and CPO is limited by the initial unimolecular enolization step. Accordingly, CPO exhibits a higher self-condensation rate than ACE due to the more favorable α-C–H abstraction by the basic O site. The thermodynamic parameters derived from the kineticmore » analysis indicate that under cross-condensation reaction conditions, the MgO surface fraction covered by CPO is significantly higher than that covered by ACE. Therefore, the product distribution is dominated by [CPO]-activated products ([CPO]CPO + [CPO]ACE). Also, for a given enolate (indicated as [CPO] or [ACE]), the higher surface coverage of CPO leads to enhanced C–C coupling with CPO as the electrophile ([CPO]CPO > [CPO]ACE; [ACE]CPO > [ACE]ACE). By contrast, for both acid catalysts investigated, the rate-limiting step is the bimolecular C–C coupling, with aspects of this step depending on the density of acid sites. That is, on the high-acid-density MCM-41-SO₃H catalyst, condensation follows a bimolecular dual-site mechanism (Langmuir–Hinshelwood model). On this catalyst surface, the ACE coverage is higher than that of CPO, which causes a higher selectivity for those products in which ACE is the electrophile ([CPO]ACE > [CPO]CPO; [ACE]ACE > [ACE]CPO). Steric hindrance is another factor that affects selectivity in the same way, favoring products in which the electrophile is ACE since it presents a lower steric hindrance to C–C coupling than CPO. Therefore, the same sequence of products is obtained for the low-acid-density MCM-41-SO₃H catalyst, which proceeds on a single-acid site with the other molecule in the liquid phase (Eley–Rideal model). In this case, the electrophile coverage is not relevant, but the steric hindrance is, yielding the same trend as above ([CPO]ACE > [CPO]CPO; [ACE]ACE > [ACE]CPO).« less

Dissolution of Mn from lithium-manganese spinel has hindered its commercialization as a cathode material in Li-ion batteries. Disproportionation of near-surface Mn(III), in the presence of acid, has been widely thought to result in dissolved divalent Mn. To what extent stray acidic water in the cell (as opposed to the organic electrolyte) acts as the solvent for Mn ions has not been established. Simulations by Leung show that a small displacement of trivalent Mn from its equilibrium site at an LiMn₂O₄ (001)/ ethylene carbonate interface leads to its reduction to Mn(II). In the present work, Thermodynamic Integration is performed, based onmore » first-principles molecular dynamics simulations within the Blue-Moon ensemble, for the detachment of Mn(III) ions at the LiMn₂O₄ (001)/water interface. The results show that reduction of Mn(III) to Mn(II) occurs also in the case of an aqueous interface. The simulations were performed for both neutral and acidic water (in the presence of HF), with the coordination number of the dissolving Mn ion with substrate oxygen ions taken as the reaction coordinate. The simulations indicate that an F^- ion strongly binds to a surface Mn(III) ion, and weakens its adhesion to the substrate. Owing to this weakening, a surface Mn-F complex traverses regions of phase space at room temperature where disproportionation becomes energetically favorable. Although this disproportionation occurs close to the substrate, where the Mn coordination number is only slightly lowered from its equilibrium value, we argue that the likelihood of reattachment after disproportionation is small (Leung arrived at a similar interpretation in the case of the LiMn₂O₄ (001)/ EC interface). We suggest that the critical role of F^- in promoting dissolution is to weaken the Mn binding to the substrate so as to enable disproportionation. The partially detached MnF complex may then undergo additional interaction with the solvent to form, e.g., MnF₂, which would enable transport away from the substrate. In conclusion, the EPR measurements by Shilina et al. which appear to show Mn(III) as the predominant solvated species are discussed.« less