| 41471. |
- Klechikov, Alexey, et al.
(författare)
-
Structure of graphene oxide membranes in solvents and solutions
- 2015
-
Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 7:37, s. 15374-15384
-
Tidskriftsartikel (refereegranskat)abstract
- The change of distance between individual graphene oxide sheets due to swelling is the key parameter to explain and predict permeation of multilayered graphene oxide (GO) membranes by various solvents and solutions. In situ synchrotron X-ray diffraction study shows that swelling properties of GO membranes are distinctly different compared to precursor graphite oxide powder samples. Intercalation of liquid dioxolane, acetonitrile, acetone, and chloroform into the GO membrane structure occurs with maximum one monolayer insertion (Type I), in contrast with insertion of 2-3 layers of these solvents into the graphite oxide structure. However, the structure of GO membranes expands in liquid DMSO and DMF solvents similarly to precursor graphite oxide (Type II). It can be expected that Type II solvents will permeate GO membranes significantly faster compared to Type I solvents. The membranes are found to be stable in aqueous solutions of acidic and neutral salts, but dissolve slowly in some basic solutions of certain concentrations, e.g. in NaOH, NaHCO3 and LiF. Some larger organic molecules, alkylamines and alkylammonium cations are found to intercalate and expand the lattice of GO membranes significantly, e.g. up to similar to 35 angstrom in octadecylamine/methanol solution. Intercalation of solutes into the GO structure is one of the limiting factors for nano-filtration of certain molecules but it also allows modification of the inter-layer distance of GO membranes and tuning of their permeation properties. For example, GO membranes functionalized with alkylammonium cations are hydrophobized and they swell in non-polar solvents.
|
|
| 41472. |
- Klechikov, Alexey, et al.
(författare)
-
Swelling of Thin Graphene Oxide Films Studied by in Situ Neutron Reflectivity
- 2018
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:24, s. 13106-13116
-
Tidskriftsartikel (refereegranskat)abstract
- Permeation of multilayered graphene oxide (GO) membranes by polar solvents is known to correlate with their swelling properties and amount of sorbed solvent. However, quantitative estimation of sorption using standard (e.g., gravimetric) methods is technically challenging for few nanometers thick GO membranes/films exposed to solvent vapors. Neutron reflectivity (NR) was used here to evaluate the amount of solvents intercalated into the film which consists of only ∼31.5 layers of GO. Analysis of NR data recorded from the GO film exposed to vapors of polar solvents provides information about change of film thickness due to swelling, amount of intercalated solvent, and selectivity in sorption of solvents from binary mixtures. A quantitative study of GO film sorption was performed for D2O, d-methanol, ethanol, dimethyl sulfoxide (DMSO), acetonitrile, dimethylformamide (DMF), and acetone. Using isotopic contrast, we estimated selectivity in sorption of ethanol/d-methanol mixtures by the GO film. Estimation of sorption selectivity was also performed for D2O/DMF, D2O/DMSO, and D2O/acetonitrile binary mixtures. Sorption of polar solvents was compared for the thin GO film, micrometer thick free standing GO membranes, and graphite oxide powders.
|
|
| 41473. |
- Kleczkowski, Leszek A, 1954-, et al.
(författare)
-
A common structural blueprint for plant UDP-sugar-producing pyrophosphorylases.
- 2011
-
Ingår i: Biochemical Journal. - Colchester : Portland Press. - 0264-6021 .- 1470-8728. ; 439:3, s. 375-379
-
Tidskriftsartikel (refereegranskat)abstract
- Plant pyrophosphorylases that are capable of producing UDP-sugars, key precursors for glycosylation reactions, include UDP-glucose pyrophosphorylases (A- and B-type), UDP-sugar pyrophosphorylase and UDP-N-acetylglucosamine pyrophosphorylase. Although not sharing significant homology at the amino acid sequence level, the proteins share a common structural blueprint. Their structures are characterized by the presence of the Rossmann fold in the central (catalytic) domain linked to enzyme-specific N-terminal and C-terminal domains, which may play regulatory functions. Molecular mobility between these domains plays an important role in substrate binding and catalysis. Evolutionary relationships and the role of (de)oligomerization as a regulatory mechanism are discussed.
|
|
| 41474. |
- Kleczkowski, Leszek A, 1954-
(författare)
-
A new player in the starch field
- 2001
-
Ingår i: Plant physiology and biochemistry (Paris). - 0981-9428 .- 1873-2690. ; 39:9, s. 759-761
-
Tidskriftsartikel (refereegranskat)abstract
- A possible role of a newly discovered ADP-glucose pyrophosphatase (AGPPase) is discussed in the context of starch synthesis. The enzyme hydrolyses ADP-glucose (starch precursor) and may potentially divert the flow of carbon from starch synthase, resulting in a 'futile cycle' when 'coupled' with ADP-glucose pyrophosphorylase. The activity of AGPPase is inversely related to starch yield in sink tissues, and may be prone to inhibition by Pi and certain other products of the starch pathway. The AGPPase likely belongs to a `nudix' family of enzymes that in animal tissues and yeast are known to regulate levels of activated sugars. Some strategies for future research are underlined. (C) 2001 Editions scientifiques et medicales Elsevier SAS.
|
|
| 41475. |
- Kleczkowski, Leszek A, 1954-
(författare)
-
A phosphoglycerate to inorganic phosphate ratio is the major factor in controlling starch levels in chloroplasts via ADP-glucose pyrophosphorylase regulation
- 1999
-
Ingår i: FEBS Letters. - 0014-5793 .- 1873-3468. ; 448:1, s. 153-156
-
Tidskriftsartikel (refereegranskat)abstract
- Purified barley leaf ADP-glucose pyrophosphorylase, a key enzyme of the starch synthesis in the chloroplast stroma, was analysed with respect to its possible regulation by factors defining the metabolic/effector status of the chloroplast during light and dark conditions. The enzyme required 3-phosphoglyceric acid for the maximal activity and was inhibited by inorganic phosphate. The optimal pH for the enzyme was at circa 7.0, regardless of the presence or absence of 3-phosphoglyceric acid, whereas the maximal activation by 3-phosphoglyceric acid was observed at pH 8.5 and higher. Changes in the concentration of Mg2+ and dithiothreitol had little or no effect on the enzymatic activity of AGPase. It has been directly demonstrated for the first time that a 3-phosphoglyceric acid/inorganic phosphate ratio, a crucial regulatory parameter, could be directly related to a defined activation state of the enzyme, allowing the prediction of a relative AGPase activity under given conditions. The predicted changes in the enzyme activity were directly correlated with earlier reported responses of starch levels to the 3-phosphoglyceric acid/inorganic phosphate ratio in chloroplasts. Consequences of this for the starch biosynthesis are discussed. (C) 1999 Federation of European Biochemical Societies.
|
|
| 41476. |
|
|
| 41477. |
- Kleczkowski, Leszek A., 1954-, et al.
(författare)
-
Effects of Magnesium, Pyrophosphate and Phosphonates on Pyrophosphorolytic Reaction of UDP-Glucose Pyrophosphorylase
- 2022
-
Ingår i: PLANTS. - : MDPI. - 2223-7747. ; 11:12
-
Tidskriftsartikel (refereegranskat)abstract
- UDP-glucose pyrophosphorylase (UGPase) carries a freely reversible reaction, using glucose-1-P and UTP to produce UDP-glucose (UDPG) and pyrophosphate (PPi ), with UDPG being essential for glycosylation reactions in all organisms including, e.g., synthesis of sucrose, cellulose and glycoproteins. In the present study, we found that free magnesium (Mg2+) had profound effects on the reverse reaction of purified barley UGPase, and was absolutely required for its activity, with an apparent Km of 0.13 mM. More detailed analyses with varied concentrations of MgPPi allowed us to conclude that it is the MgPPi complex which serves as true substrate for UGPase in its reverse reaction, with an apparent Km of 0.06 mM. Free PPi was an inhibitor in this reaction. Given the key role of PPi in the UGPase reaction, we have also tested possible effects of phosphonates, which are analogs of PPi and phosphate (Pi ). Clodronate and etidronate (PPi analogs) had little or no effect on UGPase activity, whereas fosetyl-Al (Pi analog), a known fungicide, acted as effective near-competitive inhibitor versus PPi, with Ki of 0.15 mM. The data are discussed with respect to the role of magnesium in the UGPase reaction and elucidating the use of inhibitors in studies on cellular function of UGPase and related enzymes.
|
|
| 41478. |
- Kleczkowski, Leszek A, 1954-
(författare)
-
GLUCOSE ACTIVATION AND METABOLISM THROUGH UDP-GLUCOSE PYROPHOSPHORYLASE IN PLANTS
- 1994
-
Ingår i: Phytochemistry. - 0031-9422 .- 1873-3700. ; 37:6, s. 1507-1515
-
Forskningsöversikt (refereegranskat)abstract
- Recent developments in studies on the characterization of properties and functions of UDP-glucose pyrophosphorylase (UGPase) in plant metabolism are presented. UGPase constitutes a reversible enzymatic step for interconversions between starch and sucrose metabolites, and is responsible for synthesis and metabolism of UDP-glucose, a major form of nucleoside diphosphoglucose in plant cells. The enzyme, although considered not to have any regulatory function, has attracted considerable interest due to its ubiquitous distribution in plants, high activity, especially in sink tissues, and because of the key role of UDP-glucose as a direct or indirect precursor of sucrose, starch and structural polysaccharides. The enzyme has been the subject of biotechnological manipulations to engineer its kinetic properties and gene expression in relation to metabolic processes at the sucrose/starch interface. Depending on tissue type, the UGPase reaction may be channelled in vivo, either toward UDP-glucose pyrophosphorolysis or synthesis, due to a metabolic coupling to other reactions of sugar pathways. Some strategies for future research on plant UGPase are discussed.
|
|
| 41479. |
- Kleczkowski, Leszek A, 1954-
(författare)
-
Is leaf ADP-glucose pyrophosphorylase an allosteric enzyme?
- 2000
-
Ingår i: Biochimica et Biophysica Acta - Protein Structure and Molecular Enzymology. - 0167-4838 .- 1879-2588. ; 1476:1, s. 103-108
-
Tidskriftsartikel (refereegranskat)abstract
- Barley leaf ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch synthesis in the chloroplast stroma, was analysed, in both directions of the reaction, with respect to details of its regulation by 3-phosphoglycerate (PGA) and inorganic phosphate (Pi) which serve as activator and inhibitor, respectively. AGPase was found to catalyse a close-to-equilibrium reaction, with the K-eq value of approximately 0.5, i.e. slightly favouring the pyrophosphorolytic direction. When the enzyme was analysed by substrate kinetics, PGA acted either as a linear (hyperbolic response) 'non-competitive' activator (forward reaction) or a linear near-'competitive' activator (reverse reaction). When the activation and inhibition patterns with PGA and Pi, respectively, were studied in detail by Dixon plots, the response curves to effecters also followed hyperbolic kinetics, with the experimentally determined K-a and K-i values on the order of micromolar. The results suggest that the regulation of AGPase proceeds via a non-cooperative mechanism, where neither of the effecters, when considered separately, induces any allosteric response. The evidence, discussed in terms of an overall kinetic mechanism/regulation of leaf AGPase, prompts caution in classifying the protein as an 'allosteric enzyme'. (C) 2000 Elsevier Science B.V. All rights reserved.
|
|
| 41480. |
- Kleczkowski, Leszek A, 1954-
(författare)
-
KINETICS AND REGULATION OF THE NAD(P)H-DEPENDENT GLYOXYLATE-SPECIFIC REDUCTASE FROM SPINACH LEAVES
- 1995
-
Ingår i: Zeitschrift für Naturforschung C - A Journal of Biosciences. - 0939-5075 .- 1865-7125. ; 50:1-2, s. 21-28
-
Tidskriftsartikel (refereegranskat)abstract
- Kinetic mechanism of purified spinach leaf NAD(P)H glyoxylate reductase (GR-1) was studied using either NADPH and NADH as alternative substrates with glyoxylate. The mechanism was elucidated from substrate kinetic patterns using NADH as a cofactor rather than NADPH. With NADPH varied versus glyoxylate, and with NADPH and glyoxylate varied at a constant ratio, the patterns obtained on double reciprocal plots appeared to be consistent with a ping-pong mechanism; however, kinetic patterns with NADH conclusively ruled out the ping-pong reaction in favour of the sequential addition of the reactants. Product inhibition studies with glycolate and NADP have suggested either that NADPH binds to the enzyme before glyoxylate or that the addition of substrates is a random one. Studies with active group modifiers suggested an involvement of histidine, serine and cysteine residues in GR-1 activity. Salts had little or no effect on the activity of the enzyme, with the exception of cyanide, which had an apparent K-i of ca. 2 mM. Studies with several metabolites used as possible effecters of GR-1 activity have suggested that the enzyme is modulated only by substrate availability in vivo. The apparent insensitivity of GR-1 to metabolic effecters is consistent with the proposed role of the enzyme in detoxifying glyoxylate which may act as a potent inhibitor of photosynthetic processes in plant tissues.
|
|
