The role of sulfhydryl groups in the ribulose- 1,5-bisphosphate carboxylase and oxygenase reactions.

Ribulose-l,5-bisphosphate carboxylase (E.C. 4.1.1.39) isolated from Chromatium strain D contains 64 free cysteinyl -SH groups per mol (M_r 5.11 × 10⁵) as determined using three different titrants: p-[¹⁴C]chloromercuribenzoate, the Ellman reagent, and [¹⁴C]iodoacetamide.Distribution of -SH groups in the two constituent subunits (A and B) isolated from spinach and Chromatium ribulose-1,5-bisphosphate carboxylases was determined to be for spinach, 9 in A and 3 in B; and for Chromatium, 7 in A and 1 in B.The relationship between the numbers of -SH groups blocked vs residual activities of both the ribulose-1,5-bisphosphate carboxylase and oxygenase reactions was examined by titration with p-chloromercuribenzoate. In both spinach and Chromatium enzymes, antisigmoidal curves were obtained for the degree of the enzyme activity loss in relation to the numbers of -SH groups masked. However, at alkaline pH the Chromatium enzyme shows a sharp decline in both carboxylase and oxygenase activities, apparently due to the alkali dissociation of the enzyme molecule accompanied by its structural deformation. The functional role of -SH groups in the ribulose-1,5-bisphosphate carboxylase molecule is discussed in relation to two constituent enzyme reactions, and it is concluded that in both enzyme sources the active sites are probably the same for the two reactions.     

The Association of d-Ribulose- 1,5-Bisphosphate Carboxylase/Oxygenase with Phosphoribulokinase

When Ribulose- 1,5-bisphosphate carboxylase/oxygenase was purified from spinach leaves (Spinacia oleracea) using precipitation with polyethylene glycol and MgCl₂ followed by DEAE cellulose chromatography, 75% of phosphoribulokinase and 7% of phosphoriboisomerase activities copurified with ribulose- 1,5-bisphosphate carboxylase/oxygenase. This enzyme preparation showed ribose-5-phosphate and ribulose-5-phosphate dependent carboxylase and oxygenase activities which were nearly equivalent to its corresponding ribulose- 1,5-bisphosphate dependent activity. The ribose-5-phosphate and ribulose-5-phosphate dependent reaction rates were stable and linear for much longer time periods than the ribulose- 1,5-bisphosphate dependent rates. When sucrose gradients were used to purify ribulose- 1,5-bisphosphate carboxylase/oxygenase from crude stromal extracts, phosphoribulokinase was found to cosediment with ribulose- 1,5-bisphosphate carboxylase. Under these conditions most of the phosphoriboisomerase activity remained with the slower sedimenting proteins. Ammonium sulfate precipitation resulted in separation of the ribulose- 1,5-bisphosphate carboxylase peak from phosphoribulokinase peak. Crude extracts of peas Pisum sativum and spinach contained 0.725 to 0.730 milligram of phosphoribulokinase per milligram of chlorophyll, respectively, based on an enzyme-linked immunosorbent assay.     

1,5-anhydro-D-fructose from D-fructose

1,5-Anhydro-d-fructose was efficiently prepared from d-fructose via regiospecific 1,5-anhydro ring formation of 2,3-O-isopropylidene-1-O-methyl(tolyl)sulfonyl-d-fructopyranose and subsequent deprotection.     

Metabolic engineering of Escherichia coli for high production of 1,5-pentanediol via a cadaverine-derived pathway

1,5-Pentanediol (1,5-PDO) is a high value-added chemical which is widely used as a monomer in the polymer industry. There are no natural organisms that could directly produce 1,5-PDO from renewable carbon sources. In this study, we report metabolic engineering of Escherichia coli for high-level production of 1,5-PDO from glucose via a cadaverine-derived pathway. In the newly proposed pathway, cadaverine can be converted to 1,5-PDO via 5-hydroxyvalerate (5-HV) by introducing only one heterologous enzyme in E. coli. Different endogenous genes of E. coli were screened and heterologous carboxylic acid reductase genes were tested to build a functional pathway. Compared to the previously reported pathways, the engineered cadaverine-based pathway has a higher theoretical yield (0.70 mol/mol glucose) and higher catalytic efficiency. By further combining strategies of pathway engineering and process engineering, we constructed an engineered E. coli strain that could produce 2.62 g/L 1,5-PDO in shake-flask and 9.25 g/L 1,5-PDO with a yield of 0.28 mol/mol glucose in fed-batch fermentation. The proposed new pathway and engineering strategies reported here should be useful for developing biological routes to produce 1,5-PDO for real application.     

1,5-D-anhydrofructose,the precursor of the pyrone microthecin in morchella vulgaris

The carbohydrate which, in various Discomycetes, is enzymatically converted under plasmolytic conditions to the antibiotic pyrone microthecin was isolated from a strain of Morchella vulgaris and identified as 1,5-D-anhydrofructose from X-ray analysis of its oxime.     

Regulation of ribulose 1,5-bisphosphate carboxylase-oxygenase activities by temperature pretreatment and chloroplast metabolites

Crystalline ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) was isolated from tobacco (Nicotiana tabacum L.) leaf homogenates and the two competing reactions were examined for differential regulation in vitro by temperature pretreatment and chloroplast metabolites. Both the carboxylase and oxygenase activities were inactivated 50% by storing the dissolved protein at 0 °C and fully reactivated by heating the solution at 50 °C in the absence of Mg²⁺ and a sulfhydryl reagent. When the heat-activated enzyme was preincubated with physiological levels of various chloroplast metabolites and CO₂ and the two reactions were assayed simultaneously in the same reaction vessel upon initiation with ribulose 1,5-bisphosphate, three classes of effectors were observed: (a) those which stimulated both activities (NADPH, 6-phosphogluco-bisphosphate gluconate, fructose 1,6-bisphosphate, 3-phosphoglycerate glycerate), (b) those which essentially had no effect (fructose 6-phosphate, glucose 6-phosphate), and (c) one, ribose 5-phosphate, which inhibited the two reactions. However, within the limits of experimental error, none of the metabolites examined produced a differential regulation of the ribulose 1,5-bisphosphate carboxylase-oxygenase activities. The similar response of the two competing activities to temperature pretreatment and various chloroplast metabolites is consistent with the notion that both reactions are associated with the same or adjacent catalytic sites on this bifunctional enzyme.     

Xylulose 1,5-Bisphosphate Synthesized by Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase during Catalysis Binds to Decarbamylated Enzyme

Xylulose 1,5-bisphosphate (XuBP) is synthesized from ribulose 1,5-bisphosphate (RuBP) at carbamylated catalytic sites on ribulose 1,5-bisphosphate carboxylase (Rubisco) with significant amounts of XuBP being formed at pH less than 8.0. XuBP has been separated by high performance liquid chromatography and identified by pulsed amperometry from compounds bound to Rubisco during catalysis with the purified enzyme and from celery (Apium graveolens var Utah) leaf extracts. XuBP does not bind tightly to carbamylated sites, but does bind tightly to decarbamylated sites. Upon incubation of fully activated Rubisco with 5 micromolar XuBP, loss of activator CO₂ occurred before XuBP bound to the enzyme catalytic sites, even in the presence of excess CO₂ and Mg²⁺. Binding of XuBP to decarbamylated Rubisco sites was highly pH dependent. At pH 7.0 and 7.5 with 10 millimolar MgCl₂ and 10 millimolar KHCO₃, the apparent dissociation constant for XuBP, K_d, was 0.03 micromolar, whereas at pH 8.0 and 8.5, the apparent K_d was 0.35 and 2.0 micromolar, respectively. This increase in K_d with pH was a result of a decrease in the association rate constant and an increase in the dissociation rate constant of XuBP bound to decarbamylated sites on Rubisco. The K_d of 2-carboxyarabinitol 1-phosphate binding to carbamylated sites was only slightly pH dependent.     

Cation-Exchange resin-catalyzed addition of methanol to benzoylated 1,5-anhydro-2-deoxy-d-hex-1-enitols

1,5-Anhydro-3,4,6-tri-O-benzoyl-2-deoxy-d-arabino-hex-1-enitol (1) was boiled under reflux with methanol and AG 50W-X8 cation-exchange resin. A two-product mixture of glycosides (2 and 3) was obtained in 38% yield, together with 19% of unreacted material. 1,5-Anhydro-3,6-di-O-benzoyl-2-deoxy-d-arabino-hex-1-enitol (7) was prepared from 1,5-anhydro-2-deoxy-d-arabino-hex-1-enitol by selective benzoylation, from which the corresponding 4-methanesulfonate 8 was obtained. Treatment of 8 with sodium benzoate in hexamethylphosphoric triamide for 72 h at 100° afforded 1,5-anhydro-3,4,6-tri-O-benzoyl-2-deoxy-d-lyxo-hex-1-enitol (9) in 52% yield. An unknown byproduct (B), tentatively shown to be a tri-O-benzoyl-d-hex-2-enopyranose analog, was also isolated in 14% yield. The 270-MHz n.m.r. spectrum of B was analyzed in terms of its J_1,3, J_2,4, and J_4,5 coupling constants in relation to the various configurational and conformational possibilities for hex-2-enopyranoses, and was identified as 1,4,6-tri-O-benzoyl-2,3-dideoxy-α-d-threo-hex-2-enopyranose having the ^oH₅ conformation. The analysis presented should also be applicable to pent-2-enopyranose systems. When 9 was treated with methanol in the presence of AG 50W-X8 cation-exchange resin, a mixture of glycosides 4 and 5 was obtained in 47% yield. The low yields were attributed to methanolysis of the benzoyl groups during the reaction.     

Synthesis and SAR of substituted pyrazolo[1,5-a]quinazolines as dual mGlu2/mGlu3 NAMs

Herein we report the design and synthesis of a series of substituted pyrazolo[1,5-a]quinazolin-5(4H)-ones as negative allosteric modulators of metabotropic glutamate receptors 2 and 3 (mGlu₂ and mGlu₃, respectively). Development of this series was initiated by reports that pyrazolo[1,5-a]quinazoline-derived scaffolds can yield compounds with activity at group II mGlu receptors which are prone to molecular switching following small structural changes. Several potent analogues, including 4-methyl-2-phenyl-8-(pyrimidin-5-yl)pyrazolo[1,5-a]quinazolin-5(4H)-one (10b), were discovered with potent in vitro activity as dual mGlu₂/mGlu₃ NAMs, with excellent selectivity versus the other mGluRs.     

Synthesis of 4-cyanophenyl and 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-beta-D-arabino- and -beta-D-lyxopyranosides possessing antithrombotic activity

Tri-O-acetyl-5-thio-D-ribopyranosyl bromide was converted into 3,4-di-O-benzoyl-1,5-anhydro-5-thio-D-erythro-pent-1-enitol (3,4-di-O-benzoyl-5-thio-D-ribal), the azidonitration of which afforded an unstable mixture of 2-azido-3,4-di-O-benzoyl-2-deoxy-1-O-nitro-5-thio-D-pentopyranoside++ + isomers. This was converted without separation into the corresponding 1-O-acetyl derivatives from which an alpha,beta anomeric mixture of the 1-O-acetyl-2-azido-3,4-di-O-benzoyl-2-deoxy-5-thio-D-arabinopyranose+ ++ isomers could be isolated in high yield. Glycosidation of this mixture with 4-cyano- or 4-nitrobenzenethiol, using trimethylsilyl triflate or boron trifluoride etherate, respectively, as promoters gave the corresponding D anomers exclusively. Zemplén debenzoylation afforded 4-cyanophenyl as well as 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-beta-D-arabinopyranoside, respectively. When 1-O-acetyl-2-azido-3,4-di-O-benzoyl-2-deoxy-5-thio-D-lyxopyranose was used as glycosyl donor only the corresponding 1 anomers, i.e., 4-cyanophenyl as well as 4-nitrophenyl 2-azido-2-deoxy-1,5-dithio-beta-D-lyxopyranosides, could be isolated after Zemplén debenzoylation in high yield. All four 1,5-dithioglycosides possess significant oral antithrombotic activity.     

Synthesis of some derivatives of 6-amino-1,5-anhydro-6-deoxy-d-glucitol and 2-amino-1,5-anhydro-2-deoxy-d-glucitol

6-Amino-1,5-anhydro-6-deoxy-d-glucitol (11) was prepared from 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl bromide (1) in six steps. Reduction of 1 with tributyltin hydride, followed by deacetylation, monomolar tosylation, and reacetylation, afforded 2,3,4-tri-O-acetyl-1,5-anhydro-6-O-toluene-p-sulfonyl-d-glucitol (9). Alternatively, tritylation of 1,5-anhydro-d-glucitol, followed by acetylation, detritylation, and tosylation, gave 9. Mesylation gave 8. Treatment of 8 or 9 with azide anion afforded the azide 10, reduction of which with tributyltin hydride gave 11, which was mesylated or tosylated, and then deacetylated to give the 6-methane-sulfonamido or 6-toluene-p-sulfonamido derivative. Similarly, mesylation or tosylation of 3,4,6-tri-O-acetyl-2-amino-1,5-anhydro-2-deoxy-d-glucitol (20) gave the 2-methanesulfonamido or 2-toluene-p-sulfonamido derivatives. Treatment of 11 and 20 with sulfur trioxide-pyridine afforded the sulfoamino derivatives, deacetylation of which gave sugar analogs of cyclamate-like compounds.     

New copper(II), nickel(II) and zinc(II) complexes with 1,2,4-triazolo[1,5-a]pyrimidines and the chelating ligand 1,3-propanediamine: An unexpected coordination behavior for the 7-amine-derivative

Five new metal complexes with the metal ions Cu(II), Ni(II) and Zn(II) and containing 1,2,4-triazolo[1,5-a]pyrimidine derivatives and 1,3-propanediamine (tn) are described. The structural morphology of these coordination compounds depends on the triazolopyrimidine derivative used, being mononuclear for 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) and 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO), and 1D-polymeric for 7-amine-1,2,4-triazolo[1,5-a]pyrimidine (7atp). In the 7atp case, this ligand does not coordinate through N3 atom, as expected, but through N1, N4 and N71 in a bridging fashion. This unexpected coordination mode seems to be induced by the stability of the polynuclear metal complex in presence of tn ligand. All isolated metal complexes have been characterized by single-crystal X-ray diffraction, IR and UV-Vis spectroscopies, and EPR measurements. Moreover, luminescence measurements have been carried out for 7atp ligand and its polynuclear complex with Zn(II).     

Exchange Properties of the Activator CO(2) of Spinach Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

The exchange properties of the activator CO₂ of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO₂ on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO₂. The exchange kinetics of the activator CO₂ were monitored by activation of the enzyme to steady state in the presence of ¹²CO₂, followed by addition of ¹⁴CO₂ and determination of the amount of labeled CO₂ trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants (K_obs) for exchange with both the purified enzyme (0.45 min⁻¹) and in illuminated chloroplasts (0.18 min⁻¹) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO₂ was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation.     

Discovery of pyrazolo[1,5-a]pyrimidine-based CHK1 inhibitors: a template-based approach--part 2

Previous efforts by our group have established pyrazolo[1,5-a]pyrimidine as a viable core for the development of potent and selective CDK inhibitors. As part of an effort to utilize the pyrazolo[1,5-a]pyrimidine core as a template for the design and synthesis of potent and selective kinase inhibitors, we focused on a key regulator in the cell cycle progression, CHK1. Continued SAR development of the pyrazolo[1,5-a]pyrimidine core at the C5 and C6 positions, in conjunction with previously disclosed SAR at the C3 and C7 positions, led to the discovery of potent and selective CHK1 inhibitors.     

Discovery of pyrazolo[1,5-a]pyrimidine-based Pim inhibitors: A template-based approach

The synthesis and hit-to-lead SAR development from a pyrazolo[1,5-a]pyrimidine-derived hit 5 to the identification of a series of potent, pan–Pim inhibitors such as 11j are described.     

Characteristics of ribulose-1,5-bisphosphate carboxylase/oxygenase degradation by lysates of mechanically isolated chloroplasts from wheat leaves

The lysate from intact chloroplasts mechanically isolated from primary leaves of 9 day old seedlings of wheat (Triticum aestivum L. var Aoba) was incubated in the pH range of 5.5 to 8.5 at 37°C for 5 hours. Proteolytic activity against ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was estimated by disappearance of the large subunit of Rubisco or the appearance of its degradation products. Although the activity in lysates was weak, the products were detected by applying Western blotting. The degradation products were similar to those obtained when Rubisco was incubated with the lysate of vacuoles isolated from like leaves. Although some of the products were similar to those from vacuole lysates, many were clearly different after incubation of Rubisco with trypsin, V-8 protease, or reactive oxygen (hydroxy radical). Lysates of chloroplasts, pretreated with thermolysin at 4°C for 30 minutes, had no proteolytic activity against Rubisco after incubation at 37°C for 5 hours. These results show that the proteolytic activity against Rubisco found in lysates of our mechanically isolated chloroplasts was mostly due to the contamination of vacuolar proteases adhering to the outer envelope of the chloroplasts during their isolation.     

Synthesis of 4-O-glycosylated 1,5-anhydro-d-fructose and of 1,5-anhydro-d-tagatose from a common intermediate 2,3-O-isopropylidene-d-fructose

Four novel disaccharides of glycosylated 1,5-anhydro-d-ketoses have been prepared: 1,5-anhydro-4-O-β-d-glucopyranosyl-d-fructose, 1,5-anhydro-4-O-β-d-galactopyranosyl-d-fructose, 1,5-anhydro-4-O-β-d-glucopyranosyl-d-tagatose, and 1,5-anhydro-4-O-β-d-galactopyranosyl-d-tagatose. The common intermediate, 1,5-anhydro-2,3-O-isopropylidene-β-d-fructopyranose, was prepared from d-fructose and was converted into the d-tagatose derivative by oxidation followed by stereoselective reduction to the 4-epimer. The anhydroketoses thus prepared were glycosylated and deprotected to give the disaccharides.     

The presence of ribulose 1,5-diphosphate carboxylase in the nonphotosynthetic endosperm of germinating castor beans

Ribulose 1,5-diphosphate carboxylase was detected in extracts of germinating castor bean (Ricinus communis var. Hale) endosperms. This is the first report of this enzyme in a nonphotosynthetic (no chlorophyll) plant tissue. Radioactive 3-phosphoglyceric acid has been identified as the principle product resulting from the enzymatic condensation of ¹⁴C-bicarbonate and ribulose-1,5-diP in endosperm extracts. The Km values of bicarbonate and ribulose-1,5-diP for the endosperm carboxylase are 1.14 × 10⁻²m and 7.5 × 10⁻⁵m, respectively. The carboxylase activity peaks at 4 days in endosperms of castor beans germinated in the dark. The specific activity of the carboxylase at this stage of germination is 4.3 μmoles of 3-phosphoglycerate formed/mg protein·hr. The presence of ribulose-1,5-diP carboxylase and other enzymes of the reductive pentose phosphate pathway show the potential of this pathway in castor bean endosperms.     

Partial Purification and Characterization of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Large Subunit epsilonN-Methyltransferase

The large subunit (LS) of tobacco (Nicotiana rustica) ribulose-1,5-bisphosphate carboxylase/oxygenase (ribulose-P₂ carboxylase) contains a trimethyllysyl residue at position 14, whereas this position is unmodified in spinach ribulose-P₂ carboxylase. A protein fraction was isolated from tobacco chloroplasts by rate-zonal centrifugation and anion-exchange fast protein liquid chromatography that catalyzed transfer of methyl groups from S-adenosyl-[methyl-³H]-l-methionine to spinach ribulose-P₂ carboxylase. ³H-Methyl groups incorporated into spinach ribulose-P₂ carboxylase were alkaline stable but could be removed by limited tryptic proteolysis. Reverse-phase high-performance liquid chromatography of the tryptic peptides released after proteolysis showed that the penultimate N-terminal peptide from the LS of spinach ribulose-P₂ carboxylase contained the site of methylation, which was identified as lysine-14. Thus, the methyltransferase activity can be attributed to S-adenosylmethionine:ribulose-P₂ carboxylase LS (lysine) `N-methyltransferase, a previously undescribed chloroplast enzyme. The partially purified enzyme was specific for ribulose-P₂ carboxylase and exhibited apparent K_m values of 10 micromolar for S-adenosyl-l-methionine and 18 micromolar for ribulose-P₂ carboxylase, a V_max of 700 picomoles CH₃ groups transferred per minute per milligram protein, and a broad pH optimum from 8.5 to 10.0. S-Adenosylmethionine:ribulose-P₂ carboxylase LS (lysine)^εN-methyltransferase was capable of incorporating 24 ³H-methyl groups per spinach ribulose-P₂ carboxylase holoenzyme, forming 1 mole of trimethyllysine per mole of ribulose-P₂ carboxylase LS, but was inactive on ribulose-P₂ carboxylases that contain a trimethyllysyl residue at position 14 in the LS. The enzyme did not distinguish between activated (Mg²⁺ and CO₂) and unactivated forms of ribulose-P₂ carboxylase as substrates. However, complexes of activated ribulose-P₂ carboxylase with the reaction-intermediate analogue 2′-carboxy-d-arabinitol-1,5-bisphosphate, or unactivated spinach ribulose-P₂ carboxylase with ribulose-1,5-bisphosphate, were poor substrates for tobacco LS ^εN-methyltransferase.