Synthesis of N4-aryl-β-d-glucopyranosylcytosines: a methodology study

A number of leaving groups, including arylsulfonates, triazoles, 3-nitrotriazoles, and tetrazoles, have been studied for the substitution reaction by aryl and alkyl amines at the 4-position of β-d-glucopyranosyluracils. Examination of the stability, ease of purification and reactivity in the substitution reaction led to a number of optimized conditions with the most convenient involving substitution of triazole derivatives under microwave conditions in the presence of silica gel. Under these conditions, a number of N4-aryl-substituted β-d-glucopyranosylcytosines were prepared as potential inhibitors of glycogen phosphorylase, a molecular target for type-2 diabetes mellitus

Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55–60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.Open access funding provided by Max Planck Society. Financial support of this work by the Max Planck Society and MANGAN (03EK3545) funded by the Bundesministeriums für Bildung und Forschung is gratefully acknowledged. N.C. acknowledges the support of the Australian Research Council (FT140100834

Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.Open access funding provided by Max Planck Society. Financial support of this work by the Max Planck Society and MANGAN (03EK3545) funded by the Bundesministeriums für Bildung und Forschung is gratefully acknowledged. N.C. acknowledges the support of the Australian Research Council (FT140100834)

Structured near-infrared Magnetic Circular Dichroism spectra of the Mn₄CaO₅ cluster of PSII in T. vulcanus are dominated by Mn(IV) d-d 'spin-flip' transitions

Photosystem II passes through four metastable S-states in catalysing light-driven water oxidation. Variable temperature variable field (VTVH) Magnetic Circular Dichroism (MCD) spectra in PSII of Thermosynochococcus (T.) vulcanus for each S-state are reported. These spectra, along with assignments, provide a new window into the electronic and magnetic structure of Mn₄CaO₅. VTVH MCD spectra taken in the S₂state provide a clear g=2, S=1/2 paramagnetic characteristic, which is entirely consistent with that known by EPR. The three features, seen as positive (+) at 749nm, negative (-) at 773nm and (+) at 808nm are assigned as ⁴A→²E spin-flips within the d³ configuration of the Mn(IV) centres present. This assignment is supported by comparison(s) to spin-flips seen in a range of Mn(IV) materials. S₃ exhibits a more intense (-) MCD peak at 764nm and has a stronger MCD saturation characteristic. This S₃ MCD saturation behaviour can be accurately modelled using parameters taken directly from analyses of EPR spectra. We see no evidence for Mn(III) d-d absorption in the near-IR of any S-state. We suggest that Mn(IV)-based absorption may be responsible for the well-known near-IR induced changes induced in S₂ EPR spectra of T. vulcanus and not Mn(III)-based, as has been commonly assumed. Through an analysis of the nephelauxetic effect, the excitation energy of S-state dependent spin-flips seen may help identify coordination characteristics and changes at each Mn(IV). A prospectus as to what more detailed S-state dependent MCD studies promise to achieve is outlined.We recognise the support of the Australian Research Council through grants DP110104565 and DP150103137 (E.K.), FT140100834 (N.C) and MEXT/JSPS of Japan through a Grant-in-Aid for Specially Promoted Research No. 24000018 (J.R.S.)

EstDZ3:a new esterolytic enzyme exhibiting remarkable thermostability

Lipolytic enzymes that retain high levels of catalytic activity when exposed to a variety of denaturing conditions are of high importance for a number of biotechnological applications. In this study, we aimed to identify new lipolytic enzymes, which are highly resistant to prolonged exposure at elevated temperatures. To achieve this, we searched for genes encoding for such proteins in the genomes of a microbial consortium residing in a hot spring located in China. After performing a functional genomic screening on a bacterium of the genus Dictyoglomus, which was isolated from this hot spring after in situ enrichment, we identified a new esterolytic enzyme, termed EstDZ3. Detailed biochemical characterization of the recombinant enzyme, revealed that it constitutes a slightly alkalophilic and highly active esterase against esters of fatty acids with short to medium chain lengths. Importantly, EstDZ3 exhibits remarkable thermostability, as it retained high levels of catalytic activity after exposure to temperatures as high as 95 oC for several hours. Interestingly, EstDZ3 was found to have very little similarity to previously characterized esterolytic enzymes. Computational modelling of the three-dimensional structure of this new enzyme predicted that it exhibits a typical α/β hydrolase fold, which seems to include a subdomain insertion. This insertion is similar to the one present in its closest homologue of known function and structure, the cinnamoyl esterase Lj0536 from Lactobacillus johnsonii. As it was found in the case of Lj0536, this structural feature is expected to be an important determinant of the catalytic properties of EstDZ3. The high levels of esterolytic activity of EstDZ3, combined with its remarkable thermostability and good stability against a wide range of metal ions, organic solvents, and other denaturing agents, render this new enzyme a candidate biocatalyst for high-temperature biotechnological applications

EstDZ3: A New Esterolytic Enzyme Exhibiting Remarkable Thermostability

Lipolytic enzymes that retain high levels of catalytic activity when exposed to a variety of denaturing conditions are of high importance for a number of biotechnological applications. In this study, we aimed to identify new lipolytic enzymes, which are highly resistant to prolonged exposure at elevated temperatures. To achieve this, we searched for genes encoding for such proteins in the genomes of a microbial consortium residing in a hot spring located in China. After performing a functional genomic screening on a bacterium of the genus Dictyoglomus, which was isolated from this hot spring after in situ enrichment, we identified a new esterolytic enzyme, termed EstDZ3. Detailed biochemical characterization of the recombinant enzyme, revealed that it constitutes a slightly alkalophilic and highly active esterase against esters of fatty acids with short to medium chain lengths. Importantly, EstDZ3 exhibits remarkable thermostability, as it retained high levels of catalytic activity after exposure to temperatures as high as 95 oC for several hours. Interestingly, EstDZ3 was found to have very little similarity to previously characterized esterolytic enzymes. Computational modelling of the three-dimensional structure of this new enzyme predicted that it exhibits a typical α/β hydrolase fold, which seems to include a subdomain insertion. This insertion is similar to the one present in its closest homologue of known function and structure, the cinnamoyl esterase Lj0536 from Lactobacillus johnsonii. As it was found in the case of Lj0536, this structural feature is expected to be an important determinant of the catalytic properties of EstDZ3. The high levels of esterolytic activity of EstDZ3, combined with its remarkable thermostability and good stability against a wide range of metal ions, organic solvents, and other denaturing agents, render this new enzyme a candidate biocatalyst for high-temperature biotechnological applications

Computationally motivated synthesis and enzyme kinetic evaluation of N-(β-d-glucopyranosyl)-1,2,4-triazolecarboxamides as glycogen phosphorylase inhibitors

Following our recent study of N-(β-D-glucopyranosyl)-oxadiazole-carboxamides (Polyák et al., Biorg. Med. Chem. 2013, 21, 5738) revealed as moderate inhibitors of glycogen phosphorylase (GP), in silico docking calculations using Glide have been performed on N-(β-D-glucopyranosyl)-1,2,4-triazolecarboxamides with different aryl substituents predicting more favorable binding at GP. The ligands were subsequently synthesized in moderate yields using N-(2,3,4,6-terta-O-acetyl-β-D-glucopyranosyl)-tetrazole-5-carboxamide as starting material. Kinetics experiments against rabbit muscle glycogen phosphorylase b (RMGPb) revealed the ligands to be low µM GP inhibitors; the phenyl analogue (Ki = 1 µM) is one of the most potent N-(β-D-glucopyranosyl)-heteroaryl-carboxamide-type inhibitors of the GP catalytic site discovered to date. Based on QM and QM/MM calculations, the potency of the ligands is predicted to arise from favorable intra- and intermolecular hydrogen bonds formed by the most stable solution phase tautomeric (t2) state of the 1,2,4-triazole in a conformationally dynamic system. ADMET property predictions revealed the compounds to have promising pharmacokinetic properties without any toxicity. This study highlights the benefits of a computationally lead approach to GP inhibitor design

Glucose-derived spiro-isoxazolines are anti-hyperglycemic agents against type 2 diabetes through glycogen phosphorylase inhibition

International audienceGlycogen phosphorylase (GP) is a target for the treatment of hyperglycaemia in the context of type 2 diabetes. This enzyme is responsible for the depolymerization of glycogen into glucose thereby affecting the levels of glucose in the blood stream. Twelve new d-glucopyranosylidene-spiro-isoxazolines have been prepared from O-peracylated exo-D-glucals by regio- and stereoselective 1,3-dipolar cycloaddition of nitrile oxides generated in situ by treatment of the corresponding oximes with bleach. This mild and direct procedure appeared to be applicable to a broad range of substrates. The corresponding O-unprotected spiro-isoxazolines were evaluated as glycogen phosphorylase (GP) inhibitors and exhibited IC50 values ranging from 1 to 800 μM. Selected inhibitors were further evaluated in vitro using rat and human hepatocytes and exhibited significant inhibitory properties in the primary cell culture. Interestingly, when tested with human hepatocytes, the tetra-O-acetylated spiro-isoxazoline bearing a 2-naphthyl residue showed a much lower IC50 value (2.5 μM), compared to that of the O-unprotected analog (19.95 μM). The most promising compounds were investigated in Zucker fa/fa rat model in acute and sub-chronic assays and decreased hepatic glucose production, which is known to be elevated in type 2 diabetes. This indicates that glucose-based spiro-isoxazolines can be considered as anti-hyperglycemic agents in the context of type 2 diabetes