Structural basis of purine nucleotide inhibition of human uncoupling protein 1.

Mitochondrial uncoupling protein 1 (UCP1) gives brown adipose tissue of mammals its specialized ability to burn calories as heat for thermoregulation. When activated by fatty acids, UCP1 catalyzes the leak of protons across the mitochondrial inner membrane, short-circuiting the mitochondrion to generate heat, bypassing ATP synthesis. In contrast, purine nucleotides bind and inhibit UCP1, regulating proton leak by a molecular mechanism that is unclear. We present the cryo-electron microscopy structure of the GTP-inhibited state of UCP1, which is consistent with its nonconducting state. The purine nucleotide cross-links the transmembrane helices of UCP1 with an extensive interaction network. Our results provide a structural basis for understanding the specificity and pH dependency of the regulatory mechanism. UCP1 has retained all of the key functional and structural features required for a mitochondrial carrier-like transport mechanism. The analysis shows that inhibitor binding prevents the conformational changes that UCP1 uses to facilitate proton leak.

Structure and function of a highly active Bile Salt Hydrolase (BSH) from Enterococcus faecalis and post-translational processing of BSH enzymes.

Bile Salt Hydrolase (BSH), a member of Cholylglycine hydrolase family, catalyzes the de-conjugation of bile acids and is evolutionarily related to penicillin V acylase (PVA) that hydrolyses a different substrate such as penicillin V. We report the three-dimensional structure of a BSH enzyme from the Gram-positive bacteria Enterococcus faecalis (EfBSH) which has manifold higher hydrolase activity compared to other known BSHs and displays unique allosteric catalytic property. The structural analysis revealed reduced secondary structure content compared to other known BSH structures, particularly devoid of an anti-parallel ß-sheet in the assembly loop and part of a ß-strand is converted to increase the length of a substrate binding loop 2. The analysis of the substrate binding pocket showed reduced volume owing to altered loop conformations and increased hydrophobicity contributed by a higher ratio of hydrophobic to hydrophilic groups present. The aromatic residues F18, Y20 and F65 participate in substrate binding. Thus, their mutation affects enzyme activity. Docking and Molecular Dynamics simulation studies showed effective polar complementarity present for the three hydroxyl (-OH) groups of GCA substrate in the binding site contributing to higher substrate specificity and efficient catalysis. These are unique features characteristics of this BSH enzyme and thought to contribute to its higher activity and specificity towards bile salts as well as allosteric effects. Further, mechanism of autocatalytic processing of Cholylglycine Hydrolases by the excision of an N-terminal Pre-peptide was examined by inserting different N-terminal pre-peptides in EfBSH sequence. The results suggest that two serine residues next to nucleophile cysteine are essential for autocalytic processing to remove precursor peptide. Since pre-peptide is absent in EfBSH the mutation of these serines is tolerated. This suggests that an evolution-mediated subordination of the pre-peptide excision site resulted in loss of pre-peptide in EfBSH and other related Cholylglycine hydrolases.

Molecular features of bile salt hydrolases and relevance in human health.

BACKGROUND: Bile salt hydrolase (BSH) enzyme is responsible for the de-conjugation of bile salts by commensal bacteria, thus playing a vital role in their colonization and survival in the mammalian intestine and determination of their probiotic potential. Further, bile deconjugation also leads to lowering of cholesterol and alterations in energy homeostasis, thus making BSH a clinically important enzyme. SCOPE OF THE REVIEW: Many recent observations have indicated that BSH may be involved in a multifaceted array of roles, directly or indirectly in the host and microbial physiology. BSH paralogues have now been found to occur in different microbes including free-living and pathogenic bacteria and Archaea. BSHs from various sources also show differential activity and substrate spectrum. Certain bacteria are known to possess multiple genes for BSH enzymes. BSHs have been reported to influence different metabolic phenomena, including bacterial pathogenesis and the maintenance of lipid and glucose homeostasis in the host. These observations necessitate an intense study into the biochemical, structural and regulatory features of BSH enzymes to better understand their role in regulating bacterial and host metabolism. MAJOR CONCLUSIONS: In this review, the available information on the characteristics of BSH enzymes have been organized in order to understand their interactions with a wide range of substrates and their myriad physiological roles, from bile resistance to signalling mechanisms. GENERAL SIGNIFICANCE: A detailed exploration of BSH architecture and regulation could provide insights into its evolution and a deeper appreciation of the multiple functions of this enzyme relevant to healthcare.

Electrophilic iodination: a gateway to high iodine compounds and energetic materials.

A large number of iodine atoms can be introduced into a single molecule in a one-pot reaction using trifluoroperacetic acid-mediated electrophilic iodination methodology. The scope of this reaction was investigated extensively using several pyrazole substrates which resulted in nine polyiodo pyrazole compounds with iodine content as high as 80%. This synthetic methodology was also utilized successfully for iodination of benzimidazoles. Tetraiodobenzimidazole was nitrated with 100% nitric acid to give a high yield of 4,5,6,7-tetranitro-1H-benzimidazol-2(3H)-one (14). All of these materials were fully characterized and compounds 5, 9, 10 and 14 were confirmed further with single crystal X-ray analysis. High density, positive oxygen balance, and very good impact sensitivity values characterize 14. For the first time, two 1,2,5-oxadiazole-N-oxide rings were introduced into a benzimidazole ring (11) which remarkably improves the stability of oxadiazole-N-oxide compounds.

Mono- and diiodo-1,2,3-triazoles and their mono nitro derivatives.

4-Iodo-1H-1,2,3-triazole (2) and 4,5-diiodo-1H-1,2,3-triazole (3) were synthesized using an efficient and viable synthetic route. The N-alkylation of 3 resulted in the formation of two tautomers. The N-alkyl-diiodo-triazoles were nitrated with 100% nitric acid to form monoiodo-mononitro-triazoles. The structures of 2-methyl-4,5-diiodo-1,2,3-triazole (5), 1-ethyl-4,5-diiodo-1,2,3-triazole (6), 1-methyl-4-nitro-5-iodo-1,2,3-triazole (8) and 1-ethyl-4-nitro-5-iodo-1,2,3-triazole (10) were confirmed by X-ray crystal analysis. All of the new triazoles were fully characterized via NMR, and infrared spectra, and elemental analyses as well as by their thermal and sensitivity properties. Decomposition products calculated using Cheetah 7 software show that these iodo-nitro triazoles liberate iodine.

Structural analysis of a penicillin V acylase from Pectobacterium atrosepticum confirms the importance of two Trp residues for activity and specificity.

Penicillin V acylases (PVA) catalyze the deacylation of the beta-lactam antibiotic phenoxymethylpenicillin (Pen V). They are members of the Ntn hydrolase family and possess an N-terminal cysteine as the main catalytic nucleophile residue. They form the evolutionarily related cholylglycine hydrolase (CGH) group which includes bile salt hydrolases (BSH) responsible for bile deconjugation. Even though a few PVA and BSH structures have been reported, no structure of a functional PVA from Gram-negative bacteria is available. Here, we report the crystal structure of a highly active PVA from Gram-negative Pectobacterium atrosepticum (PaPVA) at 2.5Å resolution. Structural comparison with PVAs from Gram-positive bacteria revealed that PaPVA had a distinctive tetrameric structure and active site organization. In addition, mutagenesis of key active site residues and biochemical characterization of the resultant variants elucidated the role of these residues in substrate binding and catalysis. The importance of residue Trp23 and Trp87 side chains in binding and correct positioning of Pen V by PVAs was confirmed using mutagenesis and substrate docking with a 15ns molecular dynamics simulation. These results establish the unique nature of Gram-negative CGHs and necessitate further research about their substrate spectrum.

Sequence and structure-based comparative analysis to assess, identify and improve the thermostability of penicillin G acylases.

Penicillin acylases are enzymes employed by the pharmaceutical industry for the manufacture of semi-synthetic penicillins. There is a continuous demand for thermostable and alkalophilic enzymes in such applications. We have carried out a computational analysis of known penicillin G acylases (PGAs) in terms of their thermostable nature using various protein-stabilizing factors. While the presence of disulfide bridges was considered initially to screen putative thermostable PGAs from the database, various other factors such as high arginine to lysine ratio, less content of thermolabile amino acids, presence of proline in ß-turns, more number of ion-pair and other non-bonded interactions were also considered for comparison. A modified consensus approach designed could further identify stabilizing residue positions by site-specific comparison between mesostable and thermostable PGAs. A most likely thermostable enzyme identified from the analysis was PGA from Paracoccus denitrificans (PdPGA). This was cloned, expressed and tested for its thermostable nature using biochemical and biophysical experiments. The consensus site-specific sequence-based approach predicted PdPGA to be more thermostable than Escherichia coli PGA, but not as thermostable as the PGA from Achromobacter xylosoxidans. Experimental data showed that PdPGA was comparatively less thermostable than Achromobacter xylosoxidans PGA, although thermostability factors favored a much higher stability. Despite being mesostable, PdPGA being active and stable at alkaline pH is an advantage. Finally, several residue positions could be identified in PdPGA, which upon mutation selectively could improve the thermostability of the enzyme.

Structure mediation in substrate binding and post-translational processing of penicillin acylases: Information from mutant structures of Kluyvera citrophila penicillin G acylase.

Penicillin acylases are industrially important enzymes for the production of 6-APA, which is used extensively in the synthesis of secondary antibiotics. The enzyme translates into an inactive single chain precursor that subsequently gets processed by the removal of a spacer peptide connecting the chains of the mature active heterodimer. We have cloned the penicillin G acylase from Kluyvera citrophila (KcPGA) and prepared two mutants by site-directed mutagenesis. Replacement of N-terminal serine of the ß-subunit with cysteine (Serß1Cys) resulted in a fully processed but inactive enzyme. The second mutant in which this serine is replaced by glycine (Serß1Gly) remained in the unprocessed and inactive form. The crystals of both mutants belonged to space group P1 with four molecules in the asymmetric unit. The three-dimensional structures of these mutants were refined at resolutions 2.8 and 2.5 Å, respectively. Comparison of these structures with similar structures of Escherichia coli PGA (EcPGA) revealed various conformational changes that lead to autocatalytic processing and consequent removal of the spacer peptide. The large displacements of residues such as Arg168 and Arg477 toward the N-terminal cleavage site of the spacer peptide or the conformational changes of Arg145 and Phe146 near the active site in these structures suggested probable steps in the processing dynamics. A comparison between the structures of the processed Serß1Cys mutant and that of the processed form of EcPGA showed conformational differences in residues Argα145, Pheα146, and Pheß24 at the substrate binding pocket. Three conformational transitions of Argα145 and Pheα146 residues were seen when processed and unprocessed forms of KcPGA were compared with the substrate bound structure of EcPGA. Structure mediation in activity difference between KcPGA and EcPGA toward acyl homoserine lactone (AHL) is elucidated.

Borohydride Ionic Liquids as Hypergolic Fuels: A Quest for Improved Stability.

Hydrazine and its derivatives are used as fuels in rocket propellant systems; however, due to high vapor pressure, toxicity, and carcinogenicity, handling of such compounds is extremely hazardous. Hypergolic ionic liquids have shown great promise to become viable replacements for hydrazines as fuels. Borohydride-containing ionic liquids have now been synthesized using a more efficient synthetic pathway that does not require liquid ammonia and halide precursors. Among the eight new compounds, 1-allyl-3-n-butyl-imidazolium borohydride (1) and 1, 3-diallylimidazolium borohydride (5) exhibit very short ignition-delay times (ID) of 8 and 3 ms, respectively. The hydrolytic stability of borohydride compounds has been greatly improved by attaching long-chain alkyl substituents to the imidazole ring. 1,3-Di-(n-octyl)-imidazolium borohydride (3) is a water stable borohydride-containing ionic liquid. 1,3-Di-(n-butyl)-imidazolium borohydride (2) is a unique example of a borohydride liquid crystal. These ionic liquids have some unusual advantages, including negligible vapor pressures, good ignition delay (ID) times, and reduced synthetic and storage costs, thereby showing good application potential as environmentally friendly fuels in bipropellant formulations. In addition, they also have potential applications in the form of reducing agents and hydrogen storage materials.

Versatile polyiodopyrazoles: synthesis and biocidal promise.

An efficient route to polyiodopyrazoles, 3,4,5-triiodopyrazole (1), 1-methyl-3,4,5-triiodopyrazole (2) and 1-diiodomethyl-3,4,5-triiodopyrazole (3), opens the door to prospective biocides. Nitration of 1 and 2 gives the previously inaccessible compounds, 3,4-dinitro-5-iodopyrazole (4), and 3,4-dinitro-5-iodo-1-methylpyrazole (5), respectively. These synthetic pathways will open many fronts for pyrazole chemistry.

Boronium-cation-based ionic liquids as hypergolic fluids.

Two series of boronium-cation-based ionic liquids were prepared and fully characterized by (1)H, (13)C, and (11)B NMR and infrared spectroscopy, differential scanning calorimetry (DSC), and elemental analysis. The structure of bis(1-methyl-1H-imidazole-3-yl)dihydroboronium dicyanoborohydride (5 a) was determined by single-crystal X-ray diffraction. The densities of these ionic liquids range from 1.05 to 1.28 g cm(-3), and the heats of formation, predicted on the basis of Gaussian 03 calculations, fall between -164.6 and 430.5 kJ mol(-1). Compound 5 b, bis(1-allyl-1H-imidazole-3-yl)dihydroboronium dicyanoborohydride, exhibits the lowest viscosity (35 mPa s) and shortest ignition-delay time (14 ms) in combination with 100 % HNO(3).