Computational mechanistic study of thionation of carbonyl compounds with Lawesson's reagent

The thionation reaction of carbonyl compounds with Lawesson's reagent (LR) has been studied using density functional theory methods and topological analyses. After dissociation of LR, the reaction takes place through a two-step mechanism involving (i) a concerted cycloaddition between one monomer and the carbonyl compound to form a four-membered intermediate and (ii) a cycloreversion leading to the thiocarbonyl derivative and phenyl(thioxo)phosphine oxide. Topological analyses confirmed the concertedness and asynchronicity of the process. The second step is the rate-limiting one, and the whole process resembles the currently accepted mechanism for the lithium salt-free Wittig reaction. No zwitterionic intermediates are formed during the reaction, although stabilizing electrostatic interactions are present in initial stages. Phenyl(thioxo)phosphine oxide formed in the thionation reaction is capable of performing a second thionation, although with energy barriers higher than the first one. The driving force of the thionation reactions is the formation of trimers from the resulting monomers. In agreement with experimental observations, the amides are the most reactive when compared with esters, aldehydes, and ketones and the reaction is slightly influenced by the polarity of the solvent. Whereas for amides and esters substituents have little effect, aldehydes and ketones are influenced by both steric and electronic effects.This work was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) (Project CTQ2013-44367-C2-1-P), by the Fondos Europeos para el Desarrollo Regional (FEDER), and by the Gobierno de Aragon (Zaragoza, Spain, Bioorganic Chemistry Group, E-10). M.A.C. thanks the University of Catania for partial financial support.Peer Reviewe

The small molecule luteolin inhibits N-acetyl-α-galactosaminyltransferases and reduces mucin-type O-glycosylation of amyloid precursor protein

Mucin-type O-glycosylation is the most abundant type of O-glycosylation. It is initiated by the members of the polypeptide N-acetyl-α-galactosaminyltransferase (ppGalNAc-T) family and closely associated with both physiological and pathological conditions, such as coronary artery disease or Alzheimer's disease. The lack of direct and selective inhibitors of ppGalNAc-Ts has largely impeded research progress in understanding the molecular events in mucin-type O-glycosylation. Here, we report that a small molecule, the plant flavonoid luteolin, selectively inhibits ppGalNAc-Ts in vitro and in cells. We found that luteolin inhibits ppGalNAc-T2 in a peptide/protein-competitive manner but not promiscuously (e.g. via aggregation-based activity). X-ray structural analysis revealed that luteolin binds to the PXP motif-binding site found in most protein substrates, which was further validated by comparing the interactions of luteolin with wild-type enzyme and with mutants using 1H NMR-based binding experiments. Functional studies disclosed that luteolin at least partially reduced production of β-amyloid protein by selectively inhibiting the activity of ppGalNAc-T isoforms. In conclusion, our study provides key structural and functional details on luteolin inhibiting ppGalNAc-T activity, opening up the way for further optimization of more potent and specific ppGalNAc-T inhibitors. Moreover, our findings may inform future investigations into site-specific O-GalNAc glycosylation and into the molecular mechanism of luteolin-mediated ppGalNAc-T inhibition.This work was supported by the National Basic Research Program of China Grants 2012CB822103 and 2011CB910603 (to Y. Z.); National High Technology Research and Development Program of China Grant 2012AA020203 (to Y. Z.); National Natural Science Foundation Grants 31170771 (to Y. Z.), 31370806 (to Y. Z.), and 31570796 (to Y. Z.); National Basic Research Program of China Grant 2012CB822103 (to F. W.); National Natural Science Foundation Grants 31270853 and 81102377 (to F. W.); Agencia Aragonesa para la Investigación y Desarrollo (ARAID), Ministerio de Economía y Competitividad, Grants CTQ2013-44367-C2-2-P and BFU2016-75633-P (to R. H.-G.); Diputación General de Aragón (DGA) Grant B89 (to R. H.-G.); and the EU Seventh Framework Programme (2007–2013) under BioStruct-X (Grant Agreement 283570 and BIOSTRUCTX 5186) (to R. H.-G.).Peer Reviewe

Self-acetylation at the active site of phosphoenolpyruvate carboxykinase (PCK1) controls enzyme activity

Acetylation is known to regulate the activity of cytosolic phosphoenolpyruvate carboxykinase (PCK1), a key enzyme in gluconeogenesis, by promoting the reverse reaction of the enzyme (converting phosphoenolpyruvate to oxaloacetate). It is also known that the histone acetyltransferase p300 can induce PCK1 acetylation in cells, but whether that is a direct or indirect function was not known. Here we initially set out to determine whether p300 can acetylate directly PCK1 in vitro. We report that p300 weakly acetylates PCK1, but surprisingly, using several techniques including protein crystallization, mass spectrometry, isothermal titration calorimetry, saturation-transfer difference nuclear magnetic resonance and molecular docking, we found that PCK1 is also able to acetylate itself using acetyl-CoA independently of p300. This reaction yielded an acetylated recombinant PCK1 with a 3-fold decrease in kcat without changes in Km for all substrates. Acetylation stoichiometry was determined for 14 residues, including residues lining the active site. Structural and kinetic analyses determined that site-directed acetylation of K244, located inside the active site, altered this site and rendered the enzyme inactive. In addition, we found that acetyl-CoA binding to the active site is specific and metal dependent. Our findings provide direct evidence for acetyl-CoA binding and chemical reaction with the active site of PCK1 and suggest a newly discovered regulatory mechanism of PCK1 during metabolic stress

Structural basis of trehalose recognition by the mycobacterial LpqY-SugABC transporter

The Mycobacterium tuberculosis (Mtb) LpqY-SugABC ATP-binding cassette transporter is a recycling system that imports trehalose released during remodelling of the Mtb cell-envelope. As this process is essential for the virulence of the Mtb pathogen it may represent an important target for tuberculosis drug and diagnostic development, but the transporter specificity and molecular determinants of substrate recognition are unknown. To address this, we have determined the structural and biochemical basis of how mycobacteria transport trehalose using a combination of crystallography, STD NMR, molecular dynamics, site-directed mutagenesis, biochemical/biophysical assays and the synthesis of trehalose analogues. This analysis pinpoints key residues of the LpqY substrate binding lipoprotein that dictate substrate-specific recognition and has revealed which disaccharide modifications are tolerated. These findings provide critical insights into how the essential Mtb LpqY-SugABC transporter reuses trehalose and modified analogues, and specifies a framework that can be exploited for the design of new anti-tubercular agents and/or diagnostic tools

Recent advances on asymmetric nitroso aldol reaction

The reaction of aromatic nitroso derivatives with enolizable carbonyl compounds (nitroso aldol reaction) to give either α-hydroxyamino or α-aminoxy carbonyl compounds is an important synthetic method. This review illustrates the recent advances in rendering the process regio- and enantioselective as well as catalytic. By employing metal and organic catalysts a range of α-amino (α-oxyamination) and α-hydroxy (α-aminoxylation) carbonyl derivatives can be generated with total regioselectivity and high levels of enantiomeric excess.We thank for their support of our programs: MINECO (Madrid, Spain) and FEDER Program (Project CTQ2013-44367-C2-1-P) and the Government of Aragon, Spain (Bioorganic Chemistry Group E-10).Peer Reviewe

Biosynthetic Pathways to Glycosidase Inhibitors

Glycosidase inhibitors are important compounds that can interfere with several biosynthetic processes including N-linked glycosylation and the biosynthesis of several glycoproteins. Understanding the biogenesis of naturally occurring glycosidase inhibitors would be a crucial step towards the chemical synthesis of analogues of choice. This review focuses on the current knowledge regarding the biosynthesis of a series of polyhydroxylated saturated nitrogen heterocycles including nojirimycin and swainsonine among others, with a potent biological activity as inhibitors of glycosidases and transglycosidases.This study was supported by the Ministerio de Ciencia e Innovacion (MICINN) and FEDER Program (Madrid, Spain, project CTQ2010-19606) and the Gobierno de Aragon (Zaragoza, Spain, Bioorganic Chemistry Group. E-10). F.G.- B. thanks CSIC for a JAE predoctoral grant.Peer reviewe

Recent advances on the enantioselective synthesis of c-nucleosides inhibitors of inosine monophosphate dehydrogenase (IMPDH)

This review will describe the recent advances in the synthesis of C-nucleosides with inhibitory activity of inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the biosynthesis of guanine nucleotides. The review will cover synthetic approaches of structural analogues showing modifications in the furanose ring as well as in the heterocyclic base. Heterocyclic sugar nucleoside analogues in which the furanose ring has been replaced by a different heterocyclic ring including aza analogues, thioanalogues as well as dioxolanyl and isoxazolidinyl analogues are also considered. © 2014 Bentham Science Publishers.This study was supported by the Ministerio de Ciencia e lnnovacion (MICINN) and FEDER Program (Madrid, Spain, project CTQ2010-19606) and the Gobierno de Aragon (Zaragoza, Spain, Bioorganic Chemistry Group. E-10). M.G. thanks MEC for a FPU predoctoral grant.Peer Reviewe

Stereoselective synthesis of pyrrolidinyl glycines from nitrones: complementarity of nucleophilic addition and 1,3-dipolar cycloaddition

4 pages, 4 schemes, 2 figures.-- et al.Epimeric pyrrolidinyl glycines, a sort of conformationally constrained α,β-diaminoacids, were stereoselectively prepared using complementary approaches based on nitrone chemistry. Nucleophilic additions to pyrrolidinyl nitrones and 1,3-dipolar cycloadditions of l-serine derived nitrones to form the corresponding hydroxylamines and isoxazolidines, respectively, provided key intermediates for the synthesis of the target compounds. Whereas the nucleophilic addition route afforded the syn adduct, the 1,3-dipolar cycloaddition approach furnished the precursor for the preparation of the corresponding anti compound.We thank the Ministerio de Educacion y Ciencia (MEC, Project CTQ2004-0421), the European Union (Project TRIoH, LSHB-CT-2003-503480), and the Regional Government of Aragon (DGA) for financial support. I.D. thanks DGA for a pre-doctoral grant.Peer reviewe

Synthesis of amino-acid–nucleoside conjugates

Representative derivatives of uridine-conjugated amino acids that have been suitably protected for Fmoc solid-phase chemistry have been prepared through efficient procedures that use “click” reactions as key steps, including thiol–ene radical reactions and copper-catalyzed azide alkyne cycloaddition (CuAAC) reactions. Several linkers between the amino acid and nucleoside units, including alkyl chains and a triazole ring, have been successfully employed. Alkyl chains offer retention of flexibility, to allow the bisubstrate analogues to adopt an appropriate orientation, whilst the triazole ring can promote additional interactions at the active sites of target enzymes. Furthermore, a neutral surrogate of the pyrophosphate unit has been prepared by using a Staudinger–Vilarrasa reaction as a key step. Neutral analogues are promising surrogates for avoiding difficulties owing to cell-membrane permeability.We thank MINECO (Madrid, Spain), the FEDER Program (CTQ2016-76155-R), and the Government of Aragon (Group E-10) for their support of our programs. M.G. thanks MECD for a FPU Predoctoral Grant.Peer Reviewe

One-pot organocatalytic asymmetric synthesis of nitrogenated tricyclic compounds

Trabajo presentado al 15th JCF Frühjahrssymposium (Congreso de Jóvenes Investigadores de la Sociedad Alemana de Química) celebrado en Berlín (Alemania) del 6 al 9 de marzo de 2013.Five-membered cyclic nitrones have been demonstrated to be interesting heterocycles and attractive building blocks for the synthesis of a variety of nitrogenated compounds containing a pyrrolidine ring in their structure, many of them of biological interest. Most of the reported syntheses of enantiomerically pure five-membered cyclic nitrones employ natural sugars as starting materials. A different approach was carried out in order to prepare other cyclic nitrones with different substituents, using common reactants such as nitrostyrene. In this work, a new three-step one-pot methodology was developed. The first step is a Michael addition reaction of an aldehyde to a nitroalkene. This is an organocatalytic reaction mediated by a prolinol chiral derivative. This reaction generates two chiral carbons in a γ-nitroaldehyde compound. In the following step, the reduction of the nitro group to hydroxylamine group was done and, in situ, the condensation of the latter one with the aldehyde to yield the nitrone took place. The treatment of the reaction mixture provided alkenyl nitrones (3d) gave spontaneously an intramolecular 1,3-dipolar cycloaddition leading to tricyclic derivatives.D. S. thanks the Spanish Council for Scientific Research (CSIC) for his JAE-predoctoral grant.Peer Reviewe