A detailed investigation has been undertaken on the extent and nature of torsional fluctuations about the glycosidic linkage of the model disaccharide Man alpha 1-3Man alpha 1-OMe. In particular, we sought to determine whether the three nuclear Overhauser effects and the two long-range heteronuclear 3JCH spin coupling constants measurable across the glycosidic linkage were consistent with a single conformation or multiple conformations about that linkage. Within experimental error, we have found that these five parameters can be interpreted in terms of a single, rigid geometry. Alternatively, the data are also consistent with a model in which the glycosidic torsional angles exhibit significant but restricted fluctuations about the global minimum energy conformation. Evidence from restrained molecular dynamics simulations both in vacuo and with explicit inclusion of solvent water and from 13C relaxation measurements upon an oligomannose glycan in covalent association with protein suggests that the latter model is the most accurate representation of the conformational behavior of oligosaccharides in solution.
De-N-acetylation of N-acetylglucosaminyl-phosphatidylino-sitol (GlcNAc-PI) is the second step of glycosylphosphatidylino-sitol (GPI) membrane anchor biosynthesis in eukaryotes. This step is a prerequisite for the subsequent processing of glucosaminyl-phosphatidylinositol (GlcN-PI) that leads to mature GPI membrane anchor precursors, which are transferred to certain proteins in the endoplasmic reticulum. In this article, we used a direct de-N-acetylase assay, based on the release of [14C]acetate from synthetic GlcN[14C]Ac-PI and analogues thereof, and an indirect assay, based on the mannosylation of GlcNAc-PI analogues, to study the substrate specificities of the GlcNAc-PI de-N-acetylase activities of African trypanosomes and human (HeLa) cells. The HeLa enzyme was found to be more fastidious than the trypanosomal enzyme such that, unlike the trypanosomal enzyme, it was unable to act on a GlcNAc-PI analogue containing 2-O-octyl-d- myo -inositol or on the GlcNAc-PI diastereoisomer containing l- myo -inositol (GlcNAc-P(l)I). These results suggest thatselective inhibition of the trypanosomal de-N-acetylase may be possible and that this enzyme should be considered as a possible therapeutic target. The lack of strict stereospecificity of the trypanosomal de-N-acetylase for the d- myo -inositol component was also seen for the trypanosomal GPI alpha-manno-syltransferases when GlcNAc-P(l)I was added to the trypanosome cell-free system, but not when GlcN-P(l)I was used. In an attempt to rationalize these data, we modeled the structure and dynamics of d-GlcNAcalpha1-6d- myo -inositol-1-HPO4-( sn )-3-glycerol and its diastereoisomer d-GlcNAcalpha1-6l- myo -inositol-1-HPO4-( sn )-3-glycerol. These studies indicate that the latter compound visits two energy minima, one of which resembles the low-energy conformer of former compound. Thus, it is conceivable that the trypanosomal de-N-acetylase acts on GlcNAc-P(l)I when it occupies a GlcNAc-PI-likeconformation and that GlcN-P(l)I emerging from the de-N-acetylase may be channeled to the alpha-mannosyltransferases in this conformation.
The structures in solution of the Na ϩ , K ϩ and Rb ϩ salts of monensin have been determined by the use of NOESY distance restraints and molecular modelling. The structures are similar to those obtained by X-ray diffraction with the exception of a close carboxylate oxygen-to-metal distance and only five observed close metal-to-oxygen ligating interactions. Molecular dynamics involving the derived structure of the sodium salt at increasing relative permittivity sheds light on the mechanism by which monensin binds to and unbinds from sodium ions in the membrane surface. The series of structures as the size of the metal ion increases also shows that the whole monensin molecule adapts to incorporate changes in the ionic radius including changes in torsion angles, changes in heterocyclic ring conformations and changes in the ligation and hydrogen bonding patterns.
By use of a combination of 1H nuclear Overhauser effect measurements, restrained molecular dynamics simulations, and 13C spin-lattice relaxation time measurements, the solution behavior of the glycan moiety of a complex glycoinositol phospholipid termed GIPL-6, from the protozoan parasite Leishmania major has been determined. The glycan moiety of GIPL-6 has the following structure, which is characterized by the presence of an internal beta-galactofuranose residue: [formula: see text] The glycan does not adopt a single conformation in solution, due to significant torsional variations about the two phosphodiester linkages and certain glycosidic linkages in the molecule. The presence of the internal galactofuranose residue results in an average solution conformation of the oligosaccharide, which resembles a "hairpin," with the galactofuranose residue at the apex.
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