No abstract

The rotational preferences of the hydroxymethyl group in pyranosides is known to depend on the local environment, whether in solid, solution, or gas phase. By combining molecular dynamics (MD) simulations with NMR spectroscopy the rotational preferences for the ω angle in methyl 2,3-di--methyl-α-D-glucopyranoside () and methyl 2,3-di--methyl-α-D-galactopyranoside () in a variety of solvents, with polarities ranging from 80 to 2.3 D have been determined. The effects of solvent polarity on intramolecular hydrogen bonding have been identified and quantified. In water, the internal hydrogen bonding networks are disrupted by competition with hydrogen bonds to the solvent. When the internal hydrogen bonds are differentially disrupted, the rotamer populations associated with the ω angle may be altered. In the case of in water, the preferential disruption of the interaction between HO6 and O4 destabilizes the rotamer, leading to the observed preference for gauche rotamers. Without the hydrogen bond enhancement offered by a low polarity environment, both and display rotamer populations that are consistent with expectations based on the minimization of repulsive intramolecular oxygen-oxygen interactions. In a low polarity environment, HO6 prefers to interact with O4, however, in water these interactions are markedly weakened, indicating that HO6 acts as a hydrogen bond donor to water.

Although complex modular proteins are encountered frequently in a variety of biological systems, their occurrence in biocatalysis has not been widely appreciated. Here, we describe that bacterial sialidases, which have both a catalytic and carbohydrate-binding domain, can hydrolyze polyvalent substrates with much greater catalytic efficiency than their monovalent counterparts. The enhancement of catalytic efficiency was due to a much smaller Michaelis constant and rationalized by a model in which the catalytic and lectin domains interact simultaneously with the polyvalent substrate, leading to an enhancement of affinity. Inhibition studies have shown that galactosides released by the action of the sialidase can act as the ligand for the lectin domain. This knowledge has been exploited in the design of a potent polyvalent inhibitor of the sialidase of Vibrio cholerae, which displayed exquisite selectivities for sialidases that have a lectin domain.

Negative ion tandem mass spectrometry (MS/MS) spectra of three isomeric triantennary N-linked glycans provided clear differentiation between the isomers and confirmed the occurrence of an isomer that was substituted with galactose on a bisecting GlcNAc (1 --> 4-substituted on the core mannose) residue recently reported by Takegawa et al. from N-glycans released from human immunoglobulin G (IgG). We extend this analysis of human serum IgG to reveal an analogue of the fucosylated triantennary glycan reported by Takegawa et al. together with a third compound that lacked both the sialic acid and the fucose residues. In addition, we demonstrate the biosynthesis of bisected hybrid-type glycans with the galactose modification, with and without core fucose, on the stem cell marker glycoprotein, 19A, expressed in a partially ricin-resistant human embryonic kidney cell line. It would appear, therefore, that this modification of N-linked glycans containing a galactosylated bisecting GlcNAc residue may be more common than originally thought. Negative ion MS/MS analysis of glycans is likely to prove an invaluable tool in the analysis and monitoring of therapeutic glycoproteins.

As characterization of glycosylation is required for the licensing of recombinant glycoprotein therapeutics, technique comparability must be assessed. Eleven UK laboratories (seven industrial, two regulatory or government, two academic) participated in an inter-laboratory study to analyze N-glycans present in four mixtures prepared by PNGase F cleavage of commercial glycoproteins: human alpha1-acid glycoprotein (H alpha1), bovine alpha1-acid glycoprotein (B alpha1), bovine pancreatic ribonuclease B (RNaseB), and human serum immunoglobulin G (hIgG). Participants applied their routine glycan mapping methodology using predominantly chromatography and mass spectrometry to identify and quantify components. Data interpretation focused on the relative amounts of different glycan structures present, the degree of sialylation, antennary and the galactosylation profiles, fucosylation and bisecting GlcNAc content, and the number of glycan components identified. All laboratories found high levels of sialylation for H alpha1 and B alpha1 (Z-numbers 271 +/- 24 and 224 +/- 18, respectively), but varying ratios of di-, tri-, and tetra-antennary chains. The Z-score for hIgG glycans had high variability as values obtained from mass spectrometric and chromatographic methods clustered separately. The proportion of the major penta-mannosyl chain from RNaseB was between 29 and 62%. Proportions of fucosylated and bisected GlcNAc chains from hIgG were between 58 and 96% and 9 and 23%, respectively. Mass spectrometric approaches consistently identified more glycan species, especially when both N-glycolylneuraminic acid (Neu5Gc) and N-acetylneuraminic acid (Neu5Ac) were present. These data highlight the need for well-characterized reference standards to support method validation and regulatory guidance on selection of approaches. Pharmacopoeial specifications must acknowledge method variability.

No abstract

No abstract

No abstract