Studies on the conformational behaviour of GlcNAc-Man3-GlcNAc2 oligosaccharides using molecular dynamics simulations

Tools to assist determination and validation of carbohydrate 3D structure data

The frequency of glycosylated protein 3D structures in the Protein Data Bank (PDB) is significantly lower than the proportion of glycoproteins in nature, and if glycan 3D structures are present, then they often exhibit a large degree of errors. There are various reasons for this, one of which is a comparably low support of carbohydrates in software tools for 3D structure determination and validation. This chapter illustrates the current features that assist crystallographers with handling glycans during 3D structure determination in Coot and CNS and with validation of the results.

Catalytic Mechanism of Glycosyltransferases: Hybrid Quantum Mechanical/Molecular Mechanical Study of the Inverting N-Acetylglucosaminyltransferase I

The Golgi glycosyltransferase, N-acetylglucosaminyltransferase I (GnT-I), catalyzes the transfer of a GlcNAc residue from the donor UDP-GlcNAc to the C2-hydroxyl group of a mannose residue in the trimannosyl core of the Man5GlcNAc2-Asn-X oligosaccharide. The catalytic mechanism of GnT-I was investigated using a hybrid quantum mechanical/molecular mechanical (QM/MM) method with a QM part containing 88 atoms treated with density functional theory (DFT) at the BP/TZP level. The remaining parts of a GnT-I complex, altogether 5633 atoms, were modeled using the AMBER molecular force field. A theoretical model of a Michaelis complex was built using the X-ray structure of GnT-I in complex with the donor having geometrical features consistent with kinetic studies. The QM(DFT)/MM model identified a concerted SN2-type of transition state with D291 as the catalytic base for the reaction in the enzyme active site. The TS model features nearly simultaneous nucleophilic addition and dissociation steps accompanied by the transfer of the nucleophile proton Hb2 to the catalytic base D291. The structure of the TS model is characterized by the Ob2-C1 and C1-O1 bond distances of 1.912 and 2.542 A, respectively. The activation energy for the proposed reaction mechanism was estimated to be approximately 19 kcal mol-1. The calculated alpha-deuterium kinetic isotope effect of 1.060 is consistent with the proposed reaction mechanism. Theoretical results also identified interactions between the Hb6 and beta-phosphate oxygen of the UDP and a low-barrier hydrogen bond between the nucleophile and the catalytic base D291. It is proposed that these interactions contribute to a stabilization of TS. This modeling study provided detailed insight into the mechanism of the GlcNAc transfer catalyzed by GnT-I, which is the first step in the conversion of high mannose oligosaccharides to complex and hybrid N-glycan structures.

Glycosylations versus conformational preferences of cancer associated mucin core

Synthetic oligosaccharide vaccines based on core STn (sialyl alpha2-6 GalNAc) carbohydrate epitopes are being evaluated by a number of biopharmaceutical firms as potential immunotherapeutics in the treatment of mucin-expressing adenocarcinomas. The STn carbohydrate epitopes exist as discontinuous clusters, O-linked to proximal serine and threonine residues within the mucin sequence. In an effort to probe the structure and dynamics of STn carbohydrate clusters as they may exist on the cancer-associated mucin, we have used NMR spectroscopy and MD simulations to study the effect of O-glycosylation of adjacent serine residues in a repeating (Ser)n sequence. Three model peptides/glyco-peptides were studied: a serine trimer containing no carbohydrate groups ((Ser)3 trimer); a serine trimer containing three Tn (GalNAc) carbohydrates alpha-linked to the hydroxyls of adjacent serine sidechains ((Ser.Tn)3 trimer); and a serine trimer containing three STn carbohydrates alpha-linked to the hydroxyls of adjacent serine sidechains ((Ser.STn)3 trimer). Our results demonstrate that clustering of carbohydrates shifts the conformational equilibrium of the underlying peptide backbone into a more extended and rigid state, an arrangement that could function to optimally present the clustered carbohydrate antigen to the immune system. Steric effects appear to drive these changes since an increase in the size of the attached carbohydrate (STn versus Tn) is accompanied by a stronger shift in the equilibrium toward the extended state. In addition, NMR evidence points to the formation of hydrogen bonds between the peptide backbone NH protons and the proximal GalNAc groups in the (Ser.Tn)3 and (Ser.STn)3 trimers. The putative peptide-sugar hydrogen bonds may also play a role in influencing the conformation of the underlying peptide backbone, as well as the orientation of the O-linked carbohydrate. The significance of these results will be discussed within the framework of developing clustered STn-based vaccines, capable of targeting the clustered STn epitopes on the cancer-associated mucin.