Atomic visualization of a flipped-back conformation of bisected glycans bound to specific lectins

Abstract

Glycans normally exist as a dynamic equilibrium of several conformations. A fundamental question concerns how such molecules bind lectins despite disadvantageous entropic loss upon binding. Bisected glycan, a glycan possessing bisecting N-acetylglucosamine (GlcNAc), is potentially a good model for investigating conformational dynamics and glycan-lectin interactions, owing to the unique ability of this sugar residue to alter conformer populations and thus modulate the biological activities. Here we analyzed bisected glycan in complex with two unrelated lectins, Calsepa and PHA-E. The crystal structures of the two complexes show a conspicuous flipped back glycan structure (designated ‘back-fold’ conformation), and solution NMR analysis also provides evidence of ‘back-fold’ glycan structure. Indeed, statistical conformational analysis of available bisected and non-bisected glycan structures suggests that bisecting GlcNAc restricts the conformations of branched structures. Restriction of glycan flexibility by certain sugar residues may be more common than previously thought and impinges on the mechanism of glycoform-dependent biological functions.

Biochemical and structural characterization of a mannose binding jacalin-related lectin with two-sugar binding sites from pineapple (Ananas comosus) stem

“…The same is also true for artocarpin, jacalin and the mJRLs from Morus nigra and Maclura prolifera 27 , 33 , 64 , 65 , who all share the same side-by-side dimerization as AcmJRL, but have a common tail-to-tail interaction different than AcmJRL and BanLec. The mJRL from Helianthus tuberosus (heltuba) 23 forms donuts shaped octamers made of four basic dimers similar to AcmJRL, and the mJRL from Calystegia sepium (calsepa) 66 only forms dimers different than the AcmJRL ones (Fig. 5 ).…”

The glycomic effect of N-acetylglucosaminyltransferase III overexpression in metastatic melanoma cells. GnT-III modifies highly branched N-glycans

“…A single sugar substitution of the N-glycan core is especially studied in this context, giving evidence that “bisecting” GlcNAc and core fucose can be treated as molecular switches of conformational behaviour of glycan structures [ 53 ]. Recently, it has been shown, that “bisecting” GlcNAc can restrict the conformers of branched structures by the formation of the ‘back-fold’ conformation [ 54 ]. As a consequence, the glycan binding to specific lectin can be changed.…”

3D Structure and Function of Glycosyltransferases Involved in N-glycan Maturation

“…This is probably because the presence of bisecting GlcNAc alters glycan conformation and restricts preferable conformers of N-glycan. Molecular dynamic simulation, NMR analysis, and X-ray crystallography suggest that bisected N-glycans tend to prefer back-fold conformations in which the α1-6 branch flips back to the reducing end [56][57][58][59][60]. Our MS glycan analysis revealed increases in various terminal epitopes of N-glycans in Mgat3-knockout brain, which is concomitant with complete loss of bisecting GlcNAc, suggesting that one of the physiological functions of bisecting GlcNAc is to suppress formation of mature and complex N-glycans [56].…”

“…When a bisecting GlcNAc residue is added on β-mannose, this residue lies close to OH6 of the α1-6 branched mannose (Man-2) and sterically clashes with Man-2. In addition, bisected glycan prefers to take extend-b and back-fold conformations, rather than the extend-a conformation that is observed in the complex structure [56][57][58][59][60]. Structural superposition of these two conformations shows that they cannot fit into the surface groove of GnT-V without severe steric clashes.…”