Revealing Single-Bond Anomeric Selectivity in Carbohydrate–Protein Interactions

Abstract

Non-covalent binding of proteins to glycans is amazingly selective to the isoforms of carbohydrates, including α/β anomers that co-exist in solution. We isolate in the gas phase and study at the atomic level the simplest model system: non-covalent complexes of monosaccharide α/β-GalNAc and protonated aromatic molecule tyramine. IR/UV cold ion spectroscopy and quantum chemistry calculations jointly solve the structures of the two complexes. Although the onsets of the measured UV absorptions of the complexes differ significantly, the networks of Hbonds in both complexes appear identical and do not include the anomeric hydroxyl. The detailed analysis reveals that, through inductive polarization, the α-to β-re-orientation of this group nevertheless reduces the length of one remote short intermolecular H-bond by 0.03 Å.Although small, this change substantially strengthens the bond, thus contributing to the anomeric selectivity of the binding.Protein-carbohydrate non-covalent interactions are ubiquitous in nature. The flexibility and the diversity of these weak interactions make proteins sensitive to finer structural details of the binding partners, greatly increasing their biological functionality. Despite the tremendous isomeric diversity of carbohydrates, the glycan binding proteins (GBPs), for instance, lectins, are capable of finding specific membrane carbohydrates to adhere viruses and bacteria to appropriate cells for invasion. 1 Hydrogen bonds between the hydroxyl groups of carbohydrates and the polar groups of amino acid residues play a crucial role in binding to lectins and other GBPs. Difference in the relative orientation of the hydroxyl groups in glycans has a pivotal impact on the ability of GBPs to distinguish different isoforms of carbohydrates, in particular their epimers (e.g., monosaccharaides Gal, Glu, Man, etc.) 2-3 Regardless of its epimeric isoform, a glycan in solution exists as two interconverting isomers: α-and β-anomers, which differ only by the orientation of the first OH group (1-OH) in the reducing-end monosaccharide. In contrast to epimers, which are usually recognized through binding to the 3-OH and 4-OH groups, the anomeric 1-OH often is not involved in the binding to GBPs. 2,4 Nevertheless, similar to epimers, the two anomers often exhibit very different affinities to, for instance, lectins and to some artificial receptors, which allows them a selective recognition of anomeric carbohydrates. [3][4] Despite many excellent studies in this field, the detailed mechanism of the anomeric recognition remains obscure, in particular, due to the large size of the interacting partners and the presence of solvent molecules. [5][6][7][8] Here we use cold-ion UV/IR spectroscopy and structural quantum chemistry computations to investigate at the atomic level the simplest anomeric system that models a local protein-glycan interaction: monosaccharaide anomers, α/β-GalNAc, non-covalently bound to protonated tyramine (TrmH +

Identification and Quantification of Any Isoforms of Carbohydrates by 2D UV-MS Fingerprinting of Cold Ions

“…A straightforward application of 2D UV-MS fingerprinting to glycans is challenging because they do not absorb in UV. Instead, we make use of noncovalent complexes of glycans with protonated aromatic molecules, such as phenylalanine, tyrosine, and its decarboxylated analogue tyramine. ,− These complexes can be readily prepared by dissolving a glycan and an aromatic molecule in a standard for MS methanol/water solution. The complexes appear to be strong enough to survive electrospray ionization that brings them to the gas phase.…”

“…Methylation of the anomeric 1-OH group inhibits this conversion. Noticeable differences between IR spectra of α/β anomers were earlier revealed for neutral and protonated complexes of aromatics with MeGalNAc and with GalNAc. , Figure f,g illustrates how much UVPD spectra of complexes of TrmH + with methylated glycans α-MeGalNAc and β-MeGalNAc not only differ in the onsets of UV absorption, but also in many details of highly structured absorption bands. These differences ensure an unambiguous spectral identification of the two anomers.…”

“…Instead, the isomeric information is communicated by the guest to the host through an inductive polarization of the H-bond, formed between the hydrogen of 6-OH in the glycan and the oxygen atom of the tyramine side chain. 27 A stronger polarization of the bond results in a larger blue shift of UV absorption by the aromatic, whose ring is conjugated with the hydroxyl.…”

“…Instead, we make use of non-covalent complexes of glycans with protonated aromatic molecules, such as phenylalanine, tyrosine and its decarboxylated analog tyramine. 22,[25][26][27] These complexes can be readily prepared by dissolving a glycan and an aromatic molecule in a standard for MS methanol/water solution.…”

In Silico Prediction of Inhibitory Potential of a Punicalagin β-anomer Against SARS-COV-2 Main Protease (Mpro)

“…1). Recent studies including α/ β anomers [21][22][23] revels that there are differences in the non-covalent interactions pro le between these anomers and proteins.…”