Negative Cooperativity and High Affinity in Chitooligosaccharide Binding by a Mycobacterium smegmatis Protein Containing LysM and Lectin Domains

Mevo lectin specificity towards high-mannose structures with terminal αMan(1,2)αMan residues and its implication to inhibition of the entry of Mycobacterium tuberculosis into macrophages

Mannose-binding lectins can specifically recognize and bind complex glycan structures on pathogens and have potential as anti-viral and anti-bacterial agents. We previously reported the structure of a lectin from an archaeal species, Mevo lectin, which has specificity towards terminal α1,2 linked manno-oligosaccharides. Mycobacterium tuberculosis (M. tuberculosis) expresses mannosylated structures including, lipoarabinomannan (ManLAM) on its surface and exploits C-type lectins to gain entry into the host cells. ManLAM structure has mannose capping with terminal αMan(1,2)αMan residues and is important for recognition by innate immune cells. Here, we aim to address the specificity of Mevo lectin towards high-mannose type glycans with terminal αMan(1,2)αMan residues and its effect on M. tuberculosis internalization by macrophages. ITC studies demonstrated that Mevo lectin shows preferential binding towards manno-oligosaccharides with terminal αMan(1,2)αMan structures, and showed a strong affinity for ManLAM, whereas it binds weakly to Mycobacterium smegmatis (M. smegmatis) lipoarabinomannan (MsmLAM), which displays relatively fewer and shorter mannosyl caps. Crystal structure of Mevo lectin complexed with a Man7D1 revealed the multivalent cross-linking interaction, which explains avidity-based high affinity for these ligands when compared to previously studied manno-oligosaccharides lacking the specific termini. Functional studies suggest that M. tuberculosis internalization by the macrophage was impaired by binding of Mevo lectin to ManLAM present on the surface of M. tuberculosis. Selectivity shown by Mevo lectin towards glycans with terminal αMan(1,2)αMan structures, and its ability to compromise the internalization of M. tuberculosis in vitro, underscore the potential utility of Mevo lectin as a research tool to study host-pathogen interactions.

Structural and related studies on Mevo lectin from Methanococcus voltae A3: the first thorough characterization of an archeal lectin and its interactions

Crystallographic and solution studies of Mevo lectin and its complexes, the first effort of its kind on an archeal lectin, reveal a structure similar to β-prism I fold lectins from plant and animal sources, but with a quaternary association involving a ring structure with seven-fold symmetry. Each subunit in the heptamer carries one sugar binding site on the first Greek key motif. The oligomeric interface is primarily made up of a parallel β-sheet involving a strand of Greek key I of one subunit and Greek key ΙΙΙ from a neighboring subunit. The crystal structures of the complexes of the lectin with mannose, αMan(1,2)αMan, αMan(1,3)αMan, a mannotriose and a mannopentose revealed a primary binding site similar to that found in other mannose specific β-prism I fold lectins. The complex with αMan(1,3)αMan provides an interesting case in which a few subunits have the reducing end at the primary binding site, while the majority have the nonreducing end at the primary binding site. The structures of complexes involving the trisaccharide and the pentasaccharide exhibit cross-linking among heptameric molecules. The observed arrangements may be relevant to the multivalency of the lectin. Phylogenetic analysis of amino acid sequences indicates that Mevo lectin is closer to β-prism I fold animal lectins than with those of plant origin. The results presented here reinforce the conclusion regarding the existence of lectins in all three domains of life. It would also appear that lectins evolved to the present form before the three domains diverged.

Theoretical investigation into the cooperativity effect of 1,4-dimethoxy-D-glucosamine complex with Na+ and H2O

In order to explore the essence of the hydration process of chitin or chitosan in the presence of cation, the cooperativity effects between the H-bonding and Na⁺···molecule interactions in the 1,4-dimethoxy-D-glucosamine (DMGA) complexes with H₂O and Na⁺ were investigated at the B3LYP/6-311++G(d,p), M06-2X/6-311++G(2df,2p), and ωB97X-D/6-311++G(2df,2p) levels. The result shows that the complexes in which Na⁺ or H₂O is bonded simultaneously to the -NH and -OH groups connected to the C3 atom of DMGA are the most stable. The cooperativity and anti-cooperativity effects occur in DMGA···H₂O···DMGA and DMGA···Na⁺···H₂O, while only the cooperativities are confirmed in DMGA···Na⁺···DMGA. The cooperativity occurs in the DMGA···Na⁺···H₂O complexes without the hydration, while the anti-cooperativity occurs in those with the hydration. Furthermore, the cooperativity and anti-cooperativity in DMGA···Na⁺···H₂O are far stronger than those in DMGA···Na⁺···DMGA or DMGA···H₂O···DMGA. Therefore, a deduction is given that the cooperativity and anti-cooperativity effects play an important role in the hydration of chitin or chitosan in the presence of Na⁺. When only Na⁺ is linked with -OH and -NH groups of chitosan or chitin, due to the cooperativity effect, the hydration does not occur. When both Na⁺ and H₂O are linked with -OH and -NH groups, the anti-cooperativities are dominant in controlling of the aggregation process of Na⁺, H₂O, chitosan, and chitin, leading to the possible hydration. Atoms in molecules (AIM) analysis confirms the cooperativity and anti-cooperativity effects. Graphical abstract.

Multitasking Human Lectin Galectin-3 Interacts with Sulfated Glycosaminoglycans and Chondroitin Sulfate Proteoglycans

Glycosaminoglycan (GAG) binding proteins (GAGBPs), including growth factors, cytokines, morphogens, and extracellular matrix proteins, interact with both free GAGs and those covalently linked to proteoglycans. Such interactions modulate a variety of cellular and extracellular events, such as cell growth, metastasis, morphogenesis, neural development, and inflammation. GAGBPs are structurally and evolutionarily unrelated proteins that typically recognize internal sequences of sulfated GAGs. GAGBPs are distinct from the other major group of glycan binding proteins, lectins. The multifunctional human galectin-3 (Gal-3) is a β-galactoside binding lectin that preferentially binds to N-acetyllactosamine moieties on glycoconjugates. Here, we demonstrate through microcalorimetric and spectroscopic data that Gal-3 possesses the characteristics of a GAGBP. Gal-3 interacts with unmodified heparin, chondroitin sulfate-A (CSA), -B (CSB), and -C (CSC) as well as chondroitin sulfate proteoglycans (CSPGs). While heparin, CSA, and CSC bind with micromolar affinity, the affinity of CSPGs is nanomolar. Significantly, CSA, CSC, and a bovine CSPG were engaged in multivalent binding with Gal-3 and formed noncovalent cross-linked complexes with the lectin. Binding of sulfated GAGs was completely abolished when Gal-3 was preincubated with β-lactose. Cross-linking of Gal-3 by CSA, CSC, and the bovine CSPG was reversed by β-lactose. Both observations strongly suggest that GAGs primarily occupy the lactose/LacNAc binding site of Gal-3. Hill plot analysis of calorimetric data reveals that the binding of CSA, CSC, and a bovine CSPG to Gal-3 is associated with progressive negative cooperativity effects. Identification of Gal-3 as a GAGBP should help to reveal new functions of Gal-3 mediated by GAGs and proteoglycans.