Principal Investigator Laura Kiessling
CH-p interactions -- Protein-carbohydrate interactions play pivotal roles in health and disease. However, defining and manipulating these interactions has been hindered by an incomplete understanding of the underlying fundamental forces. We quantitatively analyzed X-ray crystal structures of proteins with noncovalently bound carbohydrates to find aliphatic hydrophobic residues are disfavored, whereas aromatic side chains are enriched. The greatest preference is for tryptophan with an increased prevalence of 9-fold. Variations in the spatial orientation of amino acids around different monosaccharides indicate specific carbohydrate C-H bonds interact preferentially with aromatic residues. These preferences are consistent with the electronic properties of both the carbohydrate C-H bonds and the aromatic residues. Those carbohydrates that present patches of electropositive saccharide C-H bonds engage more often in CH-π interactions involving electron-rich aromatic partners. These electronic effects are also manifested when carbohydrate-aromatic interactions are monitored in solution: NMR analysis indicates that indole favorably binds to electron-poor C-H bonds of model carbohydrates, and a clear linear free energy relationships with substituted indoles supports the importance of complementary electronic effects in driving protein-carbohydrate interactions. Together, our data indicate that electrostatic and electronic complementarity between carbohydrates and aromatic residues play key roles in driving protein-carbohydrate complexation. Moreover, these weak noncovalent interactions influence which saccharide residues bind to proteins, and how they are positioned within carbohydrate-binding sites.
Multivalency -- Cell surface carbohydrates are uniquely poised to engage with proteins on the surfaces of other cells or pathogens. Indeed, protein–carbohydrate interactions have been implicated in physiological processes ranging from fertilization to development to immune system function. However, their affinities are typically 1,000- to 1,000,000-fold poorer than those of protein–protein interactions. To compensate for their low affinity, most protein– carbohydrate interactions are multivalent, whereby multiple binding groups (e.g.,carbohydrates) on one cell bind to multiple copies of a receptor (e.g., a protein) on another cell. Due to weak individual interactions, it is challenging to determine if a protein- carbohydrate interaction is relevant, to assess the molecular mechanisms that contribute to formation and stabilization of protein-carbohydrate complexes, and to design potent inhibitors of protein-carbohydrate interactions. To overcome this discrepancy, we have harnessed polymer chemistry to assemble defined multivalent carbohydrate derivatives, to optimize multivalent ligand activity, and to probe the mechanisms underlying multivalency.