Abstract
Protein–carbohydrate interactions play crucial roles in biology. Understanding and modifying these interactions is of major interest for fighting many diseases. We took a synthetic biology approach and incorporated noncanonical amino acids into a bacterial lectin to modulate its interactions with carbohydrates. We focused on tryptophan, which is prevalent in carbohydrate binding sites. The exchange of the tryptophan residues with analogs fluorinated at different positions resulted in three distinctly fluorinated variants of the lectin from Ralstonia solanacearum. We observed differences in stability and affinity toward fucosylated glycans and rationalized them by X-ray and modeling studies. While fluorination decreased the aromaticity of the indole ring and, therefore, the strength of carbohydrate–aromatic interactions, additional weak hydrogen bonds were formed between fluorine and the ligand hydroxyl groups. Our approach opens new possibilities to engineer carbohydrate receptors.
Highlights
G lycans and glycoconjugates are ubiquitously found on all living cells
We took a synthetic biology approach and incorporated noncanonical amino acids into a bacterial lectin to modulate its interactions with carbohydrates
While fluorination decreased the aromaticity of the indole ring and, the strength of carbohydrate−aromatic interactions, additional weak hydrogen bonds were formed between fluorine and the ligand hydroxyl groups
Summary
G lycans and glycoconjugates are ubiquitously found on all living cells. Protein receptors, such as lectins, bind to such glycans and decipher their structural code, which results in biological and physiological actions.[1]. Ralstonia solanacearum lectin (RSL) is a carbohydrate binding protein with unusually strong affinity for fucose and specificity for fucosylated oligosaccharides from plants and animals.[17] RSL is a homotrimer that forms a six-bladed β-propeller fold. It presents six carbohydrate binding sites, which are either intra- or intermonomeric (Figure 1a), and are structurally very similar.[17] Each RSL monomer contains seven Trp residues, six of which are directly involved in carbohydrate binding (Supporting Information, Table S1). ACS Chemical Biology (b) The intermonomeric binding site with three important Trp residues: W31, W36, and W53 (structurally equivalent to W76, W81, and W10 in the intramonomeric site). (c) Structures of L-tryptophan and the fluorinated L-analogs used in this study
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