Affinity ligands for glycoprotein purification based on the multi-component Ugi reaction

• The lead ligand is capable of binding glycoproteins under physiological conditions.
• The Ugi ligand displays affinity to glucose oxidase and interacts with the glycoprotein through its carbohydrate moiety.
• The lead ligand exhibits sugar specificity.
• Boronic acid group play an important role in sugar binding.
• The solid phase Ugi scaffold can generate cost-effective ligands for glycoprotein purification.

One challenge facing the purification of therapeutic glycoproteins by affinity chromatography is creating ligands specific for the glycan moiety. Affinity chromatography of glycoproteins is currently conducted with immobilized lectins or boronates, although biomimetic ligands could present a more desirable option. This work describes the rational design and combinatorial synthesis of carbohydrate-binding ligands based on the solid phase multi-component Ugi reaction. An aldehyde-functionalized Sepharose™ solid support constitutes one component (aldehyde) in the four-component reaction, while the other three components (a primary/secondary amine, a carboxylic acid and an isocyanide) are varied in a combinatorial fashion to generate a tri-substituted Ugi scaffold which provides a degree of rigidity and is functionally suitable for interacting with the glycan moiety of glycoproteins. An Ugi library containing 48 ligands was initially screened against glucose oxidase (GOx) as the model glycoprotein to identify a candidate ligand, A13C24I8, which showed affinity to GOx through its carbohydrate moiety. Immobilized ligand A13C24I8 demonstrated a static binding capacity of 16.7 mg GOx/ml resin and an apparent dissociation constant (Kd) of 1.45 × 10−6 M at pH 7.4. The adsorbent can also bind 8.1 mg AGP/ml resin and displays an apparent affinity constant Kd = 1.44 × 10−5 M. The ligand has a sugar specificity in the following sequence: sorbitol > fructose > mannitol > ribose > arabinose > xylose > galactose > mannose > glucose > fructose; however, it did not display any specificity for sialic acid or methyl α-D-glycosides. A control ligand, generated by substitution of C24 (3-carboxyphenylboronic acid) with C7 (4-hydroxyphenyl acetic acid), failed to show affinity to the carbohydrate moiety, supporting the importance of the role that boronic acid group plays in sugar binding. GOx spiked E. coli samples were loaded onto immobilized ligand A13C24I8, 3-aminophenylboronic acid (APBA) and Concanavalin A and analysis of peak elution fraction with SDS-PAGE and gel densitometry showed that ligand A13C24I8 is capable of purifying GOx to 92% (w/w) with respect to the protein specific activity of 69% under current chromatographic conditions. The solid phase Ugi scaffold provides a strategy for the development of cost-effective pseudo-biospecific ligands for glycoprotein purification.