Transforming protein-polymer conjugate purification by tuning protein solubility

Stefanie L Baker,Marie Certiat,Bibifatima Kaupbayeva,Hironobu Murata,Alan J Russell,Nin Rebecca Kang,Coray M Colina,Aravinda Munasinghe,Krzysztof Matyjaszewski,Ping Lin

doi:10.1038/s41467-019-12612-9

Abstract

Almost all commercial proteins are purified using ammonium sulfate precipitation. Protein-polymer conjugates are synthesized from pure starting materials, and the struggle to separate conjugates from polymer, native protein, and from isomers has vexed scientists for decades. We have discovered that covalent polymer attachment has a transformational effect on protein solubility in salt solutions. Here, protein-polymer conjugates with a variety of polymers, grafting densities, and polymer lengths are generated using atom transfer radical polymerization. Charged polymers increase conjugate solubility in ammonium sulfate and completely prevent precipitation even at 100% saturation. Atomistic molecular dynamic simulations show the impact is driven by an anti-polyelectrolyte effect from zwitterionic polymers. Uncharged polymers exhibit polymer length-dependent decreased solubility. The differences in salting-out are then used to simply purify mixtures of conjugates and native proteins into single species. Increasing protein solubility in salt solutions through polymer conjugation could lead to many new applications of protein-polymer conjugates.

Highlights

Almost all commercial proteins are purified using ammonium sulfate precipitation
Lysozyme–polymer conjugates were synthesized with a high grafting density and varied polymer chain lengths using grafting-from atom transfer radical polymerization (ATRP) (Fig. 1)
Five different chain lengths of two different polymer types were chosen to study the effect of polymer attachment on solubility: zwitterionic poly(carboxybetaine methacrylate) and neutral poly(oligo(ethylene glycol) methacrylate)

Summary

Introduction

Almost all commercial proteins are purified using ammonium sulfate precipitation. Proteinpolymer conjugates are synthesized from pure starting materials, and the struggle to separate conjugates from polymer, native protein, and from isomers has vexed scientists for decades. Native Lyz, Lyz-initiator, and Lyz–polymer conjugates were subjected to precipitation by ammonium sulfate at pH 7.0 to determine their salting-out points (Fig. 2).

Results

Conclusion