Hydrazin-based expressed protein ligation for sitespecific modification of maltose binding protein

Abstract

In current biotechnological applications, biofunctionalisation of substrates is mostly based on a non-oriented coupling of proteins either by physical adsorption or by random covalent coupling via functional groups of the endogeneous amino acid side chains (1). As a result, not all proteins have their active site(s) available for target binding, being a serious disadvantage, especially when aiming for miniaturisation. Nowadays, only a few oriented couplings are reported, mostly on affinity. Attaching a site-specific click functionality to proteins however, e.g. by expressed protein ligation (EPL), combines the best of both worlds, i.e. a stable covalent and oriented coupling of the protein to a substrate. Site-specific modification of proteins (in this study C-terminally) with EPL requires a nucleophile to attack the thioester bond between the protein and the intein. Generally a thiol-based nucleophile is used (2), leading to the presence of a free thiol group in modified protein, which can cause undesirable side reactions like the disruption of structural disulphide bonds, reactivity to electrophiles etc. (3). This study aims to investigate the use of a hydrazin-based nucleophile as an alternative, in order to avoid these drawbacks. Maltose binding protein (MBP) is used to demonstrate the proof of concept. The final goal is to use the proposed methodology to site-specifically functionalized nanobodies towards a covalent and oriented surface coupling.