Affinity-based covalent sialyltransferase probes enabled by ligand-directed chemistry

Sialyltransferases (ST) are key enzymes found in, among others, mammals and bacteria that are responsible for producing sialylated glycans, which play critical roles in human health and disease. However, chemical tools to study sialyltransferases have been limited to non-covalent inhibitors and probes that do not allow isolation and profiling of these important enzymes. Here we report a new class of covalent affinity-based probes (AfBP) for ST by using ligand-directed chemistry (LDchem). Our affinity-based probes are armed with a simple to synthesise but robust O-nitrobenzoxadiazole (O-NBD) warhead, which is a lysine-specific S_NAr electrophilic warhead with an advantageous turn-on fluorescence property. We chemoenzymatically synthesised a series of CMP-Neu5Ac based probes and demonstrated their high specificity in labelling a range of recombinant STs with submicromolar sensitivity. Importantly, with our LDchem ST probe, we successfully labelled the endogenous lipooligosaccharide ST (Lst) in live Neisseria gonorrhoeae, a clinically relevant human pathogen. Our results demonstrated that this new class of covalent ST probes offer a robust platform for ST profiling and future studies of STs in their native environments.