The polysialyltransferase from Neisseria meningitis serogroup B (NmB polyST) catalyzes the transfer of sialic acid from activated sialic acid donor cytidine-5´-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) onto sialic acid containing acceptors to synthesise polysialic acid (polySia). The polySia capsule protects neuroinvasive bacteria like NmB against the host immune system by molecular mimicry and is therefore an important virulence factor.
We have recently shown by STD NMR spectroscopy that the sialic acid moiety of the CMP-Neu5Ac donor makes fewer contacts with the wild type polyST than the nucleoside residue whereas a polySia trimer donor molecule was completely accommodated by the protein. 
Here we present an in-depth STD NMR spectroscopy study to interrogate the binding of CMP-Neu5Ac and polySia to NmB polyST mutants and truncations. Important structural features of the bacterial enzyme will be presented.