On the loose: We report an STD NMR spectroscopic study of the polysialyltransferase from Neisseria meningitidis serogroup B (NmB-polyST). The spectra reveal that the cytosine and ribose moiety receive more saturation than the sialic acid residue of CMP-Neu5Ac. This loose binding enables a fast and efficient sialyl transfer to the acceptor substrate. Our analysis offers a view of the structural determinants necessary for binding to NmB-polyST that provide the basis for the development of novel NmB-polyST inhibitors.