This work investigated the substrate specificity of hNEU enzymes for a glycoprotein substrate (bovine submaxillary mucin) containing 9-O-acetylated and Neu5Gc residues. Using this model substrate, we observe a general trend for hNEU tolerance of Neu5Ac>Neu5Gc>>>Neu5,9Ac 2 , consistent with our previous results with glycolipid substrates. These results expand our understanding of hNEU enzyme specificity and suggest that naturally occurring modifications of sialic acids can play a role in regulating hNEU activity. File list (2) download file view on ChemRxiv hunter.text.rev.pdf (486.29 KiB) download file view on ChemRxiv hunter.si.pdf (288.03 KiB) Human neuraminidases have reduced activity towards modified sialic acids on glycoproteins