FIGURE 2:

Determination of ST6GalNAc-II activity using mass spectrometry. IgA1 HR peptide VPSTPPTPSPSTPPTPSPSCCHPR was GalNAcosylated with GalNAc-T2 and then sialylated using ST6GalNAc-II. MS1 spectra were averaged over glycopeptide-elution retention times; the glycopeptides were observed in 3⁺ and 4⁺ charge states. Both the non-sialylated and sialylated forms of the GalNAcosylated HR glycopeptide were detected, ranging from 4 to 7 additions of GalNAc with 0 to 3 additions of sialic acid. Major glycopeptide ion species are labeled to show the number of GalNAc and sialic acid additions to the acceptor HR peptide.