TEO-GalNAc: MS Tag-Free Chemoenzymatic Enrichment and Relative Assessment of Tn/T Antigen Sialylation Occupancy via Ketone-Oxime Chemistry.

Tn/T antigens and their sialylated forms (STn/ST) are tumor-associated truncated O-glycans whose selective enrichment and MS analysis remain challenging due to limited enrichment efficiency and the MS-incompatible bulky tags in conventional chemoenzymatic enrichment methods. Here, we develop TEO-GalNAc (MS Tag-free Enrichment of O-GalNAc), a chemoenzymatic strategy that integrates ST6GalNAc1-catalyzed labeling with a ketone-modified CMP-sialic acid donor, reversible oxime capture, and mild acid-catalyzed tag removal prior to MS analysis. Using TEO-GalNAc, we identified 489 unique Tn/T glycopeptides from human serum and 1,068 unique Tn/T glycopeptides from Jurkat cell lysates, demonstrating the applicability of the workflow to complex biological samples. Importantly, this approach enables relative assessment of GalNAc-associated sialylation occupancy by comparing results with or without preservation of endogenous sialic acids prior to enzymatic tagging. Using this analytical framework, we observed differential labeling patterns between nonmetastatic MCF-7 and metastatic MDA-MB-231 cells, including distinct signals associated with several glycoproteins such as MUC1, GLU2B, MESD, and SEL1L. By enabling tag-removable chemoenzymatic enrichment and MS-compatible analysis of Tn/T-associated glycopeptides, TEO-GalNAc provides a complementary analytical strategy for studying truncated O-glycosylation in complex biological samples.