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Optimization of O-GIG for O-glycopeptide Characterization with Sialic Acid Linkage Determination.

O-glycoprotein analysis has been historically challenging due, in part, to a dearth of available enzymes active in release of O-glycans. Moreover, chemical releasing methods, such as β-elimination/Michael addition are not specific to O-glycan release and can also eliminate phosphoryl substitutions. Both of these events leave behind deaminated serine and threonine and thus can lead to ambiguous structural conclusions. Recently, the O-protease OpeRATOR, derived from intestinal bacteria and expressed in E. coli, has become commercially available. Digestion of O-glycoprotein yields O-glycopeptides cleaved at the N-terminal end of serine/threonine with O-glycan remaining intact. The enzyme has broad substrate specificity and includes mammalian Cores 1-8. However, OpeRATOR is not fully active toward sialylated glycoproteins and therefore it is suggested that this acidic residue be removed prior to digestion, thus, sacrificing structural information. In this study, we investigated the performance of OpeRATOR under a range of conditions, including buffer selection, pH, sialic acid modification, and digestion temperature in order to optimize enzymatic activity with special emphasis on sialylated glycosites. Conditions derived in this work facilitate OpeRATOR digestion of fully sialylated O-glycopeptides mass tagged to identify the sialyl linkage, thus, facilitating analysis of these charged O-glycopeptides, which are often important in biological processes.