Structural analysis of glycoprotein sialylation – part II: LC-MS based detection

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Structural characterisation of sialylated glycoproteins in time and space is a requirement for further understanding of their involvement in biology. In this second of two related reviews, our focus is on the liquid chromatography (LC) and mass spectrometry (MS) based techniques, which have become the golden analytical tools in recent decades for the analysis of N- and O-linked glycoprotein sialylation. Analytical strategies for the analysis of N- and O-linked sialoglycoproteins and pre-LC-MS aspects including enrichment, derivatisation and metabolic labelling techniques were covered in the first review. Acknowledging that glycoprotein sialylation can be studied on multiple analyte levels, LC-MS detection of sialoglycans, sialoglycopeptides and intact sialoglycoproteins are separatedly discussed. All levels have benefitted from continuous improvements of LC-MS technologies, which have gradually pushed the boundaries for separation and detection capabilities to finally allow characterisation of sialoglycoproteins more directly from biological samples. Although still not achieved from a single analysis, LC-MS facilitates the characterisation of many aspects of the sialoglycoprotein structure including the identification and quantitation of the protein carrier, the underlying glycan structure and the sialyl linkage as well as the sialic acid speciation and its further modifications. It is evident from the body of literature that the analytical glycoscientist now has a much improved, though not yet fully mature, toolbox for the analysis of glycoprotein sialylation. This capacity enables structural based investigations of the functional relevance of sialoglycoproteins. Understanding the chemistry and biology of the conjugated sialic acids is essential in order to interpret the complex protein glycosylation code.