Structural analysis of glycoprotein sialylation - Part I: pre-LC-MS analytical strategies

Sialic acids are carried by glycoproteins, proteoglycans and glycolipids as terminal entities of larger glycan structures and form a heterogeneous group of important monosaccharides in a wide range of biological systems in nature. Spatial and temporal structural characterisation of sialoglycoconjugates is required to understand their function. In this first of two related reviews we outline the available strategies for the analysis of mammalian N- and O-linked glycoprotein sialylation and summarise the associated sample handling methodologies that are a prerequisite for successful experimental designs including methods for enrichment, isolation, derivatisation and metabolic labelling. The downstream liquid chromatography (LC) mass spectrometry (MS) based separation and detection of N- and O-linked glycoprotein sialylation is covered in the second review. Since glycoprotein sialylation can be studied on multiple analyte levels, the analytical strategies and pre-LC-MS methodologies are covered separately for sialoglycans, sialoglycopeptides and intact sialoglycoproteins. Workflows to analyse glycoprotein sialylation at the glycomics level are particularly mature and the analytical chemist has multiple tools and technologies to acquire structural information on released glycans even at the system-wide level. The availability of analytical tools to study site-specific glycoprotein sialylation in the form of sialoglycopeptides or intact sialoglycoproteins is increasing through the development of sialic acid specific enrichment and labelling tools. However, the glycoproteomics route remains comparatively more challenging even when relatively simple protein mixtures are analysed. Evidenced by the wealth of available literature reviewed here, the glycoscience community has invested significant efforts to improve the analysis of glycoprotein sialylation.