Glycoproteomics of a single protein: revealing hundreds of thousands of Myozyme® glycoforms by hybrid HPLC-MS approaches

Characterisation of highly glycosylated biopharmaceuticals by mass spectrometry is challenging because of the huge chemical space of co-existent glycoforms, i.e. heterogenous glycoprotein variants. Here, we report the use of an array of HPLC-MS-based approaches at different structural levels of released glycan, glycopeptide, and hitherto unexplored intact glycoforms to scrutinize the biopharmaceutical Myozyme®, containing the highly complex lysosomal enzyme recombinant acid α-glucosidase. The intrinsic heterogeneity of recombinant acid α-glucosidase glycoforms was unraveled using a novel strong anion-exchange (SAX)-HPLC-MS approach involving a pH gradient of volatile buffers to facilitate chromatographic separation of glycoforms based on their degree of sialylation followed by the acquisition of native mass spectra in an Orbitrap mass spectrometer. The large set of interdepend data acquired at different structural levels was integrated using a set of bioinformatics tools and allowed the annotation of intact glycoforms unraveling more than 1,000,000 different intact glycoform structures. Finally, for the first time, we sought to validate the intact glycoform annotations by integrating experimental data on the enzymatically dissected proteoforms revealing the strengths but also intrinsic limitations of this approach for fully characterising such highly complex glycoproteins by mass spectrometry.