Characterization of O-linked saccharides on glycoproteins

Protein Glycosylation Investigated by Mass Spectrometry: An Overview

The protein glycosylation is a post-translational modification of crucial importance for its involvement in molecular recognition, protein trafficking, regulation, and inflammation. Indeed, abnormalities in protein glycosylation are correlated with several disease states such as cancer, inflammatory diseases, and congenial disorders. The understanding of cellular mechanisms through the elucidation of glycan composition encourages researchers to find analytical solutions for their detection. Actually, the multiplicity and diversity of glycan structures bond to the proteins, the variations in polarity of the individual saccharide residues, and the poor ionization efficiencies make their detection much trickier than other kinds of biopolymers. An overview of the most prominent techniques based on mass spectrometry (MS) for protein glycosylation (glycoproteomics) studies is here presented. The tricks and pre-treatments of samples are discussed as a crucial step prodromal to the MS analysis to improve the glycan ionization efficiency. Therefore, the different instrumental MS mode is also explored for the qualitative and quantitative analysis of glycopeptides and the glycans structural composition, thus contributing to the elucidation of biological mechanisms.

Progress in Yeast Glycosylation Engineering

While yeast are lower eukaryotic organisms, they share many common features and biological processes with higher eukaryotes. As such, yeasts have been used as model organisms to facilitate our understanding of such features and processes. To this end, a large number of powerful genetic tools have been developed to investigate and manipulate these organisms. Going hand-in-hand with these genetic tools is the ability to efficiently scale up the fermentation of these organisms, thus making them attractive hosts for the production of recombinant proteins. A key feature of producing recombinant proteins in yeast is that these proteins can be readily secreted into the culture supernatant, simplifying any downstream processing. A consequence of this secretion is that the proteins typically pass through the secretory pathway, during which they may be exposed to various posttranslational modifications. The addition of glycans is one such modification. Unfortunately, while certain aspects of glycosylation are shared between lower and higher eukaryotes, significant differences exist. Over the last two decades much research has focused on engineering the glycosylation pathways of yeast to more closely resemble those of higher eukaryotes, particularly those of humans for the production of therapeutic proteins. In the current review we shall highlight some of the key achievements in yeast glyco-engineering which have led to humanization of both the N- and O-linked glycosylation pathways.

A practical approach for O-linked mannose removal: the use of recombinant lysosomal mannosidase

The methylotrophic yeast Pichia pastoris is an attractive expression system due to its ability to secrete large amounts of recombinant protein, with the potential for glycosylation. Advances in glycoengineering of P. pastoris have successfully demonstrated the humanization of both the N- and O-linked glycosylation pathways in this organism. However, in certain cases, the presence of O-linked glycans on a therapeutic protein may not be desirable. Recently, we have reported the in vitro utility of jack bean α-1,2/3/6-mannosidase to remove O-linked mannose from intact undenatured glycoproteins produced in glycoengineered P. pastoris. However, one caveat of this strategy is that jack bean mannosidase has yet to be cloned and as such is only available as crude cellular extracts. This raises several concerns for using this reagent to treat large preparations of therapeutic proteins generated in P. pastoris. Therefore, we postulated that lysosomal mannosidases which have been cloned and demonstrated to have similar activities to jack bean mannosidase on N-linked glycans would also process O-linked glycans in a similar fashion. To this end, we screened a panel of recombinant lysosomal mannosidases from different organisms and identified several which cannot only reduce extended O-linked mannose chains but which can also hydrolyze the Man-α-O-Ser/Thr glycosidic bond on intact glycoproteins. As such, not only do we show for the first time the utility of lysosomal mannosidase for O-linked mannose processing, but since this is a recombinant enzyme, it has several benefits over the use of crude jack bean mannosidase extracts.

A tandem mass spectrometric approach to the identification of O-glycosylated glargine glycoforms in active pharmaceutical ingredient expressed in Pichia pastoris

Glycoforms of glargine expressed in Pichia pastoris were isolated by high-performance liquid chromatography and analyzed by a series of chemical and mass spectrometric methods for the identification of various glycoforms, glycosylation position, nature and structure of glycans. Reduction and alkylation, peptide mapping techniques were used to decipher the amino acid site at which glycosylation had taken place. Chemical methods were coupled with mass spectrometry techniques such as electrospray ionization and matrix-assisted laser desorption/ionization for identification of the glycosylation site.