Abstract
Glycosylation is the most common type of post-translational modification (PTM) and is known to affect protein stability, folding and activity. Inactivity of enzymes mediating glycosylation can result in serious disorders including colon cancer and brain disorders. Out of five main types of glycosylation, N-linked glycosylation is most abundant and characterized by the addition of a sugar group to an Asparagine residue at the N-X-S/T motif. Enzyme mediating such transfer is known as oligosaccharyl transferase (OST). It has been hypothesized before that a significant number of proteins serve as glycoproteins. In this study, we used programming implementations of Python to statistically quantify the representation of glycoproteins by scanning all the available proteome sequence data at ExPASy server for the presence of glycoproteins and also the enzyme which plays critical role in glycosylation i.e. OST. Our results suggest that more than 50% of the proteins carry N-X-S/T motif i.e. they could be potential glycoproteins. Furthermore, approximately 28-36% (1/3) of proteins possesses signature motifs which are characteristic features of enzyme OST. Quantifying this bias individually reveals that both the number of proteins tagged with N-X-S/T motif and the average number of motifs per protein is significantly higher in case of eukaryotes when compared to prokaryotes. In the light of these results we conclude that there is a significant bias in the representation of glycoproteins in the proteomes of all species and is manifested substantially in eukaryotes and claim for glycosylation to be the most common and ubiquitous PTM in cells, especially in eukaryotes.