# Phytoglycoproteins and Human Health: Current Knowledge and Future Applications
## 2 Structural Features and Occurrence of Phytoglycoprotein  
Phytoglycoprotein refers to glycoproteins of plant origin. Glycoprotein are a diverse class of macromolecules that contain glycan moieties covalently linked to the polypeptide side chains. They differ slightly from proteoglycans, which have a major polysaccharide backbone linked to a few amino acid residues. Both classes are composed of protein and carbohydrate moieties and, hence, express slightly similar biological properties [12]. The process of glycosylation, which is a posttranslational modification, adds glycan residues to several proteins to achieve their functions in biological systems [13]. Several types of glycosylation, such as N-, O-, P-, C- and S-glycosylation, can be found in living systems. However, PGPs are majorly N-glycosylated or O-glycosylated with one or more heteroglycans possessing two to fifteen sugar residues [14]. Glycoproteins have diverse sublocations in plants ranging from the cell wall of vascular bundles, scherenchyma cells, pollen and pollen tubes, plasma membrane, endoplasmic reticulum, Golgi bodies and intravesicular bodies. Plant-derived glycoprotein are biosynthesized and modified within plant endoplasmic reticulum and Golgi bodies and then packaged as secretory vesicles for transport to different parts of the plants [15]. The syntheses PGPs begin from the formation and translocation of activated nucleotide sugars from the cytosol to the endoplasmic reticulum and Golgi bodies. The activated sugars serve as precursors for building the glycan oligosaccharides through reactions catalyzed by sugar-specific glycosyltransferases [16]. Plant cell walls are made up of 10-15% glycoproteins and have been reported to be the largest depot of both N- and O-glycosylated proteins [15]. N-glycoprotein in plants are more conserved in higher plants and Eukaryotic organisms and have more pharmaceutical relevance than O-glycoproteins [17]. Several studies have shown that N-glycoproteins play a vital role in conserving the integrity of many protein folding events. In contrast, O-glycoproteins have mainly structural relevance, some catalytic and translocation activities in plants [18]. N-glycosylation common in plants involves the en bloc transfer of mature glycan (comprising mainly 2 GlcNAc, 9 Mannose and 3 glucose--Glc\({}_{3}\)Man\({}_{9}\)GlcNAc\({}_{2}\)) on a lipid carrier to selected asparagine residues (Asn) in the nascent polypeptide chain. The glycan used for N-glycosylation is formed in the endoplasmic reticulum (ER) in two phases (Phase I and Phase II). The steps in Phase I occurs on the cytoplasmic side of the ER and involves the attachment of 2 UDP-activated GlcNAc to Dolichol, and then 5 GDP-activated mannose to the GlcNAc, yielding a product Dolichol-GlcNAc\({}_{2}\)-Mang. Dolichol comprises of repeating chains of isoprene and is the lipid carrier of the mature glycan in the ER [17]. The Phase II for the formation of mature glycan N-glycosylation takes place in the luminal side of the endoplasmic reticulum. The product of Phase I is translocated across the membrane to the ER lumen through an unknown mechanism by an enzyme, flippase. In Phase II, 4 mannose (activated in the form of dolicol phosphate-activated mannose) and 3 glucose (activated in the form of dolicol phosphate-activated glucose) are added respectively to the growing glycan chain, yielding the matured glucan (Dolichol-GlcNAc\({}_{2}\)-Mang-Glc\({}_{3}\)) [17]. Moreover, in the lumen of the ER, the mature glycan is translocated to an asparagine residue in the nascent polypeptide chain by specific oligosaccharidestransferases (OTase). Studies have revealed that not all asparagine residues in polypeptide sequences are suitable N-glycosylation sites. For an asparagine to be glycosylated, it must first appear in a consensus sequence referred to as sequon. A plant sequon (represented as 'Asn-X-Ser/Thr') generally contains an asparagine residue (Asn) adjacent to two other residues.