*2.3.3 Glycosylation*

Glycosylation is a commonly used modification of vacuolar proteins in seeds. The asparagine residue is used for N-linked glycosylation, and vacuolar glycoproteins

#### *Seed Filling DOI: http://dx.doi.org/10.5772/intechopen.106843*

usually have both high mannose content and complex N-linked glycans [46, 48, 49]. This process occurs in the lumen of the endoplasmic reticulum because the polypeptide is still in translation and contains a lipid carrier molecule (dolichol phosphate). There are two main types of oligosaccharide side chains: simple or mannose-rich oligosaccharides, which consist of mannose and N-acetyl-glucosamine, and complex or modified oligosaccharides, which are usually rich in mannose but also contain other residues such as fucose, xylose, and galactose. These oligosaccharide side chains may be found on the same polypeptide [45]. Glycosylation is thought to increase the stability of proteins and assist them in folding and assembly [1].

### *2.3.4 Proteolytic cleavage*

Seed proteins transported by the secretory pathway are often subject to posttranslational proteolytic cleavage, including storage proteins, seed defense proteins, and various vacuolar enzymes (α-mannosidase and thiol proteases). For seed storage proteins, the process begins during the transition to the vacuole and is completed in the vacuole [51]. Polypeptides can be split into two or three smaller peptides, some peptide chains can be removed, and the N- or C-terminus of peptides can be truncated [51, 52]. These modifications, together with glycosylation, could lead to a heterogeneous pool of mature proteins derived from a single polypeptide. Among storage proteins, 2S albumins undergo the largest posttranslational modifications [1, 45].

### *2.3.5 Formation of disulfide bonds*

Many seed proteins must have disulfide bonds to stabilize their tertiary and quaternary structures. Disulfide bond formation occurs through disulfide isomerase, which promotes disulfide formation, isomerization or reduction, in newly formed proteins [53]. The enzyme interacts with unfolded proteins in the endoplasmic reticulum and catalyzes thiol oxidation and disulfide exchange reactions.
