Glycosylation

Glycosylation is a commonly conserved process in the posttranslational protein modifications of eukaryotes and bacteria [14—16]. However, glycosylation can be introduced by chemical modification in which carbohydrate molecule is conjugated to a protein, peptide, lipid, or nanocarrier in the laboratory. Thirteen monosaccharides and eight different amino acids have been described to be involved in 37 different carbohydrate—protein linkages. In this way, a large variety of glycan structures is created. Carbohydrate moieties are introduced not only to facilitate properties of a protein but also to improve physicochemical, cellular localization, turnover, protein quality control, and ligand interaction [16,17]. It can be either a co- or a postenzymatic translational modification. Glycosylation can be categorized into two groups, N-linked and O-linked. In N-linked glycosylation, the carbohydrate molecule is attached to a nitrogen molecule of arginine or asparagine side chain

[1,6]. On the other hand, in O-linked glycosylation, the oxygen on hydroxyl of tyrosine, threonine, and serine is utilized for conjugation [1,6].

Glycosylated cytochrome c (Cyt c) was immobilized in mesoporous silica nanoparticles (MSNs) for inducing apoptosis in HeLa cells by [18]. Cyt c was modified by glycosylation of lactose to increase stability and reduce proteolytic degradation. Caspase-3 assay demonstrated a 47% activation by Cyt c-SPDP (sulfosuccinimidyl 6-(3⁰-[2-pyridyldithio]-propionamido)hexanoate) relative to 87% activation observed with Cyt c-lactose bioconjugate [18]. Furthermore, MSN-SPDP-Cyt c did not induce apoptosis, whereas MSN-Cyt c-lactose induced apoptosis in HeLa cells after 72 h [18]. In addition, MSN-Cyt c-lactose demonstrated endosomal escape [18]. These findings indicate that glycosylation can stabilize and reduce enzyme degradation of Cyt c.