Effect of N-linked glycosylation on the structure, function and stability of the aspartic proteinase pepsin
This thesis is an investigation into the effects of N-linked glycosyation on the structure, function and stability of the aspartic proteinase pepsin. Through site-directed mutagenesis, the N-linked motif (asparagine-x-serine/threonine) was incorporated into four specific sites on the primary structure of pepsinogen. Two sites were located near the entrance to the active site cleft on the C-terminal domain, one near the entrance of the active site on the N-terminal domain and a fourth on the tip of the loop which covers the active site cleft. A recombinant form of pepsinogen, an O-linked recombinant and four N-linked recombinants were successfully expressed in the methylotrophic yeast ' Pichia pastoris', and purified. When compared to commercial pepsin, recombinant pepsin had similar kinetic profiles, pH/temperature stability and secondary/tertiary conformation. The O-glycosylated form was also found to exhibit similar kinetic and structural characteristics to the commercial and wild-type pepsin, however, the O-glycosylated form was slightly more thermal stable. All four N-linked recombinants exhibited similar secondary and tertiary structure to the non-glycosylated pepsin. Similar 'K' m values were obtained but the catalytic efficiency was approximately one third compared to the non-glycosylated form. Despite the change in catalytic activity, substrate specificity was not altered. Activation of pepsinogen to pepsin occurred between pH 1.0 and 6.0 for all N-linked recombinants compared to non-glycosylated pepsin, which did not activate beyond pH 4.0. Pepsin which was glycosylated on the C-terminal domain exhibited similar pH activity profiles to non-glycosylated pepsin while a decrease in activity was observed for pepsin which was glycosylated on the loop after pH 3.0 and no activity at 5.5. Activity of glycosylated pepsin on the N-domain peaked at pH 3.5 and lost activity at 5.5. Glycosylation on the N- and C-domains resulted in increased stability at pH 7.0 and 7.5, respectively, in contrast to the non-glycosylated form which was denatured at pH 7.0. Similarly, glycosylation contributed to an increase in thermal enzymatic stability as well as an increase in structural stability of the N- and/or C-domain. It is proposed that the presence of the carbohydrate residues added rigidity to the protein structure. This increase in rigidity, which was location dependent, reduced the conformational mobility of the protein, which in turn, affected its ability to adapt to changes in its environment (i.e., pH and temperature). Glycosylation on the C-domain had more of an impact on increasing structural and enzymatic stability than glycosylation on the N-domain which contributed to a decrease in stability. Finally, glycosylation on the loop increased structural stability with a slight loss in enzyme activity.