Molecular Dynamics Simulations on Pars Intercerebralis Major Peptide-C (PMP-C) Reveal the Role of Glycosylation and Disulfide Bonds in its Enhanced Structural Stability and Function

Fucosylation of Thr9 in pars intercerebralis major peptide-C (PMP-C) enhances its structural stability and functional ability as a serine protease inhibitor. In order to understand the role of disulfide bonds and glycosylation on the structure and function of PMP-C, we have carried out multiple explicit solvent molecular dynamics (MD) simulations on fucosylated and non-fucosylated forms of PMP-C, both in the presence and absence of the disulfide bonds. Our simulations revealed that there were no significant structural changes in the native disulfide bonded forms of PMP-C due to fucosylation. On the other hand, the non-fucosylated form of PMP-C without disulfide bonds showed larger deviations from the starting structure than the fucosylated form. However, the structural deviations were restricted to the terminal regions while core p-sheet retained its hydrogen bonded structure even in absence of disulfide bonds as well as fucosylation. Interestingly, fucosylation of disulfide bonded native PMP-C led to a decreased thermal flexibility in the residue stretch 29-32 which is known to interact with the active site of the target proteases. Our analysis revealed that disulfide bonds covalently connect the residue stretch 29-32 to the central beta-sheet of PMP-C and using a novel network of side chain interactions and disulfide bonds fucosylation at Thr9 is altering the flexibility of the stretch 29-32 located at a distal site. Thus, our simulations explain for the first time, how presence of disulfide bonds between conserved cysteines and fucosylation enhance the function of PMP-C as a protease inhibitor.