Folding of Pig Gastric Mucin Non-glycosylated Domains: A Discrete Molecular Dynamics Study
Mucin glycoprotein consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in polymerization of mucin via disulfide bonds and play an important role in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from autodigestion. We have examined the folding of the non-repetitive sequence of von Willebrand factor vWF-C1 domain (67 amino acids) and PGM 2X (242 amino acids) at neutral and low pH using Discrete Molecular Dynamics. A four-bead protein model with hydrogen bonding and amino acid-specific hydrophobic/hydrophilic and electrostatic interactions of side chains) was used. At neutral pH the conformation of PGM 2X was characterized by many β -strands separated by several turns and loops, while at low pH the number of turns was reduced and the β-strand propensity of the central part of the domain was significantly increased compared to the one in neutral pH conformations. The simulations reveal that the distant N- and C-terminal regions form salt-bridges at neutral pH giving a relatively compact folded structure. At low pH, the distant N- and C-terminal regions cannot form salt bridges and as a result the structure is more open and extended. We found that the calculated average value of the β-strand propensity increases from 0.23 ±0.01 at neutral pH to 0.36 ±0.01 at low pH in very good agreement with CD data, which show an increase in β-strand from 27% at neutral to 36% at low pH. Simulations of vWF C1 show 4-6 b-strands separated by turns/loops and we found that pH did not affect significantly the folded structure. The average β- strand structure of ~0.32 ± 0.03 was again in very good agreement with the CD result yielding 37% of the average β-strand structure. The implications of these simulations for describing the aggregation/gelation of PGM will be discussed.