44
Modeling of Biomolecular Systems Interactions, Dynamics, and Allostery Session IX Abstracts
Role of Histidine Protonation in the pH Induced Changes in Prion Protein Stability
Shoshana J. Wodak
, Anatoly Malevanets, Andrew Chong, Flemming Hansen, Julie Forman-
Kay.
Hospital for Sick Children, Toronto, Canada.
Nuclear Magnetic Resonance (NMR) is employed to investigate origins of the greater pH
susceptibility of the PrP protein from golden hamster (GHaPrP), relative to its homolog from
rabbit (RaPrP). Titration experiments were performed to measure the pKa of the five His
residues in the considered PrP domains (residues 90-231). In addition, proton-exchange rates of
amide groups across the two PrP domains were measured at pH 5 and pH 7, respectively, in
absence of denaturants. Results revealed a single buried His residue (H187 in GHaPrP, and H186
in RaPrP) to have a markedly down shifted pKa~5. On the other hand, significantly larger pH-
induced shifts in exchange rates were detected for the hamster protein compared to the rabbit
homolog. Using an extension of the classical proton exchange model where a mixture of protein
sub-states contribute to the observed exchange rates, evidence is provided that protonation of the
buried histidine is the primary event responsible for the pH-induced destabilization of both PrP
variants. The marked difference in the pH susceptibility between hamster and rabbit PrP is then
related to the difference between the global intrinsic stability of the two proteins. The folded
state of the more stable rabbit PrP is only marginally perturbed by His protonation, whereas
protonation significantly perturbs hamster PrP, resulting in a considerably less stable
conformational state. Our findings link the protonation of a single buried histidine to PrP
intrinsic stability, as the underlying mechanism for PrP pH-induced destabilization and its likely
implication in disease susceptibility.
