posted on 2014-07-17, 00:00authored byShu-Han Chao, Sam S. Matthews, Ryan Paxman, Aleksei Aksimentiev, Martin Gruebele, Joshua L. Price
PEGylation, or addition of poly(ethylene
glycol) chains to proteins,
is widely used to improve delivery in pharmaceutical applications.
Recent studies suggest that stabilization of a protein by PEG, and
hence its proteolytic degradability, is sequence-dependent and requires
only short PEG chains. Here we connect stabilization by short PEG
chains directly to the structural dynamics of the protein and PEG
chain. We measured the stability of human Pin1 WW domain with PEG-4
at asparagine 19 for a full mutant cycle at two positions thought
to influence PEG–protein interaction: Ser16Ala and Tyr23Phe.
We then performed explicit solvent molecular dynamics simulations
on all PEGylated and PEG-free mutants. The mutant cycle yields a nonadditive
stabilization effect where the pseudo-wild type and double mutant
are more stabilized relative to unPEGylated proteins than are the
two single mutants. The simulation reveals why: the double mutant
suffers loss of β-sheet structure, which PEGylation restores
even though the PEG extends as a coil into the solvent. In contrast,
in one of the single mutants, PEG preferentially interacts with the
protein surface while disrupting the interactions of its asparagine
host with a nearby methionine side chain. Thus, PEG attachment can
stabilize a protein differentially depending on the local sequence,
and either by interacting with the surface or by extending into the
solvent. A simulation with PEG-45 attached to asparagine 19 shows
that PEG even can do both in the same context.