Hydration-induced dynamical changes in lyophilised and weakly hydrated apoferritin: insights from molecular dynamics simulation.

The dynamics and functionality of proteins are significantly influenced by their interaction with water. For lyophilised (i.e. h ≤ 0.05 where h = g of H2O per g of protein) and weakly hydrated systems (i.e. h ≤ 0.38) hydration generally enhances protein mobility above the so-called 'dynamical transition' temperature (Td > 220 K). However, water-induced mobility hindrance at low temperatures (T < 175 K) has been reported in various proteins of varying secondary structure; namely green fluorescent protein (GFP), pig liver esterase, lysozyme, ribonuclease A (RNAse A) and apoferritin. By focussing on the dynamic behaviour of the apoferritin molecule, this study proposes mechanisms driving these hydration-induced mobility changes, particularly the less understood hindrance at low temperatures. Using atomistic molecular dynamics (MD) simulations of horse spleen apoferritin in the lyophilised (h = 0.05) and weakly hydrated (h = 0.31) states, we report here the impact of water on protein dynamics as a function of temperature. Through residue-specific mean squared displacement (MSD), radial distribution function (RDF), solvent accessible surface area (SASA), local hydration degree and hydrogen bonding analyses, we demonstrate that while water proximity directly correlates with mobility enhancement at high temperatures, the hydration-induced mobility reduction observed at temperatures below 175 K is primarily propagated through the protein backbone.