Implications of protein polymorphism on protein phase behaviour

The phase behaviour of small globular proteins is often modeled by approximating them as spherical particles with fixed internal structure. However, changes in the local environment of a protein can lead to changes in its conformation rendering this approximation invalid. We present a simple two-state model in which protein conformation is not conserved and where the high-energy, non-native state is stabilised by pair-wise attractive interactions. The resulting phase behaviour is remarkably complex, non-universal and exhibits re-entrance. The model calculations show a demarcation between a regime where conformational transitioning is largely enslaved by phase separation and one where this is not the case. In the latter regime, which is characterised by a large free energy difference between the native and the non-native state, we deduce that the kinetics of the phase transition strongly depend on the average conformation of the proteins prior to their condensation. For condensation to occur in this regime within a dispersion of native proteins, nucleation of a cluster of proteins in the non-native state is required. We argue that our theory supports the distinction between common phase separation and the nucleated assembly of non-native supramolecular aggregates in protein dispersions.