Implications of protein polymorphism on protein phase behaviour

Abstract

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.

Self-crowding induced phase separation in protein dispersions

“…28 We realise that more sophisticated and accurate approaches such as fundamental measure theory have been put forward. 28,29 However, as we have shown in previous work for a monodisperse model protein dispersion, 27 qualitatively the predicted phase behaviour is insensitive to different thermodynamic models and because of this we believe the use of the simpler BMCSL equation of state is justified.…”

“…We construct a free energy function for this model by combining the free energy of a bidisperse hard-sphere fluid that we derive from the polydisperse Boublik-Mansoori-CarnahanStarling-Leland (BMCSL) equation of state, 25,26 a van der Waals-like contribution that accounts for attractive interactions between the non-native species of protein and terms describing the "chemical" equilibrium between the two states. 27 The BMCSL equation of state is widely used and has been described as both simple and accurate. 28 We realise that more sophisticated and accurate approaches such as fundamental measure theory have been put forward.…”

Active interaction switching controls the dynamic heterogeneity of soft colloidal dispersions

“…The environment-dependent size of responsive materials can change by a factor of two or three typically 6 . Examples of this are globular proteins, in which their conformation and phase behavior can be tuned by changes in the local environment and protein-protein attractive interaction 7,8 . These colloidal systems and their responsiveness can be used for tailormade functionality such as core-shell nano-reactors for selective catalysis 3,4 or controlled drug release 1,2 , but also lay the foundation for adaptive and intelligent systems 9 .…”

“…Such a system constitutes a good model for bistable bacteria that use switching to tune structural and dynamical heterogeneities for their function 37,38 , as well as for soft active or vesicles fluctuating between two states [39][40][41][42] . It could also be applied to study the structure and phase behavior of conformationally fluctuating biopolymers [43][44][45] , in particular two-state proteins switching between native and non-native states 7,8 . In future it may be extendable to even study soft micromachines with a programmable morphology 46 .…”