I'm studying negative sense RNA virus virus fusion with host membranes. But I don't have much understanding about protein folding and unfolding, conformational stability. I need detail about this concept.
The term “protein marginal stability” is used to give account of the low values found for protein unfolding free energies (in the order of the energy needed for breaking a few hydrogen bonds). This implies that the native state is as a thermodynamic state close to the edge with “unfolded states”. In these conditions internal motions would be enough for normal protein function, e.g. catalysis.
It is important to understand the terms 'conformation' and 'entropy' separately and then put them together to understand 'conformational entropy' of proteins. Protein structures that we see in the PDB represent the lowest energy conformation but they are not the only conformation. Each protein can sample multiple conformations that structurally slightly vary from each other, such as sidechain rotameric states, backbone conformation, etc. Their individual free energies determine their population in the protein's overall conformational ensemble. Lower energy means higher population and vice versa. Now you can imagine an arbitrary threshold energy and consider conformations that have free energy only below this threshold. Proteins that have more conformations in this ensemble will have higher 'conformational entropy' than proteins that have less conformations in this ensemble.
'Conformational entropy' is important in both protein folding as well as protein-ligand interaction. In the unfolded state a protein samples much more conformations compared to the native folded state, which means a loss of conformational entropy upon folding. Similarly, upon ligand binding protein can lose conformational entropy. In such cases the loss in entropy is compensated by favorable change in enthalpy.