Protein folding in confined media has attracted wide attention over the past
decade due to its importance to both in-vivo and in-vitro applications.
It is generally believed that protein stability increases by decreasing the
size of the confining medium, if its interaction with the confining walls is
repulsive, and that maximum folding temperature occurs for a pore size only
slightly larger than the smallest dimension of protein folded state.
However, protein stability at pore sizes very close to the size of the folded
state has not received enough attention. Unfortunately, an in depth study of
this problem at atomistic level is not possible by the present experimental
apparatuses. Using 0.3 millisecond-long molecular dynamics simulations,
we show that proteins with alpha-helix native state can be destabilized at
high confinement due to entropic stabilization of the protein states that
contain the beta structures. In contradiction to the present theoretical
explanations, which do not consider entropy of the misfolded states, we find
that the folding temperature maximum occurs at larger pore sizes for smaller
alpha helices. These results shed light on many recent experimental
observations that could not be explained by the present theories.
Our work demonstrates the importance of entropic effects of proteins misfolded
states in highly confined environments. The results support the concept of
passive effect of GroEL on protein folding by preventing it from aggregation
in crowded environment of cells, and provide deeper clues to the \alpha-\beta
conformational transition, believed to contribute to Alzheimer's and
Parkinson's diseases. The strategy of protein and enzyme stabilization in
confined media may also have to be revisited in the case of high confinement.