Small-world view of the amino acids that play a key role in protein folding

We use geometrical considerations to provide a different perspective on the fact that a few selected amino acids, the so-called ``key residues,'' act as nucleation centers for protein folding. By constructing graphs corresponding to protein structures we show that they have the ``small-world'' feature of having a limited set of vertices with large connectivity. These vertices correspond to the key residues that play the role of ``hubs'' in the network of interactions that stabilize the structure of the transition state.