Amino acid residues at protein-protein interfaces:: Why is propensity so different from relative abundance?

It is known that nonpolar residues on the surface of proteins are likely to be at interfaces in protein-protein association. However, as one normalizes the relative abundance at the interface to the overall abundance, one finds that the residues with the highest propensity to be at interfaces are as follows (in decreasing order): Asn, Thr, Gly, Ser, Asp, Ala, and Cys. None of these residues is distinctly nonpolar. We show here that these residues also have the highest propensity to be engaged in hydrated backbone hydrogen bonds of the monomeric structure, acting as either proton donors or acceptors. Such preformed hydrogen bonds, in turn, are known to be determinants of protein-protein association as they are stabilized by water removal, upon formation of a complex. A linear correlation is reported between the two independently determined propensities. In addition, the seven residues with the highest propensity for being engaged in hydrated hydrogen bonds all have at most one torsional degree of freedom in their side chain. Thus, upon protein association, the thermodynamically advantageous intermolecular dehydration of preformed hydrogen bonds operates synergistically with a minimization of the entropic loss resulting from the conformational hindrance of the side chains and of contact with nearby nonpolar side chains.