The average hydrophobicity (HH) values for ENZ, nonENZ, HET and HOM are respectively 0.40, 0.37, 0.41, and 0.42. Even though average HH values are similar for different PPI types, the histogram distributions of hydrophobicity (see Supplementary Figure Two [see Additional file 2]) exhibit distinctive separation patterns between non-obligate and obligate interactions. The distribution of HH for ENZ is similar to nonENZ and that of HET is similar to HOM.