Specific protein-protein interaction is essential for the function of life systems. A variety of computational methods are being extensively used now-a-days to investigate this interaction and to identify structural features of binding sites. In this paper, the informational structure analysis method was applied to the study of protein-protein interaction interfaces in enzyme-inhibitor complexes. The analysis of amino acid sequence by informational structure analysis method reveals three types of sites (ADD+, NORMAL and ADD-) which differ in the density of first rank elements in the informational structure. ADD+, NORMAL and ADD-sites also differ in their ability towards adaptive conformational reorganization which contributes to the formation of protein-protein interaction interfaces in enzyme-inhibitor complexes. The study of hydrolytic enzymes in complex with their protein inhibitors shows that at least one of the interaction interface sites is of ADD-type. ADD-sites possess an increased ability towards adaptive conformational changes thus enabling effective protein interaction. © 2010 Taylor & Francis Group, LLC.