We combined normal mode analysis (NMA) with cavity calculations as a method to get snore insight into static crystal structures. We used nitrile hydratase (NHase) as a case study, and the crystal structure of a complex of Pseudonocardia thermophila NHase (1 UGP) with n-butyric acid was chosen as a reference structure. The reference structure wets compared with the other available NHase crystal structures. Cavity calculations of the static structures showed the entrances to the active site and also a possible function of the N-terminal in the substrate selection of the Co-type NHase. When NMA was combined with cavity calculations, a closing-opening passage was observed. Analysis of low frequency modes combined with cavity calculations led us to propose "breathing" and "flip-flop" mechanisms which might be a key part of the substrate binding mechanism. (c) 2006 Elsevier Inc. All rights reserved.