Structural basis for the difference in thermodynamic properties between the two cysteine proteinase inhibitors human stefins A and B.

Homology modelling has been used to model stefin A based on the X-ray structure of stefin B. Several models have been produced by interactive modelling or positioning of the side chains by Monte Carlo procedure with simulated annealing. The quality of models was evaluated by calculation of the free energy of hydration, 3D-1D potential or buried area of surface accessibility. Stefin A is a thermostable protein, exhibiting a two-state denaturation, while stefin B denatures at a 40 degrees C lower temperature and forms a stable molten globule intermediate under mild denaturing conditions. From the tertiary structures, thermodynamic functions were predicted, conforming closely to the experimental calorimetric results. Polar and apolar buried areas of surface accessibility were obtained by structural deconvolution of the thermograms. It is suggested that the basic difference between the stefins is the domination of hydrophobic interaction in the stabilization of stefin B, which is due to its non-specific nature leading to the formation of a molten globule intermediate. Modelling of stefin A predicts increased numbers of hydrogen bonds which stabilize it and increase the cooperativity of its denaturation.