STRUCTURAL BASIS FOR THE DIFFERENCE IN THERMODYNAMIC PROPERTIES BETWEEN THE 2 CYSTEINE PROTEINASE-INHIBITORS HUMAN STEFIN-A AND STEFIN-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 interactiv e modelling or positioning of the side chains by Monte Carlo procedure with simulated annealing. The quality of models was evaluated by calc ulation of the free energy of hydration, 3D-1D potential or buried are a of surface accessibility. Stefin A is a thermostable protein, exhibi ting a two-state denaturation, while stefin B denatures at a 40 degree s C lower temperature and forms a stable molten globule intermediate u nder mild denaturing conditions. From the tertiary structures, thermod ynamic functions were predicted, conforming closely to the experimenta l calorimetric results. Polar and apolar buried areas of surface acces sibility 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 increase d numbers of hydrogen bonds which stabilize it and increase the cooper ativity of its denaturation.