COMPARISON OF THERMOSTABILITY AND STRUCTURE OF CLOSELY HOMOLOGOUS RIBONUCLEASES OF BACILLUS-AMYLOLIQUEFACIENS AND BACILLUS-INTERMEDIUS 7P

Parameters of heat denaturation and intrinsic fluorescence of barnase and its close homologue, binase, in the acidic medium have been determ ined. Barnase is denatured at pH 2.8-5.5 according to the ''all-or-non e'' principle. The denaturation temperature is lower for barnase than for binase: the difference increases from 2.5 degrees C at pH 5 to 7 d egrees C at pH 3. The denaturation is accompanied by similar enthalpy changes only at pH 4.5-5.5; at lower pH values it is significantly dec reased in the case of barnase but not binase. Fluorescence and circula r dichroism measurements do not reveal any differences in the immediat e environment of aromatic residues in the two proteins. The observed d ifference in intrinsic fluorescence parameters is explained by quenchi ng of Trp-94 fluorescence in barnase by His-18, which is absent from b inase. Secondary structures of both native and denatured proteins do n ot differ either. Some differences, manifest in the dipole moment dist ribution, were noted in the electrostatic properties of barnase and bi nase.