COMPARATIVE-STUDY OF THERMOSTABILITY AND STRUCTURE OF CLOSE HOMOLOGS - BARNASE AND BINASE

Parameters of heat denaturation and intrinsic fluorescence of barnase and its close homologue, binase in the pH region 2-6 have been determi ned. The barnase heat denaturation (pH 2.8-5.5) proceeds according to the 'all-or-none' principle. Barnase denaturation temperature is lower than that of binase and this difference increases from 2.5-degrees-C at pH 5 to 7-degrees-C at pH 3. Enthalpy values of barnase and binase denaturation coincide only at pH 4.5-5.5, but as far as pH decreases t he barnase denaturation enthalpy decreases significantly and in this r espect it differs from binase. The fluorescence and CD techniques do n ot reveal any distinctions in the local environment of aromatic residu es in the two proteins, and the obtained difference in the parameters of intrinsic fluorescence is due to fluorescence quenching of the barn ase Trp94 by the His 18 residue, absent in binase. Secondary structure s of both native and denaturated proteins also do not differ. Some dif ferences in the barnase and binase electrostatic characteristics, reve aled in the character of the dipole moments distribution, have been fo und.