LOW-TEMPERATURE OPTICAL SPECTROSCOPY OF NATIVE AND AZIDE-REACTED BOVINE CU,ZN SUPEROXIDE-DISMUTASE - A STRUCTURAL DYNAMICS STUDY

The optical absorption spectra of native and N-3(-)-reacted Cu,Zn supe roxide dismutase (SOD) has been studied in the temperature range 300-1 0 K. The broad d-d bands observed in the room temperature spectrum, ce ntered at 14 700 cm(-1) (native enzyme) and at 15 550 cm(-1) (N-3(-)-r eacted enzyme), are clearly split at low temperature into two bands ea ch, centered at 12 835 and 14 844 cm(-1) and at 14 418 and 16 300 cm(- 1), respectively. The thermal behavior of the 23 720 cm(-1) band prese nt in the spectrum of the native enzyme indicates that this band belon gs to the His61-->Cu(IT) ligand to metal charge transfer transition. A nalysis of the zeroth, first, and second moments of the various bands as a function of temperature allowed us to obtain useful information o n the stereodynamic properties of the metal site in SOD. In particular for the native protein, it was possible to infer a variation in the m etal ligand relative position that occurs as the temperature is lowere d and that likely involves all of the ligands except His61. On the oth er hand, the site is stabilized upon N-3(-) binding, and in this case a variation in the metal ligand position is observed only at the level of the bound anion. The possible relation of these properties to the catalytic mechanism of the enzyme is discussed.