The Active Site of CU,Zn Superoxide Dismutase as Studied by EXAFS: The Binding of Chloride to the Reduced Enzyme A. DESIDERI, S. MORANTE, G. ROTILIO Dipartimento di Biologia e Fisica, II Universita di Roma, Tor Vergata Roma, Italia. INTRODUCTION Bovine erithrocyte copper-zinc superoxide dismutase Cu(II)-Zn(II)SOD is a dimer made of two equivalent subunits each containing one copper and one zinc atom. X-ray diffraction study (1) on crystals of native CU,Zn SOD have shown the copper to be coordinated to four histidines (His 44, His 46, His 61 and His 118) and one molecule of water. One of the histidines (His 61) provides an imidazolate brldge between the copper and the zinc. The biological role of this enzyme is to disproportionate the toxic by the following catalytic reactions: - 2+ + O 2 + E-Cu ---- O 2 + E-Cu + E-Cu+ + 2H+ -- H 2 0 2 + E-Cu 2 + Since is an anion and reacts with both the oxidized and reduced form of the enzyme it has been of great interest to study the inte- raction of the enzyme woth other single charged anions with regard to their mode of binding at the active site and their inhibition of enzyme activity. This problem has been extensively investigated by spectroscopic means such as EPR, NMR, electronic absorption, ENDOR, on the oxidized enzyme (2-4) but not on the reduced one, as the Cu(I) atom is magnetically and optically silent. Therefore it is reasonable to undertake an investigation of the interaction of the reduced enzyme by EXAFS, the only spectroscopic technique that may probe the surrounding of the copper irrespective of its oxidation state. In particular in this work an EXAFS investigation on the re- duced enzyme with the Cl- anion is reported. A direct binding of Cl- to the reduced form of bovine CU,Zn SOD was suggested by observing 3ge !inewidth at half height of the Cl resonance as monitored by Cl NMR spectroscopy (5). On the other hand a more recent 35Cl- NMR investigation (6) claims that A. Bianconi et al. (eds.), Biophysics and Synchrotron Radiation © Springer-Verlag Berlin Heidelberg 1987