The structure of iron superoxide dismutase (EC 1.15.1.1) from Escherichia ooli has been determined at 3. i-A resolution. The dimeric molecule is constructed from identical subunits, which are two-domain polypeptides. The NH2-terminal domain is composed of two antiparallel crossing helices and the COOH-terminal domain is a three-layered structure characterized by mixed a/.8 secondary structural features. The active center iron atoms, separated-by 18 A and located near the monomermonomer interface, are coordinated by two amino acid residues from each domain. Azide binding has been investigated by using difference Fourier techniques. Consistent with the notion of the independent evolution of the copper/zinc dismutase gene, the iron dismutase structure resembles the copper/zinc protein at neither the monomer nor the dimer level.Superoxide dismutases (EC 1.15.1.1) are metalloproteins that catalyze the dismutation of superoxide ions to oxygen and hydrogen peroxide (reaction 1).2°2+ 2H+ -H202+°2 (20). X-ray intensities were measured at 40C by using an automated diffractometer and Cu Ka radiation. Two heavy atom derivatives with anomalous contributions were used to determine the phases at 3.13-A resolution; these were obtained by first transferring the crystals to buffered, saturated lithium sulfate solutions and then soaking in saturated mercurous acetate or in 0.7 mM ethylmercury phosphate. This procedure afforded crystals having one mercurous acetate-or one ethylmercury phosphatebinding site per subunit. The intensities of Friedel pairs were measured for all mercurous acetate crystals and in the resolution range 3.56-3.13 A for the ethylmercury phosphate crystals. Intensities for a third derivative, prepared by using 5 mM silver nitrate, were also included in the phasing calculations to 5.9-A resolution.At 3.13-A resolution with the -mean figure of merit, (m), at 0.70, the polypeptide chain could be followed through each monomer unit. The clarity of the map was further enhanced by averaging the density of the two subunits forming the dimer. The coordinate transformation relating the two subunits was first approximated from the positions of the Fe and heavy atom binding sites and equivalent features such as helix termini; the transformation was then refined by comparing boxes of electron density with volumes of about 104 A3, located near the centers of the subunits. Within the error of the calculation, the monomers prove to be related by a simple dyad. (The calculated rotation about the local axis is 178.80 and the translation along the axis is 0.02 A.) Both refinement and averaging were conducted with modifications of W. A. Hendrickson's LSQMAP program. We have also inverted electron density maps in which monomer densities had been averaged and the solvent set to a uniform background level (21). Combination of the resulting 3884 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §173...

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Crystals suitable for X-ray diffraction studies of iron-containing superoxide dismutase from Escherichia COG have been grown using ammonium sulfate as the precipitant. The space group is P2,212, with a = 81.8 A, b = 752 A, c = 71.3 A. A dimer of >w, = 40,000 occupies the asymmetric unit.