Superoxide reductase is a 14 kDa metalloprotein containing a catalytic nonhaem
iron centre [Fe(His)4Cys]. It is involved in defence mechanisms against
oxygen toxicity, scavenging superoxide radicals from the cell. The oxidized form
of Treponema pallidum superoxide reductase was crystallized in the presence of
polyethylene glycol and magnesium chloride. Two crystal forms were obtained
depending on the oxidizing agents used after purification: crystals grown in the
presence of K3Fe(CN)6 belonged to space group P21 (unit-cell parameters
a = 60.3, b = 59.9, c = 64.8 A ° , = 106.9 ) and diffracted beyond 1.60 A ° resolution,
while crystals grown in the presence of Na2IrCl6 belonged to space group C2
(a = 119.4, b = 60.1, c = 65.6 A ° , = 104.9 ) and diffracted beyond 1.55 A ° . A
highly redundant X-ray diffraction data set from the C2 crystal form collected
on a copper rotating-anode generator ( = 1.542 A ° ) clearly defined the positions
of the four Fe atoms present in the asymmetric unit by SAD methods. A MAD
experiment at the iron absorption edge confirmed the positions of the previously
determined iron sites and provided better phases for model building and
refinement. Molecular replacement using the P21 data set was successful using a
preliminary trace as a search model. A similar arrangement of the four protein
molecules could be observed
