Structure of activated aconitase: formation of the 4Fe-4S cluster in the crystal.

The structure of activated pig heart aconitase citrate(isocitrate) hydro-lyase, EC 4.2.1.3 containing a 4Fe-4S cluster has been refined at 2.5-A resolution to a crystallographic residual of 18.2%. Comparison of this structure to the recently determined 2.1-A resolution structure of the inactive enzyme containing a 3Fe-4S cluster, by difference Fourier analysis, shows that upon activation iron is inserted into the structure isomorphously. The common atoms of the 3Fe-4S and 4Fe-4S cores agree within 0.1 A; the three common cysteinyl S gamma ligand atoms agree within 0.25 A. The fourth ligand of the Fe inserted into the 3Fe-4S cluster is a water or hydroxyl from solvent, consistent with the absence of a free cysteine ligand in the enzyme active site cleft and the isomorphism of the two structures. A water molecule occupies a similar site in the crystal structure of the inactive enzyme.