Geometry of the soluble methane monooxygenase catalytic diiron center in two oxidation states

The hydroxylase component of soluble methane monooxygenase (sMMO) contains a dinuclear iron center responsible for the oxidation of methane to methanol. As isolated, the center is in the oxidized, diiron(III) state. The 2.2 A resolution X-ray structure of the oxidized hydroxylase, Hox, from Methylococcus capsulatus (Bath) was previously determined at 4 degrees C. In this structure the two iron atoms are bridged by a glutamate, a hydroxide ion, and an acetate ion, and additionally coordinated to two His residues, three Glu residues, and a water molecule.

The 1.7 A resolution crystal structures of the sMMO hydroxylase from Methylococcus capsulatus (Bath) in both its oxidized diiron(III), Hox, and dithionite-treated, reduced diiron(II), Hred, oxidation states were determined at -160 degrees C. The structure of the diiron center in Hox differs from that previously reported at 2.2 A resolution and 4 degrees C. Although the hydroxide bridge is retained, the bidentate, bridging ligand assigned as acetate is replaced by a weakly coordinating monoatomic water bridge. In the resulting four-membered Fe(OH)Fe(OH2) ring, the Fe ... Fe distance is shortened from 3.4 A to 3.1 A. In protomer A of Hred, the hydroxide bridge is displaced by an oxygen atom of Glu243, which undergoes a carboxylate shift from its terminal monodentate binding mode in Hox to a mode in which the carboxylate is both monoatomic bridging and bidentate chelating. We therefore conclude that the center has been reduced to the diiron(II) oxidation state. Both iron atoms are coordinated to five ligands and weakly to a sixth water molecule in the resulting structure. The diiron center in protomer B of Hred has the same composition as those in Hox. In both the oxidized and reduced structures, the diiron core is connected through hydrogen bonds involving exogenous species to Thr213 in the active site cavity.

The diiron center in Hox can change its exogenous ligand coordination and geometry, a property that could be important in the catalytic cycle of sMMO. In Hred, a carboxylate shift occurs, extruding hydroxide ion and opening coordination sites for reaction with O2 to form the diiron(III) peroxo intermediate, Hperoxo. Residue Thr213 may function in catalysis.