Mössbauer investigation of the cofactor iron of putidamonooxin

Mononuclear non‐heme cofactor iron of putidamonooxin has been investigated in the binary oxidized ‘enzyme · substrate’ complex and in the ternary ‘enzyme · substrate · NO’ complex via Mössbauer spectroscopy. The experimental spectra were analyzed on the basis of the spin‐Hamiltonian formalism. The resulting fine and hyperfine structure parameters are compared with literature values of similar compounds. From this comparison we conclude that in the binary complex (reduced and oxidized) the mononuclear non‐heme cofactor iron has a coordination number higher than four. Additionally, the cofactor iron shows remarkable spectral similarities with iron in protocatechuate 3,4‐dioxygenase, though the catalytic properties of the iron sites in the two proteins are different. The data obtained form the ternary “enzyme · substrate · NO” complex indicate that the cofactor iron (a) is in the ferric intermediate spin state (S= 3/2) and (b) is pentacoordinated, which means that upon NO binding to the reduced cofactor iron at least one ligand has to be released. Comparing our data with literature values suggests that the cofactor iron in the binary as well as in the ternary NO complex is not directly bound to a sulfur atom, though biochemical arguments seem to indicate the opposite.