Dioxygen‐activating iron center in putidamonooxin

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Abstract
The mononuclear non-heme Fe center is the dioxyen-binding site of putidamonooxin which is the dioxygen-activating component of the 4-methoxybenzoate monooxygenate [of Pseudomonas putida]. Replacement of dioxygen by nitrosyl leads to the formation of a rather stable Fe3+ .cntdot. NO- complex which is characterized by ESR at g .apprxeq. 4 and g .apprxeq. 2. The ESR features can be composed by 2 spectral components which are characterized by different tetragonal distortions of the axial symmetry. Binding of 4-hydroxybenzoate, which is the product of the enzymatic reaction, leads to the formation of an ESR spectrum with pure axial asymmetry. After binding of 4-methoxybenzoate, i.e., the physiological substrate of the monooxygenase, only 1 spectral component, i.e., that with a small tetragonal distortion, is observed. Binding of substrate analog, like 4-aminobenzoate and 4-trifluoromethylbenzote, leads to a spectral heterogeneity with variable amounts of the ESR component with a large tetragonal distortion. Benzoate induces an ESR spectrum with only that spectral component with large tetragonal distortion. The Fe-depleted substrate-free form of the enyzme, ligated with NO, also shows ESR heterogeneity, i.e., both spectral components overlap, with 60% of the component with large tetragonal distortion. Binding of 4-methoxybenzoate leads to the occurrence of a pure spectrum, i.e., with small tetragonal distortion, whereas binding of benzoate leads to a pure spectrum with large tetragonal distortion. Thus, the structural heterogeneity is removed by binding of both the ligand (NO) and substrate. The Fe3+ .cntdot. NO+ complex is discussed as an analog of the native oxy complex Fe3+ .cntdot. O2-.

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