Abstract
The Co(II) substitution of nitrite reductases (NIRs) containing types 1 and 2 Cu from two denitrifying bacteria ( Achromobactercycloclasters IAM 1013 and Alcaligenes xylosoxidans GIFU 1051) has been carried out. The Co(II) derivatives have been characterized by electronic absorption and magnetic circular dichroism (MCD) spectroscopies. The tetrahedral Co(II) centers replaced type 1 Cu show the MCD bands at 300–500 nm (CysS −åCo(II) charge transfer transitions) and 600–700 nm (d-d transitions of Co(II), which are quite similar to those of Co(II)-substituted plastocyanin. The Co(II) ion will have a donor set of 2His, Cys and Met residues. The enzyme-substituted Co(II) ions for type 2 Cu exhibit the characteristic MCD bands due to d-d transitions in the region of 500–600 nm, which are quite similar to those of half Co(II)-substituted hemocyanin and suggest a tetrahedral Co(II) center bound to three histidyl imidazoles and one water molecule. The two types of Co(II) in NIRs are easily distinguishable by MCD spectroscopy. Moreover, the electronic absorption bands of Co(II) NIRs are assigned to types 1 and 2 Co on the basis of the MCD data.
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