Comparison of heme environment at the putative distal region of P-450s utilizing their external and internal nitrogenous ligand bound forms.

Thr-303 to Lys-mutated P-450 2E1, as well as Thr-301 to Lys-mutated P-450 2C2, had absorption spectra characteristic of a nitrogenous ligand-bound form of P-450, such as the pyridine complex of P-450 2E1; (i) in the ferric state, the red-shifted Soret band, compared with the typical low-spin type spectrum of P-450, and the more intense beta band than the alpha band and (ii) in the ferrous state, two Soret peaks at around 447 and 422 nm, the relative intensities of which depended on pH, indicating the existence of two interconvertible states. The equilibrium between the two states of the mutated P-450 2E1 appeared to be shifted toward the 422 nm state, compared with the mutated P-450 2C2. The corresponding mutant of P-450 2C14 had similar spectral properties to those of both mutated P-450s except that the shorter of the two Soret bands of its ferrous form was relatively broad and appeared at 418 nm. These findings suggest that the epsilon-amino-nitrogen of the Lys of the mutated P-450s is located in the appropriate position to occupy the sixth coordination position with the heme iron and spatial differences exist in the essentially conserved structure of the distal heme domain among the three ferrous Lys-mutated P-450s.