Interactions of various axial ligands with cytochrome P-450d wild type, proximal mutants (Lys453Glu, Ile460Ser), and putative distal mutants (Glu318Asp, Thr319Ala, Thr322Ala) expressed in yeast were studied with optical absorption spectroscopy. P-450d wild type and all five mutants were purified essentially as the high-spin form, but the putative distal mutants contained about 5% low-spin form. Bindings of metyrapone and 4-phenylimidazole to the wild type and all mutants formed nitrogen-bound low-spin forms. In contrast, binding of 2-phenylimidazole to the wild type and most of mutants formed oxygen-bond low-spin forms except for the mutant Glu318Asp in which the nitrogen-bound low-spin form was formed. By analogy with the distal structure of P-450cam, it was thus suggested that Glu318 of P-450d, which corresponds with Asp251 of P-450cam, somehow interacts with 2-phenylimidazole over the heme plane. Addition of 1-butanol and acetanilide, a substrate of P-450d, to the wild type and mutants caused the spin change to the low-spin form. The order of dissociation constants of these oxygen ligands to P-450d was wild type greater than proximal mutants greater than putative distal mutants. Spectral analyses showed that the binding of acetanilide is the same as that of another substrate, 7-ethoxycoumarin, in the putative distal mutants but is not the same in the wild type and proximal mutants. From these findings together with other spectral data, it was suggested that the region from Glu318 to Thr322 is located at the distal region of the heme in membrane-bound P-450d as suggested from the X-ray crystal structure of water-soluble P-450cam and amino acid alignments of P-450s.

Introduction:P450s have the conserved Thr(Thr252 for P450 cam , Thr319 for P450 1A2) in the (putative) distal site of the heme. This Thr seems to be important for the catalytic activities of P450s. Microsomal P450s have a conserved "Thr cluster" at the putative distal site. Thr319, Thr321, and Thr322 in P450 1A2 apparently compose the Thr cluster. An ionic amino acid such as Asp(Asp251 for P450 cam ) or Glu(Glu318 for P450 1A2) is also conserved at the adjacent amino acid position for P450s. To understand roles of those polar or ionic amino acids in the catalytic function, we made several mutants at this putative distal site of P450 1A2. Optical absorption spectra and catalytic activities of those mutants will be discussed in consideration of the activation of the O2 molecule.