Molecular mechanism of the electron transfer reaction in cytochrome p450_cam-putidaredoxin: Roles of glutamine 360 at the heme proximal site

We characterized electron transfer (ET) from putidaredoxin (Pdx) to the mutants of cytochrome P450_cam (P450_cam), in which one of the residues located on the putative binding site to Pdx, Gln360, was replaced with Glu, Lys, and Leu. The kinetic analysis of the ET reactions from reduced Pdx to ferric P450_cam (the first ET) and to ferrous oxygenated P450_cam (the second ET) showed the dissociation constants (K_m) that were moderately perturbed for the Lys and Leu mutants and the distinctly increased for the Glu mutant. Although the alterations in K_m indicate that Gln360 is located at the Pdx binding site, the effects of the Gln360 mutations (0.66-20-fold of that of wild type) are smaller than those of the Arg 112 mutants (25-2500-fold of that of wild type) [Unno, M., et al. (1996) J. Biol. Chem. 271, 17869-17874], allowing us to conclude that Gln360 much less contributes to the complexation with Pdx than Arg 112. The first ET rate (35 s^-1 for wild-type P450_cam) was substantially reduced in the Glu mutant (5.4 s^-1), while less perturbation was observed for the Lys (53 s^-1) and Leu (23 s^-1) mutants. In the second ET reaction, the retarded ET rate was detected only in the Glu mutant but not in the Lys and Leu mutants. These results showed the smaller mutational effects of Gln360 on the ET reactions than those of the Arg 112 mutants. In contrast to the moderate perturbations in the kinetic parameters, the mutations at Gln360 significantly affected both the standard enthalpy and entropy of the redox reaction of P450_cam, which cause the negative shift of the redox potentials for the Fe³⁺/Fe²⁺ couple by 20-70 mV. Since the amide group of Gln360 is located near the carbonyl oxygen of the amide group of the axial cysteine, it is plausible that the mutation at Gln360 perturbs the electronic interaction of the axial ligand with heme iron, resulting in the reduction of the redox potentials. We, therefore, conclude that Gln360 primarily regulates the ET reaction of P450_cam by modulating the redox potential of the heme iron and not by the specific interaction with Pdx or the formation of the ET pathway that are proposed as the regulation mechanism of Arg 112.