During the monooxygenase reaction catalyzed by cytochrome P450(cam) (P450(cam)), a ternary complex of P450(cam), reduced putidaredoxin, and d- camphor is formed as an obligatory reaction intermediate. When ligands such as CO, NO, and O2 bind to the heme iron of P450(cam) in the intermediate complex, the EPR spectrum of reduced putidaredoxin with a characteristic signal at 346 millitesla at 77 K changed into a spectrum having a new signal at 348 millitesla. The experiment with O2 was carried out by employing a mutant P450(cam) with Asp251 → Asn or Gly where the rate of electron transfer from putidaredoxin to oxyferrous P450(cam) is considerably reduced. Such a ligand-induced EPR spectral change of putidaredoxin was also shown in situ in Pseudomonas putida. Mutations introduced into the neighborhood of the iron-sulfur cluster of putidaredoxin revealed that a Ser44 → Gly mutation mimicked the ligand-induced spectral change of putidaredoxin. Arg109 and Arg112, which are in the putative putidaredoxin binding site of P450(cam), were essential for the spectral changes of putidaredoxin in the complex. These results indicate that a change in the P450(cam) active site that is the consequence of an altered spin state is transmitted to putidaredoxin within the ternary complex and produces a conformational change of the 2Fe-2S active center.
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