Bacterial heme-copper terminal oxidases react quickly with NO to form a heme - nitrosyl complex, which, in some of these enzymes, can further react with a second NO molecule to produce N 2O. Previously, we characterized the heme a 3-NO complex formed in cytochrome ba 3 from Thermus thermophilus and the product of its low-temperature illumination. We showed that the photolyzed NO group binds to Cu B(I) to form an end-on NO-Cu B or a side-on copper-nitrosyl complex, which is likely to represent the binding characteristics of the second NO molecule at the heme-copper active site. Here we present a comparative study with cytochrome bo 3 from Escherichia coli. Both terminal oxidases are shown to catalyze the same two-electron reduction of NO to N 2O. The EPR and resonance Raman signatures of the heme o 3-NO complex are comparable to those of the a 3-NO complex. However, low-temperature FTIR experiments reveal that photolysis of the heme o 3-NO complex does not produce a Cu B-nitrosyl complex, but that instead, the NO remains unbound in the active-site cavity. Additional FTIR photolysis experiments on the heme-nitrosyl complexes of these terminal oxidases, in the presence of CO, demonstrate that an [o 3-NO·OC-Cu B] tertiary complex can form in bo 3 but not in ba 3. We assign these differences to a greater iron-copper distance in the reduced form of bo 3 compared to that of ba 3. Because this difference in metal-metal distance does not appear to affect the NO reductase activity, our results suggest that the coordination of the second NO to Cu B is not an essential step of the reaction mechanism.
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