Heme a3-NO interactions in reduced cytochrome c oxidases: binding and recombination dynamics

Abstract : Cytochrome c oxidase (CcO) has a high affinity for nitric oxide (NO), a property in the regulation of respiration [1]. We have previously shown that recombination kinetics of reduced CcO -NO from P. denitrificans on the picosecond time scale depend strongly on the NO:enzyme stoichiometry and inferred that more than one NO can be accomodated by the active site [2], already at mildly suprastoichimetric NO concentrations. We have now largely extended these studies by studying rebinding dynamics from the picosecond up to the microsecond time scale, by performing parallel steady-state EPR characterizations under similar conditions as the optical experiments, and comparing them with molecular modeling results. A comparative study was performed on CcO ba3 from T. thermophilus, where two NO molecules cannot be copresent in the active site in steady state due to its NO reductase activity [3]. The kinetic results allow to discriminate between different models of NOdependent recombination and show that the overall NO-escape probability out of the protein is high when only one NO is bound to aa3, whereas strong rebinding on the 15-ns time scale was observed for ba3. The EPR characterizations show similar results for aa3 at substoichiometries NO:enzyme ratios and for ba3, indicating formation of a six-coordinate heme-NO complex. The presence of a second NO molecule in the aa3 active site strongly modifies the heme-NO EPR spectrum, and can be rationilized by a rotation of the NO with respect to the proximal histidine. This proposal is consistent with molecular modeling studies. The binding properties of the second NO molecule will also be discussed.
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Marten Vos, Eric Pilet, W. Nitschke, F. Rappaport, T. Soulimane, et al.. Heme a3-NO interactions in reduced cytochrome c oxidases: binding and recombination dynamics. 13th EUROPEAN BIOENERGETICS CONFERENCE, Aug 2004, Pisa, Italy. pp.159. ⟨hal-00845029⟩

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