Conformational Transition of Heme A Reconstituted Myoglobin Upon Reaction with Hydrogen Peroxide

Cheng, Wai Man
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University of Hawaii at Manoa
Cytochrome c oxidase (CcO) is responsible for catalyzing the four electron reduction of dioxygen to water associated with the terminal step of the electron transfer system in mitochondria. The reaction is proposed to involve a bridged intermediate with a peroxy linkage between ferric heme a complex and a cupric ion. In order to study the conformational changes in CcO associated with the formation of the peroxy intermediates during the reduction of dioxygen, the effects of hydrogen peroxide binding to heme A reconstituted myoglobin was examined using UV optical absorption spectroscopy. The reactivity of heme A-Mb and native myoglobin (native Mb) toward H2O2 degradation was also compared to probe the relationship between structural aspects of heme A and catalytic function essential for its role in the CcO catalytic cycle. The rate constants of heme A-Mb and native Mb toward H2O2 degradation were 34.6 M-lsec-1 and 488 M-lsec-1, respectively. Both reactions were found to be pseudo first order reaction. The data indicates the structural differences between heme A and heme b have an effect on the accessibility of the incoming ligands toward the heme center. In the spectroscopic study of catalytic intermediates associated with H2O2 degradation in heme A-Mb and native Mb, the presence of an absorption band attributed to the bridgen heme-peroxide-Cu intermediate in CcO implies a different oxidative intermediate in catalytic cycle of CcO.
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