The two oxidized forms of the trinuclear Cu cluster in the multicopper oxidases and mechanism for the decay of the native intermediate

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Abstract
Multicopper oxidases (MCOs) catalyze the 4e− reduction of O2 to H2O. The reaction of the fully reduced enzyme with O2 generates the native intermediate (NI), which undergoes a slow decay to the resting enzyme in the absence of substrate. NI is a fully oxidized form, but its spectral features are very different from those of the resting form (also fully oxidized), because the type 2 and the coupled-binuclear type 3 Cu centers in the O2-reducing trinuclear Cu cluster site are isolated in the resting enzyme, whereas these are all bridged by a μ3-oxo ligand in NI. Notably, the one azide-bound NI (NIAz) exhibits spectral features very similar to those of NI, in which the μ3-oxo ligand in NI has been replaced by a μ3-bridged azide. Comparison of the spectral features of NI and NIAz, combined with density functional theory (DFT) calculations, allows refinement of the NI structure. The decay of NI to the resting enzyme proceeds via successive proton-assisted steps, whereas the rate-limiting step involves structural rearrangement of the μ3-oxo-bridge from inside to outside the cluster. This phenomenon is consistent with the slow rate of NI decay that uncouples the resting enzyme from the catalytic cycle, leaving NI as the catalytically relevant fully oxidized form of the MCO active site. The all-bridged structure of NI would facilitate electron transfer to all three Cu centers of the trinuclear cluster for rapid proton-coupled reduction of NI to the fully reduced form for catalytic turnover.