Experimental and computational evidence of metal-O2 activation and rate-limiting proton-coupled electron transfer in...

The mechanism of O(2) reduction by copper amine oxidase from Arthrobacter globiformus (AGAO) is analyzed in relation to the cobalt-substituted protein. The enzyme utilizes a tyrosine-derived topaquinone cofactor to oxidize primary amines and reduce O(2) to H(2)O(2). Steady-state kinetics indicate that amine-reduced CuAGAO is reoxidized by O(2) >10(3) times faster than the CoAGAO analogue. Complementary spectroscopic studies reveal that the difference in the second order rate constant, k(cat)/K(M)(O(2)), arises from the more negative redox potential of Co(III/II) in relation to Cu(II/I). Indistinguishable competitive oxygen-18 kinetic isotope effects are observed for the two enzymes and modeled computationally using a calibrated density functional theory method. The results are consistent with a mechanism where an end-on (η(1))-metal bound superoxide is reduced to an η(1)-hydroperoxide in the rate-limiting step.