Mechanistic comparison of the cobalt-substituted and wild-type copper amine oxidase from Hansenula polymorpha

A recent report by Mills and Klinman [Mills, S. A., and Klinman, J. P. (2000) J. Am. Chem. Soc. 122, 9897-9904] described the preparation and initial characterization of a cobalt-substituted form of the copper amine oxidase from Hansenula polymorpha (HPAO). This enzyme was found to be fully catalytically active at saturating substrate concentrations, but with a K(m) for O(2) approximately 70-fold higher than that of the copper-containing, wild-type enzyme. Herein, we report a detailed analysis of the mechanism of catalysis for the wild-type and the cobalt-substituted forms of HPAO. Both forms of enzyme are concluded to utilize the same mechanism for oxygen reduction, involving initial, rate-limiting electron transfer from the reduced cofactor of the enzyme to prebound dioxygen. Superoxide formed in this manner is stabilized by the active site metal, facilitating the transfer of a second electron and two protons to form the product hydrogen peroxide. The elevated K(m) for O(2) at the dioxygen binding site in Co-substituted HPAO, relative to that of wild-type HPAO, is proposed to be due to a change in the net charge at the adjacent metal site from +1 (cupric hydroxide) in wild-type enzyme to +2 (cobaltous H(2)O) in cobalt-substituted HPAO.