The accessibility of large substrates to buried enzymatic active sites is dependent upon the utilization of proteinaceous channels. The necessity of these channels in the case of small substrates is questionable because diffusion through the protein matrix is often assumed. Copper amine oxidases contain a buried protein-derived quinone cofactor and a mononuclear copper center that catalyze the conversion of two substrates, primary amines and molecular oxygen, to aldehydes and hydrogen peroxide, respectively. The nature of molecular oxygen migration to the active site in the enzyme from Hansenula polymorpha is explored using a combination of kinetic, x-ray crystallographic, and computational approaches. A crystal structure of H. polymorpha amine oxidase in complex with xenon gas, which serves as an experimental probe for molecular oxygen binding sites, reveals buried regions of the enzyme suitable for transient molecular oxygen occupation. Calculated O 2 free energy maps using copper amine oxidase crystal structures in the absence of xenon correspond well with later experimentally observed xenon sites in these systems, and allow the visualization of O 2 migration routes of differing probabilities within the protein matrix. Site-directed mutagenesis designed to block individual routes has little effect on overall k cat /K m (O 2 ), supporting multiple dynamic pathways for molecular oxygen to reach the active site.Copper amine oxidases are ubiquitous copper containing enzymes that oxidize primary amines to aldehydes through the reduction of molecular oxygen to hydrogen peroxide. CAO 2 catalysis is dependent upon the protein-derived cofactor 2,4,5-trihydroxyphenylalaninequinone (TPQ). The TPQ is derived from an endogenous tyrosine through a self-catalytic process requiring only molecular oxygen and Cu(II) (see Fig. 1a) (1).Hansenula polymorpha 3 amine oxidase is the eukaryotic CAO that has been kinetically characterized in the most detail (2-7). HPAO follows a Bi Bi ping-pong reaction mechanism that can be expressed as two half-reactions, reductive and oxidative (see Fig. 1b). In the reductive half-reaction the enzyme oxidizes a primary amine to an aldehyde, generating the 2e Ϫ reduced aminoquinol form of the cofactor. In the subsequent oxidative half-reaction molecular oxygen is reduced to hydrogen peroxide via cofactor reoxidation to TPQ. Biochemical studies from several different laboratories have led to mechanistic proposals for the catalytic cycle of CAOs (8, 9). These studies have given significant insight into the mechanism for the reductive half-reaction (10). However, the details surrounding the activation of molecular oxygen both in terms of the biogenesis of the TPQ (11, 12) and of the catalytic oxidative half-reaction, remain the subject of intense study (13-16). The utilization of copper as a redox center has been the focus of recent controversy. Because CAOs contain a copper ion in their active site, chemical intuition suggests Cu(I) as the O 2 -activating species to give Cu(II)-superoxide (17). Upon a...