Oxygen activation in a copper-containing amine oxidase

The process by which molecular oxygen is activated to enable it to function as an electron acceptor in biology is poorly understood. The quinoprotein copper-containing amine oxidase (CuAO) catalyses the conversion of primary amines into aldehydes. As well as copper, the enzyme contains an organic cofactor, 2,4,5-trihydroxyphenylalanine quinone (TPQ). Following the formation of aldehyde, the enzyme is left as the two-electron reduced aminoquinol form. Reoxidation of the enzyme back to the resting state uses molecular oxygen, which is reduced to H₂O₂ in the process, with the additional release of NH₃. To understand the structural basis of oxygen activation in Escherichia coli CuAO (ECAO), catalytically competent crystals were used to trap catalytic intermediates by exposing then to amine substrate and then freeze-trapping under aerobic and anaerobic conditions. Single-crystal visible microspectrophotometry was used to probe the oxidation state of the quinone in the intermediates, as TPQ exhibits a rich palette of colour changes during catalytic turnover. This review will focus on one of these structures, that of the rate-determining species in the crystal under steady-state conditions. This structure has revealed many details regarding oxygen activation in ECAO, including the site of dioxygen binding, and the proton-transfer pathways involved in H₂O₂ formation.