ABSTRACT
Copper amine oxidases [amine:oxygen oxidoreductase (deaminating); EC 1.4.3.6; CAOs] contain a redox-active organic cofactor, 2,4,5-trihydroxyphenylalanine quinone (TPQ) that is essential for catalyzing the oxidation of primary amines by dioxygen to the corresponding aldehydes, ammonia, and hydrogen peroxide. The catalytic mechanism can be separated into two half-reactions: the enzyme reduction by the amines (reductive half-reaction; Equation 3.1) and the subsequent enzyme reoxidation by dioxygen (oxidative half-reaction; Equation 3.2):
CAOox(TPQox) + R-CH2-NH3+ → CAOred-NH2(TPQaq) + R-CHO (3.1)
CAOred-NH2 + O2 + H2O → CAOox + NH4+ + H2O2 (3.2)
Each monomer of a dimeric CAO also encompasses one Cu(II) ion as an inorganic cofactor, which is electron paramagnetic resonance (EPR)-detectable non-blue (or type 2) copper (Lindley, 2001). The yellowish pink color of CAOs stems from an intense absorption band in the 460-to 500-nm region assigned to TPQox. The weak d-d bands of the Cu(II) centers are hidden in the intense TPQox band, but the two circular dichroism (CD) extrema at 660 (+) and 810 (–) nm suggest the existence of d-d transitions (Suzuki et al., 1980). The EPR signals of CAOs have an axial character with g// > g⊥ > 2, indicating that the Cu(II) ion has a tetragonal geometry.
