Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine beta-monooxygenase (DBM) are discussed using QM/MM calculations for a whole-enzyme model of 4700 atoms. A calculated activation barrier for the hydrogen-atom abstraction by the Cu(II)-superoxo species is 23.1 kcal/mol, while that of the Cu(III)-oxo, which can be viewed as Cu(II)-O-center dot, is 5.4 kcal/mol. Energies of the optimized radical intermediate in the superoxo- and oxo-mediated pathways are 18.4 and -14.2 kcal/mol, relative to the corresponding reactant complexes, respectively. These results demonstrate that the Cu(Ill)-oxo species can better mediate dopamine hydroxylation in the protein environment of DBM. The side chains of three amino acid residues (His415, His417, and Met490) coordinate to the Cu-B atom, one of the copper sites in the catalytic core that plays a role for the catalytic function. The hydrogen-bonding network between dopamine and the three amino acid residues (Glu268, Glu369, and Tyr494) plays an essential role in substrate binding and the stereospecific hydroxylation of dopamine to norepinephrine. The dopamine hydroxylation by the Cu(Ill)-oxo species is a downhill and lower-barrier process toward the product direction with the aid of the protein environment of DBM. This enzyme is likely to use the high reactivity of the Cu(Ill)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism.