Mechanism of formation of [Fe-O-O-H, Cu-B] from [Fe-O-O, Cu-B] in active site of cytochrome c oxidase was theoretically investigated. It was found that the H2O molecule is hydrogen-bonded to both Tyr244 and His290, and is a key molecule to produce Fe-O-O-H. This H2O molecule plays a role as a carrier of a proton toward Fe-O-O to yield [Fe-O-O-H, Cu-B]. Cu-B atom is formally changed from cuprous state to cupric state through transportation of a proton by the H2O molecule. A new reaction mechanism, which we call 'water-proton transport' (WPT) mechanism, is proposed for the initial stage of reduction of the O-2 molecule. (C) 2003 Elsevier Science B.V. All rights reserved.