The validity of the proton transfer via a peptide bond in the novel proton translocation pathway of cytochrome c oxidase has been assessed with quantum mechanical calculations. When the proton affinity of heme a propionate decreases, a proton could be passed to an aspartate side-chain through a peptide bond, forming imidic acid followed by the conversion to an enol peptide bond. The enol tautomer could be converted to the keto state by a proton wiring mechanism. This proton wiring keto-enol tautomerism is the rate-determining step and is responsible for the unidirectional character of the proton translocation. (c) 2006 Elsevier B.V. All rights reserved.