My main research interest is in theoretical biochemistry, in particular reaction mechanisms of metalloenzymes. We use the tools of quantum chemistry, mainly hybrid-DFT, to study the reaction mechanism of various metalloenzymes. The emphasis is on systems with red-ox active transition metals, such as cytochrome c oxidase in the respiratory chain and photosystem II in green plants. In cytochrome oxidase molecular oxygen is reduced to water, and we are studying both the mechanism for the O-O bond cleavage step and the mechanism for the endergonic proton translocation across the membrane, which occurs coupled to the exergonic oxygen reduction.