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 redox active transition metals, such as cytochrome c oxidase (CcO) in the respiratory chain and nitric oxide reductase (NOR) involved in the denitrification process in bacteria. In CcO molecular oxygen is reduced to water, and we are studying both the mechanism and energetics for the oxygen reduction process, and the mechanisms for the proton pumping across the membrane, which occurs coupled to the exergonic oxygen reduction. In NOR nitric oxide is reduced to nitrous oxide and water, and we are studying the mechanism and energetics of this reaction, with particular emphasis on comparisons between the two enzymes, CcO and NOR, which belong to the same family of heme-copper oxidases.