The general goal of my research is to elucidate enzyme mechanisms, in particular for redox active enzymes containing transition metals. For this purpose quantum chemical methods (mainly hybrid Density Functional Theory, DFT) are used to study biochemical model systems. My main project concerns mechanisms for enzymes that reduce molecular oxygen and/or nitric oxide. Therefore I have studied the reactions in heme-copper oxidases (cytochrome oxidase and nitric oxide reductase) for many years. In recent years I study also the mechanisms for another group of enzymes which perform the same reactions, the flavin dependent non-heme diiron proteins (FDPs). I have determined the mechanisms for both NO and O2 reduction in the FDPs from Thermotoga maritima and Desulfovibrio gigas, as well as in variants of Thermotoga maritima FDP. Papers reporting the results from those studies were published in 2023 in the journal ACS Catalysis and in 2024 and 2025 in the Journal of Inorganic Biochemistry. One purpose of these studies is to try to explain differences in reduction activities between the different FDPs. During the last year I have therefore extended the study to include an FDP from still another species, Escherichia coli, which seems to be the only FDPs having a larger NO reduction activity compared to the O2 reduction activity. This is an ongoing project, which include also studies of variants of Escherichia coli, for which there are experimental results available. For this purpose larger models of the active sites are needed, which makes the calculations more time-consuming. To further continue the investigation on the role of the second sphere residues I am also performing calculations on an FDP-model from still another species, Methanohermobacter marburgensis. This FDP is of particular interest because it is missing a second sphere tyrosine that is conserved in all other FDPs, and from the available experimental information it seems to only reduce molecular oxygen and not nitric oxide. A complication of this part of my studies is that there seems to be some problems with the crystal structure of this species. I have a close collaboration with the experimental groups working on these types of reactions at Stockholm University (in particular Pia Ädelroth), which has turned out to be very fruitful.