Cysteine dioxygenase is a unique nonheme iron enzyme involved in the metabolism of cysteine in the body. It contains an iron active site with an unusual 3 His ligation to the protein, which contrasts the structural features of common nonheme iron dioxygenases. Recently, some of us reported the first truly biomimetic model for this enzyme, namely a trispyrazolylborato iron II cysteinato complex, which not only has a structure very similar to the enzyme substrate complex but also represents a functional model Treatment of the model with dioxygen leads to cysteine dioxygenation as shown by isolating the cysteine part of the product in course of work up. However, little is known on the catalytic mechanism and so far not even the structure of the actual product complex had been characterized, which is unknown in case of the enzyme, too. In a multidisciplinary approach including Density Functional Theory calculations, and X ray absorption spectroscopy XAS , we have now determined the structure of the actual sulfinato complex for the first time. The Cys SO2 functional group was found to be bound in an amp; 61544;2 O,O coordination mode, which based on the excellent resemblance between model and enzyme also provides first support for a corresponding binding mode within the enzymatic product complex. Indeed this is confirmed again by theory, which had predicted a amp; 61544;2 O,O binding mode for