We are interested in understanding how proteins, in particular hemeproteins, work within their cellular environment in health and disease. The three-dimensional structures of hemeproteins are designed by nature to perform a variety of biological functions raging from delivering electrons, transporting oxygen, sensing small gas molecules, to oxidizing inert organic substances. These biological activities are typically finely tuned by translational / post-translational regulations and/or by intermolecular interactions with small cellular metabolites or macromolecules in response to environmental stimuli. Our current research is focused on the comprehension of structure-and-function relationships of a group of human hemeproteins called dioxygenases, including tryptophan dioxygenase (TDO) and two isoforms of indoleamine 2,3-dioxygenase (IDO1 and IDO2), at the molecular level.