Heme is an essential cofactor which binds to a diverse group of proteins. Many of these proteins bind heme with high affinity and are well-characterized due to their involvement in essential activities such as oxygen transport, catalysis of oxygen-dependent reactions, and participation in electron transfer. In contrast, much less is known about proteins that bind heme with lower affinity. Many of these proteins are involved in heme trafficking and cellular signaling, suggesting that study of these proteins is essential to our understanding of heme homeostasis in human health and disease. My research focuses on characterizing the heme binding properties and the functional aspects of a specific set of heme-binding proteins that contain heme regulatory motifs (HRMs), which are short amino acid sequences containing a Cys-Pro core that in many, but not all, cases bind heme with the Cys of the HRM acting as an axial ligand. A protein that contains HRMs and is directly involved in regulating cellular heme levels is heme oxygenase-2 (HO2), the protein that has been the primary focus of my research. HO2 binds heme in its catalytic core and, with cytochrome P450 reductase and NADPH, degrades heme to biliverdin. HO2 contains two HRMs that we propose function as “heme storage sites”, holding heme in reserve to transfer it in a rapid and specific manner to the catalytic core as needed in a direct, protein mediated transfer of heme from the HRMs to the catalytic site. Humans and other amniotes express HO2 as well as the other heme oxygenase isoform, HO1, which does not contain HRMs. As the HRMs are the major distinguishing feature between the two HO isoforms, my research has also focused on investigating aspects of HO2 that distinguish it from HO1 to further our understanding of why humans might express both HO2 and HO1.