Heme oxygenases (HOs) oxidize heme to biliverdin, iron, and carbon monoxide (CO) via a complex, three-stage process that requires seven electrons and three molecules of O2. Interest in the heme oxygenases and their mechanisms has grown exponentially over the past few years as persuasive evidence has accumulated that these enzymes and their products, particularly CO and biliverdin, are physiologically important and clinically significant. CO functions as a messenger akin to nitric oxide, in part through activation of guanylyl cyclase, but also through action at other receptor and target sites. Furthermore, CO and biliverdin provide strong protection against inflammation, organ rejection in heart, kidney, and liver transplants, and neurological and cardiovascular disorders. HO itself has become a target of potential anticancer therapeutics due to its demonstrated antiapoptotic role. In previous work, we have used crystallography and NMR to elucidate the structure and mechanism of HO-1, one of the two active forms of the enzyme in humans. These studies have provided a cinematographic view of the conformations of the enzyme as it proceeds through the three-step oxidative process. In contrast, relatively little is known definitively about the structure and mechanism of HO-2, the second, constitutive form of human heme oxygenase. This enzyme differs from HO-1 in that it is a longer polypeptide and incorporates three heme regulatory motifs (HRMs), each of which has one cysteine residue. HO-1 has no cysteines. HO-2 has been specifically shown to be involved in the regulation of a potassium channel and control of a circadian rhythm clock. We now propose to elucidate the structural and mechanistic features of HO-2 that distinguish it from HO-1 and that enable it to fulfill unique biological functions. Heme oxygenases degrade the heme group of hemoglobin and other hemoproteins to free iron, carbon monoxide, and biliverdin, which is, in turn, converted to bilirubin. The past decade has shown that the human heme oxygenases and the products they generate play critical roles as antioxidants, signaling molecules, and in maintenance of the body's iron supply. As a result the heme oxygenases suppress inflammation, help to prevent tissue rejection in transplants, are antiatherosclerotic, and play other important roles. At the same time, their antiapoptotic activity makes them targets for the design of inhibitors of potential utility in anticancer therapy.