(Figure Presented) Heme oxygenase (HO) is an enzyme that catalyzes the regiospecific conversion of heme to biliverdin IXα, CO, and free iron. In mammals, HO has a variety of physiological functions, including heme catabolism, iron homeostasis, antioxidant defense, cellular signaling, and O2 sensing. The enzyme is also found in plants (producing light-harvesting pigments) and in some pathogenic bacteria, where it acquires iron from the host heme. The HO-catalyzed heme conversion proceeds through three successive oxygenations, a process that has attracted considerable attention because of its reaction mechanism and physiological importance. The HO reaction is unique in that all three O2 activations are affected by the substrate itself. The first step is the regiospecific self-hydroxylation of the porphyrin α-meso carbon atom. The resulting α-meso-hydroxyheme reacts in the second step with another O2 to yield verdoheme and CO. The third O2 activation, by verdoheme, cleaves its porphyrin macrocycle to release biliverdin and free ferrous iron. In this Account, we provide an overview of our current understanding of the structural and biochemical properties of the complex self-oxidation reactions in HO catalysis. The first meso-hydroxylation is of particular interest because of its distinct contrast with O2 activation by cytochrome P450. Although most heme enzymes oxidize exogenous substrates by high-valent oxo intermediates, HO was proposed to utilize the Fe-OOH intermediate for the self-hydroxylation. We have succeeded in preparing and characterizing the Fe-OOH species of HO at low temperature, and an analysis of its reaction, together with mutational and crystallographic studies, reveals that protonation of Fe-OOH by a distal water molecule is critical in promoting the unique self-hydroxylation. The second oxygenation is a rapid, spontaneous auto-oxidation of the reactive α-meso-hydroxyheme; its mechanism remains elusive, but the HO enzyme has been shown not to play a critical role in it. Until recently, the means of the third O2 activation had remained unclear as well, but we have recently untangled its mechanistic outline. Reaction analysis of the verdoheme-HO complex strongly suggests the Fe-OOH species as a key intermediate of the ring-opening reaction. This mechanism is very similar to that of the first meso-hydroxylation, including the critical roles of the distal water molecule. A comprehensive study of the three oxygenations of HO highlights the rational design of the enzyme architecture and its catalytic mechanism. Elucidation of the last oxygenation step has enabled a kinetic analysis of the rate-determining step, making it possible to discuss the HO reaction mechanism in relation to its physiological functions.
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