Heme Catabolism by Heme Oxygenase and Model Reaction: What We Have Learned About Reaction Mechanism

HO enzyme catalyzes the degradation of free heme to biliverdin, CO, and Fe in three consecutive steps. The up-regulated activity of heme oxygenase is thought to be correlated with the antioxidant role of HO-1 in an oxidative stress environment. HO enzyme regiospecificity oxidizes heme at α meso position in a three oxygenation steps process. Although hydroperoxy-ferric heme, generated in the first step, has been indicated as an intermediate in most heme enzyme, heme degradation is only occurred in HO reaction. Therefore, a different mechanism from the other heme enzymes must be responsible for the heme hydroxylation in HO-catalyzed reaction in the first step. In the second step of the process, there are also uncertainties regarding electron requirement and a binding site for O2 molecule. In this article, we review researchers’ attempts to elucidate complicated heme catabolism mechanism, especially in the less known process steps which have been done to shed light on the determinants of specificity and better contribute to developing new medicines. Special focus is placed on the experimental and theoretical studies on the structural characteristics and electronic configurations of heme catabolism intermediates catalyzed by HO. The fascinating electronic communication between the metal and the ring in this process has been discussed, as well. An overview of the non- precedent route for the direct conversion of oxophlorin to biliverdin is also presented.