Hydrogen Bonding Activates Ferric Porphyrin Hydroperoxo Species and Drives the Regioselective Heme Oxygenase Reaction.

Heme oxygenase (HO) is responsible for the controlled breakdown of heme in most living organisms, which avoids heme toxicity and provides essential biochemical components like biliverdin and the signaling molecule CO. The reactive oxidant in HO is proposed to be a ferric hydroperoxide species, which attacks a specific meso carbon center of the heme. Both site-directed mutagenesis and density functional theory (DFT) calculations have suggested a dominant role of the hydrogen bonding present in the second sphere of the HO active site in the reactivity of HO. An iron porphyrin with a guanidine group in the second sphere, mimicking the arginine group present in the distal site of HO, is used to generate a ferric hydroperoxide species by sequential electron and proton transfer to a ferrous dioxygen (or ferric superoxide) adduct in a fashion similar to that of HO. Spectroscopic data on these species indicate that the hydroperoxide ligand is in direct hydrogen-bonding interaction with the distal guanidine residue. This ferric hydroperoxide species of this porphyrin is found to regioselectively hydroxylate the meso carbon adjacent to the guanidine group, which gradually degrades further to generate verdoheme and biliverdin, mimicking the function of HO. Experimental data and DFT calculations indicate that the hydrogen bonding from the second sphere guanidine group (1) activates the O-O bond of the ferric hydroperoxide for homolytic cleavage and (2) directs the resultant hydroxyl radical to the closest meso carbon by stabilizing it through hydrogen bonding with the second sphere guanidine group.