Crystal structures of ferrous and ferrous–NO forms of verdoheme in a complex with human heme oxygenase-1: catalytic implications for heme cleavage

Abstract

Heme oxygenase oxidatively degrades heme to biliverdin resulting in the release of iron and CO through a process in which the heme participates both as a cofactor and substrate. One of the least understood steps in the heme degradation pathway is the conversion of verdoheme to biliverdin. In order to obtain a better understanding of this step we report the crystal structures of ferrous–verdoheme and, as a mimic for the oxy–verdoheme complex, ferrous–NO verdoheme in a complex with human HO-1 at 2.20 and 2.10 Å, respectively. In both structures the verdoheme occupies the same binding location as heme in heme–HO-1, but rather than being ruffled verdoheme in both sets of structures is flat. Both structures are similar to their heme counterparts except for the distal helix and heme pocket solvent structure. In the ferrous–verdoheme structure the distal helix moves closer to the verdoheme, thus tightening the active site. NO binds to verdoheme in a similar bent conformation to that found in heme–HO-1. The bend angle in the verodoheme–NO structure places the terminal NO oxygen 1 Å closer to the α- meso oxygen of verdoheme compared to the α- meso carbon on the heme–NO structure. A network of water molecules, which provide the required protons to activate the iron–oxy complex of heme–HO-1, is absent in both ferrous–verdoheme and the verdoheme–NO structure.