Distal Histidine Conformational Flexibility in Dehaloperoxidase from Amphitrite Ornata

Abstract

The enzyme dehaloperoxidase (DHP) from the terebellid polychaete Amphitrite ornata is a heme protein, which has a globin fold, but can function as both a hemoglobin and a peroxidase. As a peroxidase, DHP is capable of converting para-halogenated phenols to the corresponding quinones in the presence of hydrogen peroxide. As a hemoglobin, DHP cycles between the oxy and deoxy states as it reversibly binds oxygen for storage. Herein, we report that the distal histidine shows a large conformational flexibility in the deoxy form. Crystals of deoxy ferrous DHP were obtained by reducing the ferric wild-type DHP in sodium dithionite solution and the structure was determined at 100K to a resolution of 1.22Ã…. The heme iron in the deoxy ferrous DHP is five-coordinate and has an out-of-plane displacement of 0.23 Ã… for the heme iron relative to the oxy form. The distal histidine, H55 is observed in conformations, which are analogous to the open and closed forms of myoglobin. In the closed conformation, H55 is located inside the distal pocket, but does not penetrate as deeply into the distal pocket as in the metaquo ferric or oxy ferrous structures. This observation is consistent with the hypothesis that H55 interacts with heme iron ligands through hydrogen bonding in the closed conformation. There are two open or solvent-exposed conformations, in which H55 is more than 9.5 Ã… away from the heme. The comparison of the deoxy structure with the other structures provides new insight into the correlation between the heme iron ligation and the conformation of distal histidine in the DHP.