Dehalopeoxidase A (DHP A) is a detoxifying enzyme found in the marine worm Amphitrite ornata. This enzyme converts halophenols found in the environment where the worm lives, into quinones by dehalogenation. The enzyme has globin structure and function, but works also as a peroxidase in the presence of H2O2 which binds to the iron present in the heme group. The initial step in the enzymatic reaction path is the transformation of the heme Fe(III) ion into a ferryl (Fe = O) moiety. A distal histidine, His55, is crucial for this process. His55 can occupy two positions, either in the distal pocket of the active center (“closed”), or exposed to the solvent (“open”). NMR experiments show that His55 moves between those positions in the resting state of the enzyme. For this process to occur it is necessary that a gate, composed of a triad Asn37-Lys36-Lys51 and two carboxylates on the heme group, suffer a conformational change before and after the passage of the histidine. We examined computationally this process at the B3LYP/6-31G(d,p) level, within a PCM simulated aqueous environment. This analysis leads us to propose a correction of the experimental structure of the enzyme determined by X-ray crystallography and offers an explanation for different conformations of the twin carboxylates at the heme group observed in the crystals. This new proposal agrees with the experimentally determined electron density distributions and explains the role of the His55 as a functional hook for the peroxide in the aqueous media.