Crystal structure of Nitrosomonas europaea cytochrome c peroxidase and thestructural basis for ligand switching in bacterial Di-heme peroxidases

The crystal structure of the fully oxidized di-heme peroxidase from Nitroso monas europaea has been solved to a resolution of 1.80 Angstrom and compare d to the closely related enzyme from Pseudomonas aeruginosa. Both enzymes c atalyze the peroxide-dependent oxidation of a protein electron donor such a s cytochrome c. Electrons enter the enzyme through the high-potential heme followed by electron transfer to the low-potential heme, the site of peroxi de activation. Both enzymes form homodimers, each of which folds into two d istinct heme domains. Each heme is held in place by thioether bonds between the heme vinyl groups and Cys residues. The high-potential heme in both en zymes has Met and His as axial heme ligands. In the Pseudomonas enzyme, the low-potential heme has two His residues as axial heme ligands [Fulop et al . (1995) Structure 3, 1225-1233]. Since the site of reaction with peroxide is the low-potential heme, then one His ligand must first dissociate. In sh arp contrast, the low-potential heme in the Nitrosomonas enzyme already is in the "activated" state with only one His ligand and an open distal axial ligation position available for reaction with peroxide. A comparison betwee n the two enzymes illustrates the range of conformational changes required to activate the Pseudomonas enzyme. This change involves a large motion of a loop containing the dissociable His ligand from the heme pocket to the mo lecular surface where it forms part of the dimer interface. Since the Nitro somonas enzyme is in the active state, the structure provides some insights on residues involved in peroxide activation. Most importantly, a Glu resid ue situated near the peroxide binding site could possibly serve as an acid- base catalytic group required for cleavage of the peroxide O-O bond.