title: Crystal structure of the di-haem cytochrome c peroxidase from Pseudomonas aeruginosa.
abstract: BACKGROUND
  Cytochrome c peroxidase from Pseudomonas aeruginosa (PsCCP) represents a new class of peroxidases which work without the need to create a semi-stable free radical for catalysis. The enzyme is located in the bacterial periplasm where its likely function is to provide protection against toxic peroxides. The soluble 323-residue single polypeptide chain contains two covalent c-type haems with very different properties: one of them is a low-potential (-330 mV) centre where hydrogen peroxide is reduced (the peroxidatic site); the other is a high-potential (+320 mV) centre which feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c and azurin.
  
  
  RESULTS
  The crystal structure of the oxidized form of PsCCP has been determined to 2.4 A resolution by multiple isomorphous replacement, and refined to an R-factor of 19.2%. PsCCP is organized into two domains, both of them containing a covalent c-haem in a structure reminiscent of class 1 cytochromes c. The domains are related by a quasi-twofold axis. The domain interface holds a newly discovered calcium-binding site with an unusual set of ligands.
  
  
  CONCLUSIONS
  The likely function of the calcium site is to maintain the structural integrity of the enzyme and/or to modulate electron transfer between the two haem domains. The low-potential haem has two histidine axial ligands (His55 and His71) and the high-potential haem is ligated by His201 and Met275. There are no polar residues at the peroxidatic site in the inactive oxidized enzyme. The structure suggests that, in the half-reduced functional form of the enzyme, the low-potential haem has to shed His71 in order to make the enzyme catalytically competent. This process is likely to trigger a reorganization of the active site, and may introduce a new residues into the haem pocket.
authors.affiliation: Laboratory of Molecular Biophysics, University of Oxford, UK.
authors.forename: V
authors.initials: V
authors.lastname: Fülöp
authors.forename: C J
authors.initials: CJ
authors.lastname: Ridout
authors.forename: C
authors.initials: C
authors.lastname: Greenwood
authors.forename: J
authors.initials: J
authors.lastname: Hajdu
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002384:Catalysis; D018360:Crystallography, X-Ray; D003578:Cytochrome-c Peroxidase / Q000737:chemistry*; D004579:Electron Transport; D005609:Free Radicals; D006418:Heme / Q000737:chemistry; D007501:Iron / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000145:classification; D011487:Protein Conformation; D011550:Pseudomonas aeruginosa / Q000201:enzymology*; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D005609:Free Radicals; D006418:Heme; D007501:Iron; D010544:Peroxidases; D003578:Cytochrome-c Peroxidase; D002118:Calcium