title: Crystal structure of protoporphyrinogen IX oxidase: a key enzyme in haem and chlorophyll biosynthesis.
abstract: Protoporphyrinogen IX oxidase (PPO), the last common enzyme of haem and chlorophyll biosynthesis, catalyses the oxidation of protoporphyrinogen IX to protoporphyrin IX. The membrane-embedded flavoprotein is the target of a large class of herbicides. In humans, a defect in PPO is responsible for the dominantly inherited disease variegate porphyria. Here we present the crystal structure of mitochondrial PPO from tobacco complexed with a phenyl-pyrazol inhibitor. PPO forms a loosely associated dimer and folds into an FAD-binding domain of the p-hydroxybenzoate-hydrolase fold and a substrate-binding domain that enclose a narrow active site cavity beneath the FAD and an alpha-helical membrane-binding domain. The active site architecture suggests a specific substrate-binding mode compatible with the unusual six-electron oxidation. The membrane-binding domains can be docked onto the dimeric structure of human ferrochelatase, the next enzyme in haem biosynthesis, embedded in the opposite side of the membrane. This modelled transmembrane complex provides a structural explanation for the uncoupling of haem biosynthesis observed in variegate porphyria patients and in plants after inhibiting PPO.
authors.affiliation: Max-Planck-Institut für Biochemie, Abteilung Strukturforschung, Martinsried, Germany. mikoch@biochem.mpg.de
authors.forename: Michael
authors.initials: M
authors.lastname: Koch
authors.forename: Constanze
authors.initials: C
authors.lastname: Breithaupt
authors.forename: Reiner
authors.initials: R
authors.lastname: Kiefersauer
authors.forename: Jörg
authors.initials: J
authors.lastname: Freigang
authors.forename: Robert
authors.initials: R
authors.lastname: Huber
authors.forename: Albrecht
authors.initials: A
authors.lastname: Messerschmidt
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D002462:Cell Membrane / Q000201:enzymology; D002734:Chlorophyll / Q000096:biosynthesis*; D018360:Crystallography, X-Ray; D019281:Dimerization; D005294:Ferrochelatase / Q000378:metabolism; D005420:Flavoproteins; D006418:Heme / Q000096:biosynthesis*; D006801:Humans; D008928:Mitochondria / Q000201:enzymology; D024101:Mitochondrial Proteins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry* / Q000378:metabolism*; D046350:Porphyria, Variegate / Q000201:enzymology; D020836:Protein Structure, Quaternary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D050768:Protoporphyrinogen Oxidase; D016415:Sequence Alignment; D014026:Nicotiana / Q000201:enzymology
chemical_list: D005420:Flavoproteins; D024101:Mitochondrial Proteins; D021122:Protein Subunits; D002734:Chlorophyll; D006418:Heme; D044925:Oxidoreductases Acting on CH-CH Group Donors; C493978:PPOX protein, human; D050768:Protoporphyrinogen Oxidase; D005294:Ferrochelatase