title: Organization of the terminal two enzymes of the heme biosynthetic pathway. Orientation of protoporphyrinogen oxidase and evidence for a membrane complex.
abstract: Protoporhyrinogen oxidase (EC 1.3.3.4), the penultimate enzyme of the heme biosynthetic pathway, catalyzes the removal of six hydrogens from protoporphyrinogen IX to form protoporphyrin IX. The enzyme in eukaryotes is associated with the inner mitochondrial membrane. In the present study we have examined requirements for solubilization of this enzyme and find that it behaves as an intrinsic membrane protein that is solubilized only with detergents such as sodium cholate. The in situ orientation of the enzyme with respect to the inner mitochondrial membrane places the active site on the cytosolic face of this membrane rather than the matrix side where the active site of ferrochelatase, the terminal pathway enzyme, is located. Examination of the kinetics of the two terminal enzymes in mitochondrial membranes demonstrates that substrate channeling occurs between these terminal two-pathway enzymes. However, examination of solubilized and membrane-reconstituted enzymes shows no evidence for a stable complex. Based upon these and previous data a model for the terminal three-pathway enzymes is presented.
authors.affiliation: Department of Microbiology, University of Georgia, Athens 30602.
authors.forename: G C
authors.initials: GC
authors.lastname: Ferreira
authors.forename: T L
authors.initials: TL
authors.lastname: Andrew
authors.forename: S W
authors.initials: SW
authors.lastname: Karr
authors.forename: H A
authors.initials: HA
authors.lastname: Dailey
mesh_terms: D000818:Animals; D005294:Ferrochelatase / Q000302:isolation & purification / Q000378:metabolism*; D005420:Flavoproteins; D006418:Heme / Q000096:biosynthesis*; D007425:Intracellular Membranes / Q000201:enzymology / Q000648:ultrastructure; D008190:Lyases / Q000378:metabolism*; D051379:Mice; D008930:Mitochondria, Liver / Q000201:enzymology*; D024101:Mitochondrial Proteins; D009097:Multienzyme Complexes / Q000302:isolation & purification / Q000378:metabolism*; D010088:Oxidoreductases / Q000378:metabolism*; D044925:Oxidoreductases Acting on CH-CH Group Donors; D050768:Protoporphyrinogen Oxidase; D013367:Submitochondrial Particles / Q000201:enzymology*
chemical_list: D005420:Flavoproteins; D024101:Mitochondrial Proteins; D009097:Multienzyme Complexes; D006418:Heme; D010088:Oxidoreductases; D044925:Oxidoreductases Acting on CH-CH Group Donors; C493979:Ppox protein, mouse; D050768:Protoporphyrinogen Oxidase; D008190:Lyases; D005294:Ferrochelatase