Slide 1Slide 2Slide 3An Overview of Iron MetabolismRoles of Iron in the CellSlide 6Slide 7Slide 8Slide 9HEME SYNTHESISHEME SYNTHESIS: Red blood cellsSlide 12Slide 13Slide 14HEME SYNTHESIS: LiverFormation of 5-aminolevulinate (5-ALA)Slide 17Disorders of Heme SynthesisLEAD TOXICITYHEME SYNTHESIS (CONT.)PORPHYRIASSlide 22Slide 23ACUTE INTERMITTENT PORPHYRIAPORPHYRIA CUTANEA TARDAPathophysiology of Heme SynthesisBeth A. BouchardBIOC 212: Biochemistry of Human DiseaseSpring 2006HEME-CONTAINING PROTEINS- Hemoglobin- Myoglobin- Cytochromes- Catalase- Some peroxidasesSTRUCTURE OF HEMEAn Overview of Iron MetabolismGutBloodCells• Low pH of stomach solubilizes Fe-containing ionic compounds.• Fe transporters facilitate absorption into blood stream• Fe3+ ions are bound and chelated by Transferrin (Tf).• Transferrin transports Fe to tissues•Maintains solubility•Keeps Fe ions unreactive • Transferrin endocytosis is receptor-mediated (TfR)• Endocytosis results in Fe3+ release• Fe is distributed to topologically distinct regions of the cell via Fe transporter and/or channels (?)• Usage: Protein components (Heme)• Storage: Ferritin (Fe2+)• ToxicityRoles of Iron in the CellTransferrin Receptors (TfR)Fe(III)2-Tf TfProteins: Catalysis Electron, oxygen transport Structural stabilization Sensor of Fe, ROS Formation of protein-bound radicalsStorage and Sequestration: FerritinFerritinToxicity: Oxidative stress[Fe][Fe][Fe]Iron Control of Translation•IREs are found in the 5’-UTR or the 3’-UTR of mRNAs•Regulate mRNA translation via IRBP•Decreased cellular iron levels: –IRBP is free of iron and can therefore, interact with the IREs in the 3'-UTR of the transferrin receptor (TfR) mRNA, which prevents its degradation. –IRBP binds to the IRE in the 5’-UTR of the ferritin mRNA preventing its translation.•Increased cellular iron levels: –IRBP binds iron and cannot interact with the IREs in the TfR mRNA resulting in an increase in its degradation.–IRBP cannot bind to the IRE in the ferritin mRNA allowing for its translation. IRESTRUCTURE OF HEME•Ferrous iron (Fe2+)•Protoporphyrin IX: contains 4 pyrrole rings linked together by methenyl bridgesHeme88Succinyl CoAGlycine**HEME SYNTHESIS** Amino acid (building blocks of protein) synthesized in your bodyHeme synthesisHEME SYNTHESISHEME SYNTHESIS: Red blood cells•85% of total heme synthesis occurs in red blood cells (RBC)•Ceases when RBC’s mature•Erythroid-specific ALA synthase is regulated by an IRE in the mRNA – binding of IRBP inhibits mRNA translationHeme stimulates hemoglobin synthesis in reticulocytesHCI = heme controlledinhibitorReduced initiation of translation***In RBCS, heme synthesis is also regulated at the level of the en-zymes ferrochelatase* and porphobilinogen deaminase**HEME SYNTHESIS: Liver•The liver is the main non-RBC source of heme synthesis•Heme produced in the liver is used mainly for the synthesis of the cytochrome P450 class of enzymes that are involved in detoxificationRegulated at level of ALA synthase: Formation of 5-ALA is the rate-limiting step in heme synthesis in the liverFormation of 5-aminolevulinate (5-ALA)5-ALA- 5-ALA is formed in the mitochondria and transported to the cytoplasmRegulation of ALA SynthaseLevel of enzyme synthesisEnzyme synthesis, as well as its transport to the mitochondria, is inhibited by elevated levels of heme and hemin, the Fe3+ oxidation product of hemeEnzyme synthesis is upregulated by a large number of drugs including barbiturates, steroids with a 4,5 double bond (e.g. testosterone) and some oral contraceptives: These drugs are metabolized by the microsomal cytochrome P450 mono-oxygenase system, a heme-containing protein. Level of enzyme activityHeme and hemin inhibit ALA synthase activityRequires pyridoxal phosphate (Vitamin B6) as a coenzymeDisorders of Heme Synthesis• Acquired: Lead poisoning• Congenital: Porphyrias• Deficiency of heme has far-reaching effects (hemoglobin, cytochromes, etc.)LEAD TOXICITYSymptoms- Irritibility - Poor appetite- Lethargy - Abdominal pain (with or - Sleeplessness without vomiting)- Headaches - ConstipationPathophysoiology- Binds to any compound with a sulfhydryl group- Inhibits multiple enzyme reactions including those involved in heme biosynthesis (PBG synthase & ferrochelatase)- One symptom of lead toxicity is increases in 5-ALA without concomitant increases in PBGHEME SYNTHESIS (CONT.)leadVitamin B6PORPHYRIAS- A group of rare disorders caused by deficiencies of enzymes of the heme biosynthetic pathway-The majority of the porphyrias are inherited in a autosomal dominant fashion - thus, affected individuals have 50% normal levels of the enzymes, and can still synthesize some heme-Affected individuals have an accumulation of heme precursors (porphyrins), which are toxic at high concentrations-Attacks of the disease are triggered by certain drugs, chemicals, and foods, and also by exposure to sun- Treatment involves administration of hemin, which provides negative feedback for the heme biosynthetic pathway, and therefore, prevents accumulation of heme precursorsScriver et al., The Metabolic & Molecular Basis of Inherited Disease, 8th edition, 2001.ACUTE INTERMITTENT PORPHYRIA- Hepatic, autosomal dominant- Caused by a deficiency in porphobilinogen deaminase, which is involved in the conversion of porphobilinogen (PBG) to uroporphyrinogen III-PBG, uroprophryin, and 5-ALA accumulate in the plasmaand the urine-Patients have neuropyschiatric symptoms and abdominal pain (neurovisceral)PORPHYRIA CUTANEA TARDA-Most common porphyria-Hepatic, autosomal dominant-Disease is caused by a deficiency in uroporphyrinogen decarboxylase, which is involved in the conversion of uroporphyrinogen III to coproporphyrinogen III - Uroporphyrinogen accumulates in urine- Patients are photosensitive (cutaneous photosensitivity)Accumulation of porphyrinogens results in their conversion to porphyrins by lightPorphyrins react with molecular oxygen to form oxygen radicalsOxygen radicals can cause severe damage to the