Transcript PORPHYRINS - New York University
David Hart Dec 12, 2006
Heme
Porphyrins
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Cyclic compounds that bind metal ions Chlorphyll (Mg 2+ )
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Central to solar energy utilization Heme (Fe 2+ )
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Most prevalent metalloporphyrin in humans
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Central to oxygen sensing and utilization Cobalamin (Cobalt)
The Heme Pocket in Hemoglobin
Heme
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One ferrous (Fe 2+ ) atom in the center of the tetrapyrrole ring of Protoporphyrin IX Prosthetic group for
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Hemoglobin and Myoglobin
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The Cytochromes Catalase and Tryptophan pyrrolase Nitric Oxide Synthase Turnover of Hemeproteins (Hemoglobin, etc) is coordinated with synthesis and degradation of porphyrins Bound iron is recycled
Lecture Outline
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Heme function Heme synthesis and regulation Iron metabolism Porphyrias Heme degradation
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Heme Function
Oxygen sensing (heme and hemoproteins) Oxygen transport (hemoglobin) Oxygen storage (myoglobin) Electron transport (cytochromes) Oxidation (cyrochrome p450, tryptophan pyrrolase, guanylate cyclase …) Decomposition and activation of H 2 O 2 (catalase and peroxidase) Nitric Oxide Synthesis Regulation of cellular processes Effector of apoptosis
Porphyrin: Cyclic molecule formed by linkage of four pyrrole rings through methenyl bridges A N D NH HN B N C
Porphyrin Side Chains
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M = Methyl (-CH 3 ) V = Vinyl (-CH=CH 2 ) P = Propionyl (-CH 2 -CH 2 -COO ) A = Acetyl (-CH 2 -COO )
Biosynthesis of Heme
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Synthesized in every human cell Liver (15%):
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65% Cytochrome P450
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Synthesis fluctuates greatly Alterations in cellular heme pool Bone Marrow (80%)
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Erythrocyte precursors: Hemoglobin
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Synthesis relatively constant Matched to rate of globin synthesis Largely unaffected by other factors
All Carbon and Nitrogen atoms provided by 2 building blocks: COOH CH 2 CH 2 COSCoA CH 2 COOH NH 2 SUCCINYL CoA GLYCINE
COOH CH 2 CH 2 COSCoA CH 2 COOH NH 2 - CO 2 SUCCINYL CoA GLYCINE is Decarboxylated
AMINOLEVULINIC ACID SYNTHASE IN MITOCHONDRIA
COOH CH 2 CH 2 C=O CH 2 NH 2
Condense to form: AMINOLEVULINIC ACID (
ALA) MOVES OUT OF THE MITOCHONDRION
COOH CH 2 CH 2 C=O CH 2 NH 2 COOH CH 2 CH 2 C=O CH 2 NH 2 -2 H 2 O 2 Molecules dehydrated by
ALA DEHYDRATASE
COOH CH 2 CH 2 C COOH CH 2 C C C NH CH 2 NH 2 To form Porphobilinogen (PBG)
Propionate CH 2 CH 2 COO COOH CH 2 CH 2 COOH Acetate CH 2 COO CH 2 N H CH 2 NH 2 Porphobilinogen (PBG)
A P NH 2 CH 2 N H Porphobilinogen (PBG)
Hydroxymethylbilane synthase & Uroporphyrinogen III synthase
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Four PBG molecules condense Ring closure Isomerization
P A A A A B NH HN Uroporphyrinogen III D NH HN C P A P P
COOH CH 2 CH 2 COOH CH 2 HOOC-H 2 C NH HN Uroporphyrinogen III NH HN -CH 2 -CH 2 -COOH -CH 2 -COOH HOOC-H 2 C CH 2 CH 2 COOH CH 2 CH 2 COOH
Series of decarboxylations & oxidations
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Porphyrinogens:
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Chemically reduced
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Colorless intermediates Porphyrins:
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Intensely colored Fluorescent
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Uroporphyrinogen III
Coproporphyrinogen III
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Moves back into Mitochondrion Protoporphyrinogen IX
Protoporphyrin IX
CH=CH 2 CH 3 H 3 C NH N Protoporphyrin IX N HN H 3 C CH 2 CH 2 COOH CH 2 CH 2 COOH -CH=CH 2 -CH 3
HEME Fe 2+ chelated by Protoporphyrin IX Assisted by Ferrochelatase
CH 3 -
Regulation of Heme Synthesis
AMINOLEVULINIC ACID SYNTHASE
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Two tissue-specific isozymes Coded on separate genes In Liver , heme represses synthesis and activity of
ALAS
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Heme can be used for treatment of acute porphyric attack In RBC heme synthesis regulation is more complex
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Coordinated with globin synthesis
IN MITOCHONDRIA COOH COOH SUCCINYL CoA CH 2 CH 2 COSCoA CH 2 CH 2 C=O
GLYCINE CH 2 COOH NH 2 CH 2 NH 2
AMINOLEVULINIC ACID SYNTHASE RATE-CONTROLLING STEP IN HEPATIC HEME SYNTHESIS ALA
Bonkovsky ASH Education Book December 2005
Disorders of Heme Synthesis
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X-linked Sideroblastic Anemia Lead Poisoning Iron Deficiency Anemia The Porphyrias
ALAS Requires Pyridoxal Phosphate as Coenzyme Some Sideroblastic Anemias improve with Pyridoxine (B 6 ) X-linked Sideroblastic Anemia
ALA moves out of the mitochondrion COOH CH 2 CH 2 C=O CH 2 NH 2 COOH CH 2 CH 2 C=O -2 H 2 O A CH 2 N H P CH 2 NH 2 PBG NH 2
ALA DEHYDRATASE Inhibited by Heavy Metal: LEAD POISONING
Lead Poisoning
Lead Poisoning Lead Poisoning
ALAD and Ferrochelatase Are particularly sensitive to Lead inhibition Fe + PPIX Ferrochelatase Heme
Iron Metabolism
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Reactive Transition Metal (Fe 2+
Fe 3+ ) Normally present complexed with proteins that limit its reactivity Both iron deficiency and iron overload cause cellular defects and disease Most available iron generated by macrophages that recycle red cell iron Dietary Fe 3+ in duodenum converted to Fe 2+ and absorbed by duodenal enterocyte
nasa Iron 35% of Earth’s mass
GUT Contents Blood Hepatocyte Macrophage Erythroid Cell Fe 3+ Heme Fe 2+ diFe 3+ Transferrin Fe 2+ Mitochondrial Heme Synthesis
NEJM June 2004
Fe 2+ Blood Macrophage Hemoglobin Haptoglobin Heme Hemopexin RBC Fe 2+
?
Syed, Hemoglobin 2006
http://walz.med.harvard.edu
Hentze, Muckenthaler & Andrews Cell, Vol 117, 285-297, April 30, 2004
Hepcidin: 25 Amino Acids J Med Genet 2004
Beutler, Science Dec 2004
Ferroportin Hentze, Muckenthaler & Andrews Cell, Vol 117, 285-297, April 30, 2004
Genetic Hemochromatosis Disruption of Hepcidin / Ferroportin
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Autosomal Recessive
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HFE C282Y/C282Y
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TfR2
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Hemojuvelin
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Hepcidin
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Autosomal Dominant
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Ferroportin
Normal Liver medlib.med.utah.edu
Granular, Dark Reddish Brown Surface of Liver in Hemochromatosis
www.med.niigata-u.ac.j
Iron Accumulation in Chronic Disease
http://eduserv.hscer.washington.edu
Ring Sideroblast Prussian Blue stains Iron In Mitochondria
www.uchsc.edu
Iron Deficiency Anemia Hypochromic, Microcytic
Normal Red Blood Cells
http://eduserv.hscer.washington.edu
Spinach: Non-Heme Iron Less Readily Absorbed Oxalates Phytates Tannins Fiber Calcium
www.lsuagcenter.co
Heme Iron is More Readily Absorbed
www.mcgil.com/food/pics
Iron Deficient Spinach “Chlorosis”
www.agnet.org/library
www.geoimagery.com
Harvesting Latex
Geophagia
www.sentientkinetics.com
Pagophagia
www.awesomedrinks.com
Solemnity Scale: 0 = No smiles/hour 5 = “wreathed” In smiles
Spoon Nails
www.drmhijazy.com
Blue Sclera
Disorders of Heme Synthesis
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X-linked Sideroblastic Anemia Lead Poisoning Iron Deficiency Anemia The Porphyrias
Heme
porphuros (purple)
Heme Synthesis: Porphyrias
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8 Enzymatic Reactions 7 Deficiencies: “Porphyrias” Most are Autosomal Dominant Hepatic or Erythroid depending on main site of synthesis / accumulation
Porphyrias
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Accumulation and excretion of porphyrins
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Pattern depends on which enzyme affected Multiple alleles Acute and Chronic
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Acute: Neurovisceral attacks Porphyrin accumulation: Photosensitivity
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Formation of reactive oxygen species Damage tissues, Release lysosomal enzymes
Lead Poisoning ALA-D Porphyria Very Rare Recessive Porphyria
ALA-D Porphyria Acute Hepatic Lead Poisoning Hydroxymethylbilane Synthase PBG and
ALA Accumulate in Urine PBG in Urine: Diagnostic Screen Urine darkens with exposure NOT photosensitive Neuro-visceral attacks Precipitated by Drugs, EtOH which induce cytochrome P450