Download PORPHYRINS

David Hart
Dec 12, 2006
Heme
Porphyrins
• Cyclic compounds that bind metal ions
• Chlorphyll (Mg2+)
– Central to solar energy utilization
• Heme (Fe2+)
– Most prevalent metalloporphyrin in
humans
– Central to oxygen sensing and utilization
• Cobalamin (Cobalt)
•
The Heme Pocket in Hemoglobin
Heme
• One ferrous (Fe2+) atom in the center of the
tetrapyrrole ring of Protoporphyrin IX
• Prosthetic group for
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Hemoglobin and Myoglobin
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
• Heme function
• Heme synthesis
and regulation
• Iron metabolism
• Porphyrias
• Heme degradation
Heme Function
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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 H2O2
(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
B
A
N
NH
D
HN
N
C
Porphyrin Side Chains
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M = Methyl (-CH3)
V = Vinyl (-CH=CH2)
P = Propionyl (-CH2-CH2-COO-)
A = Acetyl (-CH2-COO-)
Biosynthesis of Heme
• Synthesized in every human cell
• Liver (15%):
– 65% Cytochrome P450
– Synthesis fluctuates greatly
– Alterations in cellular heme pool
• Bone Marrow (80%)
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Erythrocyte precursors: Hemoglobin
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
CH2
SUCCINYL CoA
CH2
COSCoA
CH2
NH2
COOH
GLYCINE
COOH
CH2
SUCCINYL CoA
CH2
COSCoA
CH2
NH2
COOH - CO2
GLYCINE is
Decarboxylated
 AMINOLEVULINIC ACID SYNTHASE
IN MITOCHONDRIA
COOH
CH2
CH2
C=O
CH2
NH2
Condense to form:
 AMINOLEVULINIC ACID (ALA)
MOVES OUT OF THE MITOCHONDRION
COOH
CH2
COOH
CH2
CH2
C=O
CH2
CH2
C=O
NH2
CH2
-2 H2O
NH2
2 Molecules dehydrated by
ALA DEHYDRATASE
COOH
CH2
COOH
CH2
CH2
C
C
C
C
NH
CH2
NH2
To form Porphobilinogen (PBG)
COOH
Propionate
CH2CH2COO- CH2
Acetate
CH2COO-
COOH
CH2
CH2
N
H
CH2
NH2
Porphobilinogen (PBG)
A
CH2
P
N
H
NH2
Porphobilinogen (PBG)
Hydroxymethylbilane synthase
& Uroporphyrinogen III synthase
• Four PBG molecules condense
• Ring closure
• Isomerization
P
A
A
B
A
NH
P
HN
Uroporphyrinogen III
NH
A
D
P
HN
C
P
A
COOH
CH2
CH2
COOH
CH2
-CH2-CH2-COOH
HOOC-H2CNH
HN
Uroporphyrinogen III
NH
HN
-CH2-COOH
HOOC-H2C-
CH2
CH2
COOH
CH2
CH2
COOH
Series of
decarboxylations & oxidations
• Porphyrinogens:
– Chemically reduced
– Colorless intermediates
• Porphyrins:
– Intensely colored
– Fluorescent
• Uroporphyrinogen III
• Coproporphyrinogen III
Moves back into Mitochondrion
• Protoporphyrinogen IX
• Protoporphyrin IX
CH=CH2
CH3
-CH=CH2
H3CNH
N
Protoporphyrin IX
N
HN
-CH3
H3C-
CH2
CH2
COOH
CH2
CH2
COOH
HEME
Fe2+ chelated by Protoporphyrin IX
Assisted by Ferrochelatase
CH3-
Regulation of Heme Synthesis
 AMINOLEVULINIC ACID
SYNTHASE
• Two tissue-specific isozymes
• Coded on separate genes
• In Liver, heme represses synthesis and
activity of ALAS
– Heme can be used for treatment of acute
porphyric attack
• In RBC heme synthesis regulation is
more complex
– Coordinated with globin synthesis
IN MITOCHONDRIA
SUCCINYL CoA
GLYCINE
COOH
COOH
CH2
CH2
CH2
CH2
COSCoA
C=O
CH2
CH2
COOH
NH2
ALA
NH2
AMINOLEVULINIC ACID SYNTHASE
RATE-CONTROLLING STEP IN
HEPATIC HEME SYNTHESIS
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
X-linked
Sideroblastic Anemia
ALAS Requires
Pyridoxal Phosphate as Coenzyme
Some Sideroblastic Anemias
improve with Pyridoxine (B6)
ALA moves out of the mitochondrion
COOH
CH2
COOH
CH2
CH2
C=O
CH2
CH2
C=O
NH2
CH2
A
-2 H2O
CH2
NH2
P
N
H
PBG
NH2
ALA DEHYDRATASE
Inhibited by Heavy Metal: LEAD POISONING
Lead
Poisoning
Lead
Poisoning
Lead Poisoning
ALAD and Ferrochelatase
Are particularly sensitive
to Lead inhibition
Ferrochelatase
Fe + PPIX
Heme
Iron Metabolism
• Reactive Transition Metal (Fe2+  Fe3+)
• 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 Fe3+ in duodenum converted to Fe2+
and absorbed by duodenal enterocyte
Iron
35% of
Earth’s mass
nasa
Blood
GUT
Contents
Fe3+
Heme
Fe2+
diFe3+
Transferrin
Hepatocyte
Macrophage
Erythroid Cell
Fe2+
Mitochondrial
Heme
Synthesis
NEJM
June 2004
Blood
Macrophage
RBC
Hemoglobin
Haptoglobin
Fe2+
Heme
Hemopexin
Fe2+
?
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
Nemeth et al, Science, Dec 2004
Beutler,
Science
Dec 2004
Hentze, Muckenthaler
& Andrews
Cell, Vol 117, 285-297,
April 30, 2004
Ferroportin
Genetic Hemochromatosis
Disruption of Hepcidin / Ferroportin
• Autosomal Recessive
– HFE C282Y/C282Y
– TfR2
– Hemojuvelin
– Hepcidin
• Autosomal Dominant
– 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
Harvesting Latex
www.geoimagery.com
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
• Accumulation and excretion of porphyrins
– Pattern depends on which enzyme affected
• Multiple alleles
• Acute and Chronic
– Acute: Neurovisceral attacks
• Porphyrin accumulation: Photosensitivity
– 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