16. porphyrins
TRANSCRIPT
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PORPHYRINSHEME SYNTHESIS AND DEGRADATION
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Heme
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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)
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PORPHYRINS
HC
HCNH
CH
CH
Pyrrole ring
Porphyrin: Cyclic molecule formed by
linkage of four pyrrole rings through methenyl
bridges
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PORPHYRIN SIDE CHAINS
M = Methyl (-CH3)
V = Vinyl (-CH=CH2)
P = Propionyl (-CH2-CH2-COO-)
A = Acetyl (-CH2-COO-)
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HEME
One ferrous (Fe2+) atom in the center of the tetrapyrrole ring of Protoporphyrin IX
Prosthetic group forHemoglobin and MyoglobinThe CytochromesCatalase and Tryptophan pyrrolaseNitric Oxide Synthase
Turnover of Hemeproteins (Hemoglobin, etc) is coordinated with synthesis and degradation of porphyrins
Bound iron is recycled
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Heme
Protoporphyrin III
prefix or suffix ring substituents between rings uro- acetate, propionate -- copro- methyl, propionate -- proto- methyl, propionate, vinyl -porphyrinogen -- methylene -porphyrin -- methene
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BIOSYNTHESIS OF HEME
Synthesized in every human cell Liver (15%): Bone Marrow (80%)
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COOH
CH2
CH2
COSCoACH2 NH2
COOH
SUCCINYL CoA
GLYCINE
All Carbon and Nitrogen atoms provided by 2 building blocks:
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COOH
CH2
CH2
COSCoACH2 NH2
COOH
SUCCINYL CoA
GLYCINE isDecarboxylated
IN MITOCHONDRIA
AMINOLEVULINIC ACID SYNTHASE
- CO2
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COOH
CH2
CH2
C=OCH2
NH2
Condense to form: AMINOLEVULINIC ACID (ALA)
MOVES OUT OF THE MITOCHONDRION
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COOH
CH2
CH2
C=OCH2
NH2
COOH
CH2
CH2
C=OCH2
NH2
2 Molecules dehydrated byALA DEHYDRATASE
-2 H2O
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COOH
CH2
CH2
CC
NH
COOH
CH2
C
CCH2
NH2
To form Porphobilinogen (PBG)
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COOH
CH2
CH2
NH
COOH
CH2
CH2
NH2
Porphobilinogen (PBG)
AcetateCH2COO-
PropionateCH2CH2COO-
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Porphobilinogen (PBG)
NH
CH2
NH2
A P
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NH
CH 2
NH 2
A
PN
H
CH
2
NH2
A
P
NH
CH2
NH2
A
P
NH
CH2
NH2
A
P
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HYDROXYMETHYLBILANE SYNTHASE& UROPORPHYRINOGEN III SYNTHASE
Four PBG molecules condense Ring closure Isomerization
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NH
NH HN
HN
A B
CD
A
A
P
A
P
P
P
A
Uroporphyrinogen III
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NH
NH HN
HNHOOC-H2C-
HOOC-H2C-
-CH2-CH2-COOH
-CH2-COOH
CH2
CH2
COOH
CH2
CH2
COOH
COOHCH2
CH2
COOHCH2
Uroporphyrinogen III
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SERIES OF DECARBOXYLATIONS & OXIDATIONS
Porphyrinogens: Chemically reducedColorless intermediates
Porphyrins: Intensely coloredFluorescent
Uroporphyrinogen III Coproporphyrinogen IIIMoves back into Mitochondrion Protoporphyrinogen IX Protoporphyrin IX
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NH
N HN
NH3C-
H3C-
-CH=CH2
-CH3
CH2
CH2
COOH
CH2
CH2
COOH
CH3
Protoporphyrin IX
CH=CH2
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HEMEFe2+ chelated by Protoporphyrin IX
Assisted by Ferrochelatase
CH3-
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REACTIONS FORPROTOPORPHYRIN IX
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REGULATION OF HEME SYNTHESIS
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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
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COOH
CH2
CH2
COSCoACH2 NH2
COOH
SUCCINYL CoA
GLYCINE
IN MITOCHONDRIA
AMINOLEVULINIC ACID SYNTHASERATE-CONTROLLING STEP IN
HEPATIC HEME SYNTHESIS
COOH
CH2
CH2
C=OCH2
NH2
ALA
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BonkovskyASH Education BookDecember 2005
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DISORDERS OF HEME SYNTHESIS
Acquired: Lead poisoning
Congenital: Porphyrias
Deficiency of heme has far-reaching effects (hemoglobin, cytochromes, etc.)
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LEAD TOXICITY
Symptoms Irritibility Poor appetite Lethargy Abdominal pain (with or Sleeplessness without vomiting) Headaches Constipation
Pathophysoiology Binds to any compound with a sulfhydryl group Inhibits multiple enzyme reactions including those involved in heme biosynthesis (ALA dehydratase & ferrochelatase)
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COOH
CH2
CH2
C=OCH2
NH2
COOH
CH2
CH2
C=OCH2
NH2
-2 H2O
ALA moves out of the mitochondrion
ALA DEHYDRATASEInhibited by Heavy Metal: LEAD POISONING
PBG
NH
CH2
NH2
A P
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Lead Poisoning
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Lead PoisoningALAD and FerrochelataseAre particularly sensitive
to Lead inhibition
Lead Poisoning
Fe + PPIX
Ferrochelatase
Heme
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PORPHYRIAS 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 precursors
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Scriver et al., The Metabolic & Molecular Basis of Inherited Disease, 8th edition, 2001.
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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 plasma and the urine
Patients have neuropyschiatric symptoms and abdominal pain (neurovisceral)
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PORPHYRIA CUTANEA TARDAMost 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 light
Porphyrins react with molecular oxygen to form oxygen radicals
Oxygen radicals can cause severe damage to the skin
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HEME DEGRADATION
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REACTIONSFIG. 44.8
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Heme oxygenaseBiliverdin reductaseSerum albuminBilirubin UDP-glucuronyl transferase
Spleen MacrophagesBloodLiver
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HEME DEGRADATION
Features Reactions Jaundice
hemolytic obstructive Neonate kernicterus liver disease Gilbert’s disease
•Blood Proteins–serum albumin–haptoglobin–hemopexin