1. Porphyrin Metabolism

2. Structure and Properties of Iron Protoporphyrin IX
propionate
methyl
pyrrole ring
vinyl
Derived from protoporphyrin IX
Pattern of side chains defines isomer
Binds metals: Heme- Fe2+ (ferrous)
Hemin- Fe3+ (ferric)
Zinc protoporphyrin (ZnPP)- Zn2+
Extended conjugation across ring system
Photoreactive generation of Reactive Oxygen Species (ROS)

3. Amino Acid Pools are in Steady State
Majority of amino acids used for de novo protein synthesis (80%) derives from the degradation of existing proteins
Only 30 g (6%) used for synthesis of specialized products
Only 70 g (14%) of total amino acid utilization is used for energy or stored as glycogen/fatty acids in the well-fed state (nitrogen balance)

4. Overview of Heme Synthesis
Protoporphyrin IX
Succinyl CoA + Glycine
ALA synthase
Protoporphyrinogen IX
-aminolevulinic acid
Coproporphyrinogen III
mitochondrial matrix
cytoplasm
-aminolevulinic acid
Porphobilinogen
Uroporphyrinogen III
Coproporphyrinogen III
Uroporphyrinogen I
Coproporphyrinogen I
Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group on enzymes and electron transport chain. By weight, the major locations of heme synthesis are the liver and the erythroid progenitor cells of the bone marrow.

5. -Aminolevulinate (ALA) Synthase
is the Committed Step for Heme Biosynthesis
Rate limiting committed step; requires pyridoxal-5’-phosphate as coenzyme
Transcriptional regulation is the principal form of control since the enzyme has a short half life (t1/2 = 1 hr). Heme and hemin repress transcription
In erythrocytes heme synthesis is coordinated with that of the globin chains, all of which are stimulated by erythropoietin (Epogen©, Procrit©, and congeners)
Heme and hemin allosterically inhibit ALA synthase
Aromatic drugs, xenobiotics, and steroids induce synthesis of ALA synthase and can exacerbating certain porphyrias (later)

6. -Aminolevulinate (ALA) Dehydratase
Asymmetry of the reaction results in acetate and propionate side chains
The enzyme active site contains a required cysteine, making the enzyme sensitive to inactivation by lead (Pb2+) and other heavy metals
Increased urinary excretion of -aminolevulinate is a leading indicator of heavy metal poisoning

7. Formation of the Final Ring Requires a Bi-functional Enzyme
This step occurs by elimination of the primary amines as the methylene adds across the double bond of the pyrrole ring.
Note: Uroporphyrinogen I synthase is alternate name for Hydroxymethylbilane synthase

8. Uroporphyrinogen Decarboxylase Remodels the Acetate Side Chains
Spontaneously oxidizes to the biologically inactive Coproporphyrin I and III which are subsequently excreted in the urine

9. Coproporphyrinogen III Oxidase Catalyzes the Oxidative Decarboxylation of Specific Propionate Side Chains

10. Protoporphyrinogen IX Oxidase
This reaction oxidizes the methylene bridge carbons between the pyrrole rings to methenyl bridge carbons, allowing extended conjugation through the entire tetrapyrrole ring system for the first time.

11. Ferrochelatase Inserts Fe2+ into Protoporphyrin IX to yield heme
The reaction also requires ascorbic acid and cysteine as reducing agents
Lead (Pb2+) acts as a competitive inhibitor of Fe2+ but does not insert into protoporphyrin IX
Iron deficiency leads to insertion of Zn2+ to yield zinc protoporphyrin (ZnPP), an important clinical indicator of iron deficiency

12. Porphyrias Caused by hereditary or acquired defects in heme synthesis
- Accumulation and increased excretion of metabolic
precursors (each unique)
- Most porphyrias show a prevalent autosomal dominant pattern, except congenital eythropoietic porphyria, which is recessive
Can be hepatic or erythropoietic, reflecting the two major locations of heme synthesis
- hepatic can be acute or chronic
Those with tetrapyrrole intermediates show photosensitivity due to extended conjugated double bonds
- Formation of superoxide radicals
- Skin blisters, itches (pruritis)
- Skin may darken, grow hair (hypertrichosis)

14. Acquired Porphyrias
Lead poisoning
- inhibition of ferrochelatase and ALA dehydratase
- displaces Zn+2 at enzyme active site
Children
- developmental defects
- drop in IQ
- hyperactivity
- insomnia
- many other health problems
Adults
- severe abdominal pain
- mental confusion
- many other symptoms

15. Most heme from RBCs (85%) - rest from turnover of cytochromes, p450s, immature erythrocytes.
RBCs last 120 days, degraded by reticuloendothelial (RE) system [liver and spleen].
Microsomal heme oxygenase hydroxylates methenyl bridge carbon and oxidizes Fe2+ to Fe3+. Second reaction open ring and release methenyl carbon as CO.
The resulting biliverdin is poorly soluble due to ring stacking and aggregation.
Serum albumin carries bilirubin in circulation, ligandin in hepatocytes.

17. Types of Jaundice Hemolytic jaundice
- Liver can handle 3000 mg bilirubin/day - normal is 300
- Massive hemolysis causes more than can be processed
- cannot be conjugated
- increased bilirubin excreted into bile, urobilinogen
is increased in blood, urine
- unconjugated bilirubin in blood increases = jaundice
Obstructive jaundice
- Obstruction of the bile duct
- tumor or bile stones
- gastrointestinal pain - nausea
- pale, clay-colored stools
- can lead to liver damage and increased unconjugated bilirubin

18. Hepatocellular jaundice
- Liver damage (cirrhosis or hepatitis) cause increased
bilirubin levels in blood due to decreased conjugation
- Conjugated bilirubin not efficiently exported to bile
so diffuses into blood
- Decreased urobilinogen in enterohepatic circulation
so urine is darker and stool is pale, clay-colored
- AST and ALT levels are elevated due to hepatic damage
- Nausea and anorexia

19. Jaundice in Newborns
Premature babies often accumulate bilirubin due to
late onset of expression of bilirubin glucuronyltransferase
- Maximum expression (adult level) at ~ 4 weeks
- Excess bilirubin can cause toxic encephalopathy
(kernicterus)
- Treated with blue fluorescent light
- converts bilirubin to more polar compound
- can be excreted in bile without conjugation
- Crigler-Najjar syndrome is deficiency in bilirubin
glucuronyltransferase