Heme Biosynthesis And Porphyrias

Question 1

Heme

Answer 1

• protoporphyrin IX
• 4 pyrrole rings with either mthyl, vinyl, or carboxylic sidechains
• cofactor in transcription, translation, miRNA processing

Question 2

Functions of Cytochrome P450 Enzymes in Liver

Answer 2

•  Phase I Liver Detoxification
•  Detoxify xenobiotic sources of toxicity, chemicals,
alcohols and carcinogens - converting them into water and oxygen
•  Bilirubin metabolism
•  Synthesis of Vitamin D
•  Cholesterol synthesis
•  Synthesis of bile and bile acids

Question 3

Porphyrin precursors

Answer 3

–  ALA and PBG
–  Water soluble and are excreted and measured in urine
–  Biologically inactive

Question 4

Porphyrinogens

Answer 4

–  Larger molecules whose aqueous solubility varies in part
based on the number of carboxylic acid side chains in the
molecule.
–  Biologically active
–  Reduced form

Question 5

Porphyrins

Answer 5

–  Molecules detected and measured in clinical laboratories –  Oxidized
–  Oxidation creates an extended conjugation system that
allows the molecules to absorb visible light.
–  This property results in clinical manifestations of disease
because release of the absorbed energy produces reactive
oxygen species that damage tissue. In the lab the
spectrophotometric and fluorescent properties are used to
detect the compounds in body fluid specimens.

Question 6

First Step of Heme Synthesis

Answer 6

-δ-ALA is formed in the mitochondria and transported to the
cytoplasm
-pyridoxal phosphate (vit b6 is necessary cofactor)
•  Committed step, highly regulated
•  Housekeeping or ALAS1
•  Erythroid-specific or ALAS2
-Sideroblastic anemia results from mutation in ALAS2

Question 7

Second Step of Heme Biosynthesis

Answer 7

-2 molecules of ALA condense to form Porphobilinogen
(PBG) by ALA dehydratase (ALAD)
-Cytoplasmic
-Zinc-containing enzyme
-1st precursor pyrole synthesized
-Inhibited by lead (Pb)

Question 8

Third Step of Heme Biosynthesis

Answer 8

-Synthesis of Hydroxymethylbilane by Porphobilinogen deaminase-PBGD
-4 PBG are joined into an extended linear tetrapyrole intermediate
by porphobilinogen deaminase
-Acute Intermittent Porphyria occurs when PBGD is mutated

Question 9

Fourth Step of Heme Biosynthesis

Answer 9

-either UroIII synthase catalyzes an intramolecular rearrangement and ring closure into uroporphyrinogen III
OR
-at high [hydroxymethylbilane], spontaneous arrangement into uroI–>coproI

Question 10

Fifth Step of Heme Biosynthesis

Answer 10

• uroporphyrinogenIII converted to coproporphyrin via UROIII decarboxylase
• mutations in this enzyme leads to porphyria cutanea tarda

Question 11

Final Step of Heme Biosynthesis

Answer 11

• protoporphyrin IX gets Fe2+ added via ferrochelatase
• mitochondrial
• inhibited by Pb
• Associated disease: erythropoietic protoporphyria

Question 12

Mechanisms of Regulation of ALAS1 in LIVER

Answer 12

1)Allosteric feedback inhibition of the enzyme by Heme, Hemin, hematin

2) Inhibition of newly synthesized ALAS protein
transport from cytosol to mitochondria by heme

3) Repression of transcription of ALAS by heme,
insulin and glucose.

4) Induction by Peroxisome proliferator-activated receptor γ coactivator 1α

5) Induction by 4 Ms (menstruation, medication, malady, malnutrition)
- medication can be alcohol, barbs, steroids, sulfa drugs

Question 13

PGC-1α

Answer 13

• Peroxisome proliferator-activated receptor γ coactivator 1α
• induces ALAS1 transcription

Question 14

Mechanism of Induction of ALAS in the Liver

Answer 14

Increased p450 usage–> decrease [Heme]—> induction of heme synthesis

Question 15

Mechanisms of Regulation of ALAS2 in Erythroid cells

Answer 15

-Repression of translation of ALAS2 by low iron
cellular content through an iron response element sequence, IRE.
-Fe deficiency leads to binding of IRP to IRE, inhibiting ALAS2 translation, since there is no point in making heme without Fe.

Question 16

Sideroblastic Anemia

Answer 16

-Heterogeneous group of disorders
with 2 common features:
–  Ring sideroblasts in the bone marrow 
–  Impaired heme biosynthesis
- microcytic and hypochromic
- Disturbed mitochondrial metabolism
- Fe granules in cytoplasm

Causes:
X-linked mutation, mtDNA mutation, isoniazid, alcohol, Pb, B6 deficiency

Question 17

Diagnosis of Lead Poisoning

Answer 17

-Accumulation of ALA in urine due to ALAD
inhibition.
-Zinc protoporphyrin (ZPP) in blood
-Basophilic stippling in peripheral smear

Question 18

Congenital Erythropoietin Porphyria

Answer 18

Autosomal recessive

Question 19

AIP

Answer 19

-partial deficiency is PBGD (hydroxymethylbilane synthase or uroporphyrinogen I synthase)
-accumulation of ALA and/or PBG is neurotoxic
-no skin lesions
-Only 10 % develop disease, all are at risk for primary liver cancer
•  periodic attack of abdominal colic, gastrointestinal disturbances, paralyses, psychiatric disorders
-Triggered by 4 Ms and low heme, since low heme turns off inhibition of ALA synthase
-PBG in urine is oxidized to porphobilin upon standing, which gives a dark-brown color to urine, and often referred to as ‘port-wine reddish urine’

Question 20

Treatment for AIP

Answer 20

1.  Avoid precipitating factors
2.  Glucose loading (carbohydrate)
3.  Administer Hemin/Hematin
•  End product / Feedback
inhibition of ALAS by heme

Question 21

PCT

Answer 21

• autosomal dominant, most common porphyria
• can also be caused by environmental trigger
• UROD deficiency (the decarboxylase)
• clinical onset in 40’s-50’s

Symptoms: bullae, milia, dark pink fluorescent urine, Hypertrichosis in the malar region, heliotrope, sclerodermoid plaque on scalp.

Clinical Expression Influenced By
Alcohol, Hepatic iron overload, Exposure to sunlight, Presence of hepatitis B or C, or HIV infections, Hydrochlorobenzene (1952 incident in Turkey)
•  Fungicide used for agricultural wheat

Question 22

Detection of PCT

Answer 22

Coral red fluorescence of urine
•  Uroporphyrins to coproporphyrins 3:1 to 5:1
•  Fractionation of serum porphyrins.
•  A predominance of 7-8-
carboxyl porphyrin fractions reflecting a uroporphyrinogen decarboxylase (UROD) defect
•  UROD enzyme activity assay in
red blood cells 
•  Mutation analysis of genes
encoding UROD

Question 23

Treatment of PCT

Answer 23

Remove environmental exposures
•  Avoid alcohol, tobacco,
estrogen therapy
•  Sunscreens 
•  Chelation with desferrioxamine
(iron chelator) 
•  Phlebotomy (BLEEDING)
•  Reduces iron stores, which
improves heme synthesis
disturbed by iron-mediated
inhibition of uroporphyrinogen
decarboxylase (UROD).

Question 24

EPP (erythropoietic protoporphyria)

Answer 24

Mutations in Ferrochelatase
•  Autosomal dominant inheritance 
•  Presentation in early childhood 
•  Severe cutaneous photosensitivity 
•  Burning, stinging pain with sunlight 
•  Subsequent skin changes 
•  Chronic liver disease in later life