2. Outline of this lecture topic…….
a. Porphyrins & structure of Heme
b. Synthesis of Heme
c. Regulation of Heme synthesis
d. Porphyrias
e. Degradation of Heme
f. Jaundice
g. Genetic disorders related to Bilirubin
metabolism 2
3. Porphyrins
Porphyrins are cyclic compounds composed by 4 pyrrole
rings held together by methenyl ( =CH-) bridges.
Metal ions can bind with nitrogen atoms of pyrrole ring to
form a complex
Heme is a iron containing porphyrins
3
4. Structure of Heme
Heme contains
1. A porphyrin molecule namely protoporphyrin IX which
consist of
- four pyrrole rings
- with pyrrole rings 4 methyl (-CH3), 2 vinyl (-CH=CH2)
and 2 propionyl (CH2-CH2-COO) groups are attached
2. Ferrous form of iron (Fe2+) are present in center.
4
5. Biosynthesis of Heme
- Biosynthesis of heme primarily occurs in
- Liver
- Erythrocytes producing cells of bone marrow
(erythroid cells)
- Stages of Biosynthesis of heme
1. Formation of δ-aminolevulinic acid
2. Synthesis of porphobilinogen
3. Formation of porphyrin ring
4. Conversion of uroporphyrinogen III to
protoporphyrin IX
5. Synthesis of heme from protoporphyrin IX
5
6. Outline of Heme synthesis
6
Mitochondrion Cytoplasm
Succinyl-CoA
Glycine
δ-amino
levulinic acid
porphobilinogen
hydroxymethylbilane
uroporphyrinogen III
coproporphyrinogen IIIProtoporphyrinogen III
Heme
Protoporphyrin IX
Hydroxymethylbilane
synthase (PBG
deaminase)
Uroporphyrinogen III synthase
Uroporphirinogen III decarboxylase
Coproporphyrigen III oxidase
Protoporphyrinogen III oxidase
Ferrochelatase
δ ALA-synthase ALA-dehydratase
+
7. Regulation of Heme synthesis
Two different mechanisms exist in regulation of heme
biosynthesis in the Liver and the erythroid cells
1. Regulation in liver
δ-ALA
The first committed step in heme biosynthesis catalyzed
by ALA synthase is regulatory.
7
8. 2. Regulation in Erythroid cells
Heme synthesis in Erythroid cells are controlled by
a. Uroporphyrinogen III synthase
b. Ferrochelatase
- Heme or its oxidized product hemin (Fe+3) controls
this enzyme activity by three mechanisms
a. Feedback inhibition
b. Repression of ALA synthase
c. Inhibition of transport of ALA synthase
from the cytosol to mitochondria(the site of action)
8
9. PORPHYRIAS
-A group of rare metabolic disorders caused by
deficiencies of enzymes of the heme biosynthetic
pathway.
-Affected individuals have an accumulation of heme
precursors (porphyrins), which are toxic at high
concentrations
-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
-However, congenital erythropoietic porphyria is an
exception ( Autosomal recessive)
9
11. PORPHYRIAS
GLYCINE + SuccinylCoA
-aminolevulinic acid(ALA)
Porphobilinogen(PBG)
hydroxymethylbilane
uroporphyrinogen III
coprophyrinogene III
Protoporphyrinogene IX
protoporphyrin IX
Heme
ALA synthase
ALA dehydratase
Hydroxymethylbilane
Synthase(PBG deaminase)
Uroporphyrinogen III
cosynthase
Coproporphyrinogen
oxidase
Protoporphyrinogen
oxidase
Ferrochelatase
ALA-dehydratase
deficiency porphyria
Acute intermittent
porphyria
Uroporphyrinogen
decarboxylase
Congenital erythropoietic
porphyria
Prophyria
cutanea tarda
Herediatary
coproporphyria
Variegate
porphyria
Erythropoietic
protoporphyria
DEFICIENCY of ENZYME PORPHYRIAs
11
12. Acute Intermittent Porphyria
-Autosomal dominant mode of inheritance
-Caused by a deficiency in Hydroxymethylbilane synthase
(PBG deaminase)which is involved in the conversion of
porphobilinogen (PBG) to uroporphyrinogen III
-PBG and δ-ALA accumulate in the plasma and the urine and
urine get darkened on exposure to air due to the
conversion of PBG to porphobilin and porphyrin.
-Characteristics features are
- Usually expressed after puberty in humans
- Symptoms include abdominal pain, vomiting, cardiovascular
abnormality and neuropsychiatric disturbance
- patients are not photosensitive 12
13. Congenital erythropoietic porphyria
- Due to deficiency of Uroporphyrinogen III cosynthase.
- Characteristic features are
a. Rare congenital disorder caused by autosomal
recessive mode of inheritance.
b. Patients excrete uroporphyrinogen I and
coproporphyrinogen I which oxidized respectively to
uroporphyrine I and corpoporphyrin I.
c. The patients are photosensitive due to accumulation
of abnormal porphyrin
13
14. Porphyria cutanea tarda
-Most common porphyria
-Autosomal dominant mode of inheritance
-Disease is caused by a deficiency in uroporphyrinogen
decarboxylase, which is involved in the conversion of
uroporphyrinogen III to coproporphyrinogen III
-Characteristic features are
1. Increased excretion of uroporphyrin I and III and
rarely porphobilinogen
2. Patients are photosensitive (cutaneous photosensitivity)
3. Liver exhibits fluorescence due to high concentration
of accumulated porphyrins
14
16. FATE OF RED BLOOD CELLS
-Life span of RBCs are about 120 days
-At the end of this period, RBCs are phagocytosed
and/or lysed
-Normally, lysis occurs extravascularly by the
macrophages of the reticuloendothelial system in liver
and spleen.
-Lysis can also occur intravascularly (in blood stream)
-About 80% of heme subjected for degradation comes
from RBC. 20% comes from immature RBC, myoglobin
and cytochromes.
16
19. DEGRADATION OF HEME TO BILIRUBIN (Unconjugated)
“unconjugated” bilirubin
-About 80%% of heme is derived
from RBCs
-In normal adults this results in
a daily load of 250-300 mg of
bilirubin
Hydrophobic – transported by
albumin to the liver for further
metabolism prior to its excretion
19
20. NORMAL BILIRUBIN METABOLISM
In liver unconjugated bilirubin is
conjugated with glucoronic acid
catalyzed by UDP glucuronyl
tranferase
“Conjugated” bilirubin is water
soluble and is secreted by the
hepatocytes into the biliary
canaliculi
Converted to urobilinogen
(colorless) by bacteria in the
gut
Oxidized to stercobilin which is
colored and excreted in feces
Some stercobilin may be re-
adsorbed by the gut and re-
excreted by either the liver or
kidney 20
21. HYPERBILIRUBINEMIA
-Recognized clinically as jaundice
-Jaundice is classified into three major types
a. Prehepatic or hemolytic jaundice
b. Hepatic jaundice
c. Posthepatic or obstructive jaundice
Normal value of serum bilirubin
a. Total Bilirubin 0.2-1 mg/dl
b. Conjugated 0.1-0.4 mg/dl
c. Unconjugated 0.1-0.6 mg/dl
-Hyperbilirubinemia is defined as increased plasma
concentrations of bilirubin occurs when there is an
imbalance between its production and excretion
21
22. Prehepatic (hemolytic) jaundice
-Results from excess production of
bilirubin (beyond the livers ability
to conjugate it) following hemolysis
-Excess RBC lysis is commonly the
result of
a. Rh- or ABO incompatibility
b. structurally abnormal
RBCs (Sickle cell disease)
c. Malaria
-High plasma concentrations of
unconjugated bilirubin (normal
concentration 0.1-0.6 mg/dL)
22
23. Intrahepatic jaundice
-Impaired uptake, conjugation,
or secretion of bilirubin by liver
-Reflects a generalized liver
(hepatocyte) dysfunction or
damage ie
a. Viral Hepatitis
b. Cirrhosis of Liver
c. Poisons and toxins
which damage the liver cells
- Increased serum level of
conjugated and unconjugated
bilirubin is usually accompanied
by other abnormalities in
biochemical markers of liver
function
23
24. Posthepatic jaundice 1.Caused by an obstruction of
the biliary tree.
2. Characterized by
- Increased concentration of
conjugated bilirubin in
serum.
- Serum Alkaline
phosphatase is elevated.
- Pale colored stools (due to
absence of faecal
stercobilin
- dark urine ( due to
increased conjugated
bilirubin) 24
25. Neonatal Jaundice (Physiological jaundice of newborn)
-Common, particularly in premature infants
-It occurs due to
a. immaturity of the enzymes involved in bilirubin
conjugation ( UDP glucuronyl Transferase)
b. limitation of availability of the substrate UDP-
glucoronic acid.
- Unconjugated bilirubin is highly elevated in serum.
High levels of unconjugated bilirubin are toxic to the new-
born – due to its hydrophobicity it can cross the blood-brain
barrier and cause a type of mental retardation known as
kernicterus.
-If bilirubin levels are judged to be too high, then
phototherapy with blue light is used to convert it to a water
soluble, non-toxic isomer called Lumirubin which can easily
excreted through urine 25
