Porphyrins (Structure of Porphyrins)
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Objective:
In addition to serving as building blocks
for proteins, amino acids are precursor
of many nitrogen-containing
compounds that have important
physiologic functions, these molecules
include porphyrin.
Porphyrins (Structure of Porphyrins)
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Porphyrins are cyclic compounds that readily
bind metal ions, usually Fe2+, Fe3+.
The most prevalent metallopophyrin in
human is heme, which consists of one Fe2+
coordinated in the center of tetrapyrrol ring
protoporphyrin lX through methenyl bridges.
Heme is the prothetic group for hemoglopin,
myoglopin, the cytochromes, catalase and
tryptophane pyrrolase
Porphyrins (Structure of Porphyrins)
Structure of Porphyrins
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(1) Side chains :Ex., uroporphyrin contains
acetate (-ch2-coo-), and propionate (-ch2-ch2coo-). while coproporphyrin is substituted
with methyle (-ch3) and propionate group.
Distribution of side chains: The side chains of
porphyrins can be ordered around the
tetrapyrrol nucleus in four different ways
designated by Roman numerals I & IV. Only
type III Porphyrins which contains
asymmetric substitution on ring D are
physiologically important in humans.
Biosynthesis of heme
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Liver is the major site of heme
biosynthesis.
Liver synthesizes a number of heme
proteins, cytochrome P 450, and the
erythrocyte-producing cells of the bone
marrow which are active in hemoglobin
synthesis.
Biosynthesis of heme.
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In the liver: The rate of heme synthesis is
highly variable, responding to alterations in
the cellular heme pool caused by fluctuating
demands for heme proteins.
In contrast, heme synthesis in erythroid cells
is relatively constant , and is matched to
globin synthesis
The initial reaction and the last three steps
occur in the mitochondria, whereas the
intermediate steps occur in the cytosol.
Biosynthesis of heme
(1) Formation of δ-aminolevulinic acid (ALA).
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All the carbon of the porphyrin molecule are
provided by by two simple blocks: (glycine
and succinyl CoA ).
glycine and succinyl CoA condense to form
ALA in a reaction catalyzed by ALA synthase.
This reaction requires pyridoxal phosphate as
a coenzyme.
It is a rate limiting steps.
Biosynthesis of heme
(2) Formation of porphobilinogen.
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In this reaction: The dehydration of two
molecules of ALA to form
porphobilinogen by ALA dehydratase (
this enzyme is extremely sensitive to
inhibition by heavy metal ions).
(1) Formation of δ-aminolevulinic acid (ALA).
Biosynthesis of heme
(3) Formation of uroporphybilinogen
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The condensation of four molecules of
porphobilinogen results in the formation
of uroporphybilinogen lll
This reaction requires
hydroxymethybilane synthase and
uroporphybilinogen lll synthase.
Biosynthesis of heme
(4) Formation of heme.
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uroporphybilinogen lll is converted to
heme by a series of decarboxylation
and oxidation reactions
The introducing of Fe+2 into
protoporphyrin spontaneously, but the
rate is enhanced by ferrochelatase ( an
enzyme inhibited by lead.
Regulation of heme synthesis
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(1) The rate controlling step is the formation of ALA
in which, when porphyrin production exceeds the
availability of globin or other apoproteins. Heme
accumulates and is converted to hemin by oxidation
of Fe+2 to Fe+3.
Hemin decreases the activity of hepatic ALA
synthase by causing decreased the synthesis of the
enzyme
Note: in erythroid cells, heme synthesis is under the
control of erythropoietin and the availability of
intracellular iron.
Regulation of heme synthesis
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(2) Effect of drugs on ALA Synthase activity: Ex, phenobarbital,
griseofulvin or hydantions. These drugs are metabolized
microsomal cytochrome P450 monooxygenase system (a hemeprotein oxidase system found in the liver)
In response to this drugs, the synthesis of cytochrome P450
increases, leading to an enhanced consumption heme.(heme a
component of cytochrome P450 )
This in turn, cause a decrease concentration of heme in the liver
cells.
The lower intracellular heme concentration leads to an increase
in the synthesis of ALA Synthase , and prompts a corresponding
increase in ALA synthesis.
Degradation of heme.
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After approximately 120 days in the
circulation, red blood cells are taken up
and degraded by the reticuloendothelial (RE) system, particularly in
the liver and spleen.
Degradation of heme.
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(1) Formation of bilirubin:
Microsomal heme oxygenase (HO) system of the RE
cells responsible for the first step of heme
degradation. (a) In presence of NADH & O2, the
enzyme adds hydroxyl group to the methenyl bridge
between two pyrrole rings with a concomitant
oxidation of Fe+2 to Fe+3 (b) by the same enzyme
system a second oxidation results in cleavage of
porphyrin ring, Fe+3 & Co is released in addition to
production of green pigment Biliverdin.( c) Biliverdin
is reduced by biliverdin reductase forming bilirubin
( red-orange pigment, bile pigment)
Degradation of heme.
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(2) Uptake of bilirubin by the liver.
Bilirubin is slightly soluble in plasma, so
it transported to the liver by binding
non-covalently to albumin.
Bilirubin dissociated from albumin and
enter hepatocyte, where it binds to
intracellular proteins (ligandin)
Degradation of heme.
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(3)Formation of bilirubin diglucuronide:
In hepatocytes bilirubin is conjugated to
2 molecules of glucuronic acid in
presence of bilirubin
glucuronyltransferas using UDPglucuroic acid (as a glucuronate donor)
Degradation of heme
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(4) Excretion of bilirubin into bile: Bilirubin
diglucuronide is actively transported against
concentration gradient into the bile canaliculi
and then into the bile.
This energy –dependant, rate limiting step is
susceptible to impairment in the liver disease
Unconjugated bilirubin is normally not
excreted.
Degradation of heme
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(5)Formation of urobilins in the intestine. In the gut,
bilirubin diglucuronide is hydrolyzed by bacteria to
yield urobilinogen ( a colourless compound) , (a)
Most urobilinogen is oxidized to sterbilin( brown
color & excreted in feces). (b) Some is reabsorbed
from the gut and enter portal blood to participate in
the enterohepatic urobilinogen cycle. © The
remainder urobilinogen is transported by the blood
to the kidney where it is converted to yellow
urobilinogen (give urine its characteristic color)
Disorders in heme metabolism
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In heme synthesis:
(1) Porphyrias are classified as
erythropoietic or hepatic, depending on
location of the enzyme deficiency
occurs in the erythropoietic cells or in
the liver.
Porphyrias
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(1) Porphyria cutanea tarda:
A chronic disease caused by a deficiency in
uroporphyrinogen decarboxylase .
Uroporphyrin accumulates in the urine
It is the most common Porphyria
Patients are photosenstive, their skin itches
and burn (pruritis) when exposed to visible
light.
(1) Porphyria cutanea tarda:
Porphyrias
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(2) Acute hepatic Porphyrias: they are
characterized by acute attacks of gastrointestinal ,
neurologic/ psychatric and cardiovascular symptoms.
(A) Acute intermittent Porphyria: An acute disease
caused by a deficiency in hydroxymethylbilane
synthase.
Porphobilinogen and δ-aminolevulinic acid
accumulate in the urine.
Urine darkens on expoure to light and air.
Patients are not photosenstive
Porphyrias :
Acute hepatic Porphyrias
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(2) Hereditary coproPorphyria: An acute
disease caused by a deficiency in
coproPorphyrinogen oxidase
CoproPorphyrinogen lll and other
intermediates prior to the block
accumulate in the urine.
Patients are photosenstive.
Porphyrias :
Acute hepatic Porphyrias
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(3) Varigate Porphyria:
An acute disease caused by a deficiency
in protoporphyrinogen oxidase .
ProtoPorphyrinogen lX and other
intermediates prior to the block
accumulate in the urine.
Patients are photo-senstive.
Porphyrias :
Erythropoietic Porphyrias
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Erythropoietic Porphyrias are characterized
by skin rashes and blisters that appear
in the early childhood. The disease are
complicated by cholestatic liver cirrhosis
and progressive hepatic failure.
Porphyrias :
Erythropoietic Porphyrias
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(1) Erythropoietic Porphyrias:
The disease caused by a deficiency in
ferrochelates.
ProtoPorphyrin accumulate in the
erythrocytes, bone marrow and plasma
Patients are photosenstive.
Porphyrias :
Erythropoietic Porphyrias
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(2) Congenital erythropoietic Porphyrias:
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The disease caused by a deficiency in
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uroporphyrinogen lll synthase.
Uroporphyrinogen l &
coproPorphyrinogen l accumulate in
urine
Patients are photosenstive.
Porphyrias
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Increased in ALA synthase activity:
One of the common feature of the Porphyrias
is a decreased in heme synthesis.
In the liver heme functions as a repressor of
ALA synthase .Therefore, the absence of this
end product results in an increase in the
synthesis of ALA synthase (derepression).
This leads to an increased in synthesis of
intermediates (they are the major
pathophysiology of the Porphyrias
Porphyrias
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Treatment:
Treatment of vomiting and pain.
I.V injection by hemin, which decrease
the synthesis of ALA synthase
Avoidance of sunlight and ingestion of
β-carotene ( free-radical scavenger) are
also helpful.
Disorders in heme metabolism
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In Degradation of Heme
Jaundice: it is also called icterus, which refers
yellow color of skin, nail beds, and sclerae
caused by deposition of bilirubin.
Types of Jaundice:
(1) Hemolytic Jaundice
(2) obstructive Jaundice
(3) Hepatocellular Jaundice
Jaundice
Disorders in heme metabolism
Jaundice
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(1) Hemolytic Jaundice: In massive lysis of
RBCs (in patients with sickle cell anemia, PK,
G-6-P dehydrogenase deficiency or malaria)
Bilirubin produced faster than it can be
conjugated. So more bilirubin is excreted in
the bile, the amount of urobilinogen enter
enterohepatic circulation is increased.
Uncojugated bilirubin level is increased.
Disorders in heme metabolism
Jaundice
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(2) obstructive Jaundice:
Jaundice results from obstruction of the bile
duct ( due to hepatic tumor or bile stone may
block the bile duct), preventing passage of
bilirubin in the intestine.
Patients with obstructive jaundice suffer from
gastrointestinal pain and nausea and produce
stools that are a pale ,clay color
The liver regurgitates bilirubin into the blood.
The compound is excreted in the blood.
Disorders in heme metabolism
Jaundice
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(3) Hepatocellular Jaundice.
Damage of the liver cells ( Ex. Liver cirrhosis or
hepatitis) can cause increased in the unconjugated
bilirubin level in the blood
The bilirubin that conjugated is not efficiently
secreted in the bile but diffuses into the blood.
The urine becomes dark ( urobilinogen increased) ,
whereas stools are a pale ,clay color. Patients nausea
Plasma level of (SGOT), (SGPT) are elevated.
Jaundice
Jaundice
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(4) Jaundice in new born:
Premature babies often accumulate bilirubin (
activity of bilirubin glucuronyl transferase is
low)
Elevated bilirubin, more than albumin
capacity , can diffuse into the basal ganglia
and cause toxic encephalopathy
Treatment: By blue fluorescent light, which
converts bilirubin to more polar and watersoluble isomers.
Jaundice in new born
Determination of bilirubin
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By Van de Bergh reaction: in which diazotized
sulfanilic acid reacts with bilirubin to form red
azodipyrroles that measured colorimetry.
Direct bilirubin: In aqueous solution
conjugated bilirubin reacts rapidly with the
reagent
Indirect bilirubin: when the reaction carried
out in methanol , both conjugated bilirubin
and unconjugated bilirubin are soluble and
reacted with the reagent and give total
bilirubin