Download Metabolism of heme

Molecular Biochemistry II
Metabolism of heme
xiaoli
White
cell
serum
platelet
Blood
cell
Read
cell
hemoglobin
globin
heme
N
H
pyrrole
pyrrole rings
heme
Heme is a derivative of the porphyrin. Porphyrins are
cyclic compounds formed by fusion of 4 pyrrole rings linked
by methenyl bridges.
Heme is the prosthetic group of hemoglobin,
myoglobin, & cytochromes and so on.
- most common porphyrin in humans is heme
- one ferrous goup in tetrapyrole ring
- heme proteins (hemoproteins) are rapidly synthsized and
degraded
- 6 to 7 g per day hemoglobin turned over
- cyclic compounds that bind metals
- usually iron
- Fe+2 = ferrous
- Fe+3 = ferric
Synthesis of heme
★ The substrates mainly include succinyl-CoA,
glycine, Fe2+ .
★ Heme can be synthesized by almost all the
tissues in the body which require hemoproteins.
★major sites of synthes is liver and bone marrow
(erythroblasts: reticulocyte, prorubricyte)
- cytochrome p450 in liver
- hemoglobin in bone marrow
- heme production equal to globin synthesis in marrow
- variable in liver dependent on heme pool balance
Inside RBCs, heme is synthesized in the normoblasts,
but not in the matured ones .
Inside RBCs, heme is
synthesized in the
normoblasts, but not in
the matured ones .
Step 1
In mitochondrion
COOH
COOH
H2C
CH 2
+
C¡«SCoA
O
CH 2NH 2
COOH
HSCoA + CO2
H2C
CH 2
ALA synthase
（pyridoxal phosphate）
C
O
CH 2NH 2
① Heme synthesis begins with condensation of
glycine & succinyl-CoA, with decarboxylation, to
form d-aminolevulinic acid (ALA).
ALA Synthase is the committed step of the heme
synthesis pathway, & is usually rate-limiting for the
overall pathway.
Regulation occurs through control of gene
transcription.
Heme functions as a feedback inhibitor, repressing
transcription of the ALA Synthase gene in most
cells.
A variant of ALA Synthase expressed only in
developing erythrocytes ,is regulated instead by
availability of iron in the form of iron-sulfur clusters.
H
Pyridoxal phosphate (PLP)
serves as coenzyme for dAminolevulinate Synthase
(ALA Synthase), an enzyme
evolutionarily related to
transaminases.
O
O
P
C
H2
C
OH
O
O

N
CH3
H
Pyridoxal phosphate (PLP)
Condensation with succinylCoA takes place while the
amino group of glycine is in
Schiff base linkage to the
PLP aldehyde.
CoA & the glycine carboxyl
are lost following the
condensation.
O
H2C
COO
N+
HC
O
O
H2
C
P
O
H
O
O

N
H
CH3
glycine-PLP Schiff base (aldimine)
Step 2
COOH
O
HO
CH 2
ALA dehydratase
O
C
H
C
H
H
N
H
O
2H2O
H2N
★ The
OH
porphobilinogen
CH 2
N
H
succeeding few reactions occur in the cytoplasm.
one ALA condenses with another molecule of ALA to form
porphobilinogen(PBG).
★ the
condensation involves removal of 2 molecules of water
and the enzyme is ALA dehydratase .
the enzyme contains zinc and is very sensitive to lead
and other heavy metals.
Inhibition of Porphobilinogen Synthase by Pb++ results in
elevated blood ALA, as impaired heme synthesis leads to derepression of transcription of the ALA Synthase gene.
High ALA is thought to cause some of the neurological
effects of lead poisoning, although Pb++
COO
COO
also may directly affect the nervous system.
ALA is toxic to the brain, perhaps due to:
•
Similar ALA & neurotransmitter GABA
(g-aminobutyric acid) structures.
•
ALA autoxidation generates reactive
oxygen species (oxygen radicals).
CH2
CH2
CH2
CH2
C
O
CH2
CH2
NH3+
NH3+
ALA
GABA
COO
COO
CH2
H2C
NH3+
CH2
CH2
N
H
Porphobilinogen
(PBG) is the first
pathway intermediate
that includes a
pyrrole ring.
N
H
pyrrole
Porphobilinogen (PBG)
The porphyrin ring is formed by condensation of 4
molecules of porphobilinogen.
Porphobilinogen Deaminase catalyzes successive PBG
condensations, initiated in each case by elimination of
the amino group.
COOCOO-
CH2
COO- COO-CH
COO-
Enz
S
COO- CH2
COO- CH2
CH2
CH2
N
H
CH2
CH2
2
CH2
CH2
CH2
N
H
NH
HN
NH
HN
CH2 COO-
CH2 COOCH2
CH2 CH2
CH2
COO-COO-
CH2
COO-
PBG units are added to the dipyrromethane until a
linear hexapyrrole has been formed.
COO-
hydroxymethylbilane
-
OOC
COO-
uroporphyrinogen
III
-
CH2
COO-
CH2
COO
CH2
CH2
CH2
CH2
CH2
CH2
NH
HN
NH
HN
CH2 COO-
-
OOC CH2
NH
HN
NH
HN
CH2
CH2 COO-
CH2
COO-
HO
C
C
CH2
C
COO-
-
OOC CH2
C
CH2
CH2 CH2
CH2
COO-COO-
CH2
COO-
Uroporphyrinogen III
Synthase
CH2
CH2
CH2
CH2
COO-
COO-
Uroporphyrinogen III Synthase converts the linear
tetrapyrrole hydroxymethylbilane to the macrocyclic
uroporphyrinogen III.
-
COO
-
OOC
protoporphyrin IX
uroporphyrinogen III
CH2
COO-
CH2
CH2
CH2
CH
CH2
CH2
CH2
-
COO
CH3
CH CH2
H3C
NH
HN
NH
NH
HN
N
-
CH2
OOC CH2
COO-
N
HN
H3C
CH3
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
COO-
COO-
COO-
COO-
- uroporphyrinogen II converted to
coproporphyrinonogen III
- transported back into mitochondria
- converted to protoporphyrinogen IX, then
to protoporphyrin IX
CH2
protoporphyrin IX
CH2
CH
CH3
CH
CH CH2
H3C
NH
N
N
++
Fe
heme
CH3
CH CH2
H3C
2H
N
N
+
Fe
HN
N
H3C
CH3
CH2
CH2
CH2
COO-
N
H3C
Ferrochelatase
CH3
CH2
CH2
CH2
CH2
CH2
COO-
COO-
COO-
- heme is formed by incorporation of iron (Fe+2)
- partly spontaneous
- ferrochelatase enhances rate
- also inhibited by lead
Fe++ is added to protoporphyrin IX via Ferrocheletase, a
homodimeric enzyme containing 2 iron-sulfur clusters.
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summary
① major sites of synthes is liver and bone marrow
(erythroblasts). Matured red blood cells have no
mitochondria, so can’t make heme.
② The substrates mainly include succinyl-CoA,
glycine, Fe2+ .
③ first and last 3 reactions take place in mitochondria
- others in cytoplasm
Regulation of heme synthesis
Regulation of heme synthesis
1. ALA synthase
Major site of regulation is at the level of ALA synthase.
① It is regulated by repression mechanism.
Heme inhibits the synthesis of ALA synthesis by
acting as a corepressor. The feedback regulatory
effect is a typical example of end-product inhibition.
② ALA synthase is also allosterically inhibited by
hematin.
When there is excss of free heme without globin
chains to bind with, the Fe++ is oxidized to Fe+++
forming hematin. Hematin will inhibit ALA synthase
to prevent excessive unwanted production of heme.
Hematin will also inhibit the translocation of ALA
synthase from the cytoplasm into the mitochondria where
its substrate, succinyl CoA is formed. thus heme synthesis
is inhibited till there are sufficient globin chains to bind with.
③ Lack of Vit B6 will decrease the synthesis of ALA.
Drugs like INH (isonicotinic acid hydrazide) that
decrease the availability of pyridoxal phosphate may
also affect heme synthesis.
2. Heme synthesis may be inhibited by heavy
metals. the steps catalyzed by ALA dehydratase
and ferrochelatase are inhibited by lead.
3. erythropoietin, EPO
The kidneys also secrete a hormone called
erythropoietin.
The function of erythropoietin is to stimulate the
production of red blood cells. The kidney produces 85~95%
of the body's erythropoietin so when the kidney is damaged
(kidney disease or failure), not enough erythropoietin is
produced to maintain normal red blood cell levels. This
leads to anemia.
anemia
normal
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Porphyria

Porphyria is a name given to a group of metabolic
disorders. These disorders cause the individual to
accumulate "porphyrins" or "porphyrin precursors" in
their body. which in turn causes an abundance of the
porphyrins.

In porphyria, the cells do not convert porphyrins to
heme in a normal manner.
Porphyrias
Porphyrias are genetic diseases in which
activity of one of the enzymes involved in heme
synthesis is decreased (e.g., PBG Synthase,
Porphobilinogen Deaminase, etc…).
Symptoms vary depending on
★ the enzyme
★ the severity of the deficiency
★ whether heme synthesis is affected primarily in
liver or in developing erythrocytes.
common symptom of Porphyrias
common symptom of Porphyrias
1. Occasional episodes of severe neurological
symptoms are associated with some porphyrias.
◆had acute bouts of abdominal pain and mental
confusion
Permanent nerve damage and even death can
result, if not treated promptly.
Elevated d-aminolevulinic acid (ALA), arising from
derepression of ALA Synthase gene transcription, is
considered responsible for the neurological symptoms.
2. Photosensitivity is another common symptom.
◆formation
of superoxide radicals.
◆ Skin damage may result from exposure to light.
This is attributable to elevated levels of lightabsorbing pathway intermediates and their
degradation products.
3. porphyrins build up in the body and are
excreted in the urine and stool in excessive
amounts. When present in very high levels, they
cause the urine to have a spectacular port wine
color.
normal
King George III - Mad King George
had acute bouts of abdominal pain and
mental confusion
- may have been porphyria sufferer
- complicated by all the drugs his doctors gave him
vampires and wherewolves?
- some have put forth that porphyrias misinterpreted
in Middle Ages
- consider photosensitivity, red blood (even teeth)
hypertrichosis
Porphyrias
can be grouped into erythropoietic porphyria
and hepatic porphyria
- hepatic can be acute or chronic
caused by hereditary or acquired defects in heme synthesis
— genetic diseases: the enzymes of heme synthesis
— Liver desfunction, lead posioning
Acute hepatic porphyrias
Each acute hepatic porphyria is a result of a deficiency
of one of the enzymes in the heme biosynthesis pathway.
These deficiencies result in an accumulation of the
precursors of porphyrins in the liver (delta-aminolevulinic
acid, ALA and porphobilinogen, PBG) and also, in the case
of variagate porphyria and hereditary coproporphyria, an
accumulation of porphyrins resulting in cutaneous
manifestations.
- similar symptoms - acute attacks of gastrointestinal
pain, neurologic / psychologic, cardiovascular.
When an acute attack is confirmed, urgent treatment with an injection
of human hemin and/or perfusion of carbohydrates is required.
Management includes the prevention of attacks (by avoiding causal
factors) and the protection of skin from the light in cases of cutaneous
manifestations.
erythropoietic porphyrias
- congenital erythropoietic porphyria (uroporphyrinogen III
synthase)
- erythropoietic protoporphyria (ferrochelatase)
symptoms include:
- skin rashes and blisters early in childhood
- cholestatic liver cirrhosis and progressive liver failure
Treatment for Porphyrias
- medical support for vomiting and pain
- hemin, decreases ALA synthase synthesis
- avoidance of sunlight and precipitating drugs, factors
porphyria cutanea tarda
-
a chronic porphyria
liver and erythroid tissues
deficiencey in uroporphyrinogen decarboxylase
often no symptoms until 4th or 5th decade
clinical expression determined by many factors:
- hepatic iron overload
- exposure to sunlight
- hepatitis B or C
- HIV
symptoms include:
- cutaneous rashes, blisters
- urine that is red to brown in natural light, or
pink to red in UV light
Acquired Porphyrias
- hexochlorobenzene used as a fungicide in Turkey in 1950s
- thousands of children ate bread from treated wheat
- they acquired porphyria cutanea tarda
due to inhibition of uroporphyrinogen
decarboxylase
- due to hypertrichosis - referred to locally
as the “monkey children”
Acquired Porphyrias
lead poisoning
-inhibition of ferrochelatase 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