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Molecular Biochemistry II. Metabolism of heme. xiaoli. White cell. serum. platelet. Blood cell. Read cell. globin. hemoglobin. heme. pyrrole rings. heme.

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Molecular Biochemistry II

Metabolism of heme

xiaoli


White cell

serum

platelet

Blood cell

Read cell


globin

hemoglobin

heme


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 .


+

HSCoA + CO2

ALA synthase

(pyridoxal phosphate)

Step 1

In mitochondrion

①Heme 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, repressingtranscription 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.


Pyridoxal phosphate (PLP) serves as coenzyme for d-Aminolevulinate Synthase (ALA Synthase), an enzyme evolutionarily related to transaminases.

Condensation with succinyl-CoA 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.


ALA dehydratase

2H2O

Step 2

porphobilinogen

★The 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 de-repression of transcription of the ALA Synthase gene.

High ALA is thought to cause some of the neurological effects of lead poisoning, although Pb++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 oxygenspecies (oxygen radicals).


Porphobilinogen (PBG) is the first pathway intermediate that includes a pyrrole ring.

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.


PBG units are added to the dipyrromethane until a linear hexapyrrole has been formed.


Uroporphyrinogen III Synthase converts the linear tetrapyrrole hydroxymethylbilane to the macrocyclic uroporphyrinogen III.


  • converted to protoporphyrinogen IX, then

  • to protoporphyrin IX


Fe++ is added to protoporphyrin IX via Ferrocheletase, a homodimeric enzyme containing 2 iron-sulfur clusters.

  • heme is formed by incorporation of iron (Fe+2)

  • - partly spontaneous

  • - ferrochelatase enhances rate

  • - also inhibited by lead


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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

1. ALA synthase

Major site of regulation is at the level of ALA synthase.

①It 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.


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.


  • Lack of Vit B6willdecrease 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

  • 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

Porphyriasare 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

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 light-absorbing 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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

- 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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

  • - 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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

- 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 the urine and stool in excessive amounts. When present in very high levels, they cause the urine to have a spectacular port wine color

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


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