Metabolism of heme

Molecular Biochemistry II

xiaoli

serum

Blood cell

Read cell

platelet

White cell

hemehemoglobin

globin

heme

Heme is a derivative of the porphyrin. Porphyrins are cycli

c compounds formed by fusion of 4 pyrrole rings linked by m

ethenyl bridges.

pyrrole rings

NH

pyrrole

- heme proteins (hemoproteins) are rapidly synthsized and degraded - 6 to 7 g per day hemoglobin turned over

- most common porphyrin in humans is heme

- one ferrous goup in tetrapyrole ring

Heme is the prosthetic group of hemoglobin, myoglobin, & cytochromes and so on.

- cyclic compounds that bind metals - usually iron - Fe+2 = ferrous - Fe+3 = ferric

Synthesis of heme ★ The substrates mainly include succinyl-CoA, gly

cine, 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

H2C

CH2

C¡«SCoA

O

CH2NH2

COOH

HSCoA + CO2

ALA synthase

COOH

H2C

CH2

C

CH2NH2

O（ pyridoxal phosphate ）

① Heme synthesis begins with condensation of glycine & succinyl-CoA, with decarboxylation, to form -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.

Pyridoxal phosphate (PLP) serves as coenzyme for -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.

P y rid o x a l p h o sp h a te (P L P )

NH

CO

P

O O

O

O H

C H 3

CH O

H 2

NH

CO

P

OO

O

O

CH3

HC

H2

N

H2C

H

+

COO

glycine-PLP Schiff base (aldimine)

ALA dehydratase

2H2ONH

OH

O

OH

O

NH2

COOH

CH2

CH2

C

C

N

O

H H

H H

porphobilinogen

Step 2

★ 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

(-aminobutyric acid) structures. • ALA autoxidation generates reactive

oxygen species (oxygen radicals).

CH2

CH2

COO

C

CH2

O

NH3+

CH2

CH2

COO

CH2

NH3+

ALA GABA

NH

pyrrole

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.

NH

CH2

COO

CH2

CH2

COO

H2C

NH3+

Porphobilinogen (PBG)

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

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

NH

CH2

CH2

COO-

CH2

COO-

HN

CH2

COO-

CH2 CH2

HN

CH2

CH2

COO-

CH2 COO-

NH

CH2

COO-

CH2

CH2

COO-

COO-

NH

CH2

COO-

CH2

CH2

COO-

SEnz NH

CH2

COO-

CH2

CH2

COO-

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

HO

NH

CH2

CH2

COO -

CH2-OOC

HN

CH2

COO -

CH2 CH2

HN

CH2

CH2

COO -

CH2 COO -

C NH

C

CH2

COO -

CH2

CH2

COO -

COO -

NH

CH2

CH2

COO -

CH2-OOC

HN

CH2

COO -

CH2 CH2

HN

CH2

CH2

COO -

CH2 COO -

COO -

C NH

C

CH2

CH2

COO -

CH2-OOC

Uroporphyrinogen III Synthase

hydroxy- uroporphyrinogen methylbilane III

NH

CH

CH2

H3C

N

CH3

CH CH2

HN

CH2

CH2

COO-

CH3

N

CH2

CH2

COO-

H3C

protoporphyrin IX

NH

CH2

CH2

COO-

CH2-OOC

HN

CH2

COO-

CH2 CH2

HN

CH2

CH2

COO-

CH2 COO-

COO-

NH

CH2

CH2

COO-

CH2-OOC

uroporphyrinogen III

- uroporphyrinogen II converted to coproporphyrinonogen III - transported back into mitochondria

- converted to protoporphyrinogen IX, then to protoporphyrin IX

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

NH

CH

CH2

H3C

N

CH3

CH CH2

HN

CH2

CH2

COO-

CH3

N

CH2

CH2

COO-

H3C

N

CH

CH2

H3C

N

CH3

CH CH2

N

CH2

CH2

COO-

CH3

N

CH2

CH2

COO-

H3C

Fe

Ferrochelatase

protoporphyrin IX heme

2H+ Fe++

- heme is formed by incorporation of iron (Fe+2) - partly spontaneous - ferrochelatase enhances rate - also inhibited by lead

URO

COPROPROTO

①major 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

summary

Regulation of heme synthesis

① It is regulated by repression mechanism. Hem

e inhibits the synthesis of ALA synthesis by acting

as a corepressor. The feedback regulatory effect is

a typical example of end-product inhibition.

1. ALA synthase

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

Regulation of heme synthesis

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 synthas

e from the cytoplasm into the mitochondria where its substr

ate, succinyl CoA is formed. thus heme synthesis is inhibit

ed 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 decr

ease the availability of pyridoxal phosphate may als

o 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 erythropoieti

n.

The function of erythropoietin is to stimulate the producti

on of red blood cells. The kidney produces 85~95% of the b

ody's erythropoietin so when the kidney is damaged (kidne

y disease or failure), not enough erythropoietin is produced

to maintain normal red blood cell levels. This leads to anem

ia.

normal

anemia

URO

COPROPROTO

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 are genetic diseases in which activi

ty of one of the enzymes involved in heme synthe

sis is decreased (e.g., PBG Synthase, Porphobilin

ogen Deaminase, etc…).

Porphyrias

Symptoms vary depending on ★ the enzyme

★ the severity of the deficiency

★ whether heme synthesis is affected primarily in liv

er 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 -aminolevulinic acid (ALA), arising from derepression of ALA Synthase gene transcription, is considered responsible for the neurological symptoms.

common symptom of Porphyrias

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-absorbin

g pathway intermediates and their degradation product

s.

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 andmental 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

caused by hereditary or acquired defects in heme synthesis

— genetic diseases: the enzymes of heme synthesis

— Liver desfunction, lead posioning

can be grouped into erythropoietic porphyria a

nd hepatic porphyria

- hepatic can be acute or chronic

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.

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.

Acute hepatic porphyrias

- similar symptoms - acute attacks of gastrointestinal pain, neurologic / psychologic, cardiovascular.

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