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7/28/2019 HEME SYNTHESIS AND PORPHYRIAS

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Biologically important porphyrins are usually conjugated

proteins consisting of a metalloporphyrin linked to aprotein.

Some biologically important compounds containing

porphyrins are the following:

Haemo lobin

Myoglobin

Cytochromes

Catalase

Peroxidase

Tryptophan pyrrolase

Nitric oxide synthase


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Haemoglobin In haemoglobin, iron-

porphyrin is linked withglobin

Each of the four subunits,

a ferrous ion and apolypeptide chain

Functions of Hemoglobin ??


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Myoglobin

Structure similar to that of haemoglobin

with the difference that it is a monomer

Cytochromes

Contain iron-porphyrin conjugated to proteins

Components of respiratory chain in mitochondria and

transport electrons


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Catalase

An iron-porphyrin containing enzyme that acts on

hydrogen peroxide

Peroxidase

Iron-porphyrin containing enzyme that acts on hydrogen

peroxide

Tryptophan pyrrolase

Iron-porphyrin containing enzyme

oxidises tryptophan


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HEME: INTRODUCTION

Heme is a derivative of the

porphyrin.

Heme is a ferroprotoporphyrin.

Porphyrins are a group of

compounds formed by the

union of four pyrrole rings

through methenyl Bridges.

The porphyrins usually contain a metal ion linked to the

nitrogen atoms of the pyrrole rings.


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The Pyrrole rings are

named as I, II, III, IV.

The bridges that connects

pyrrole rings are named as

alpha, beta, gamma and

delta methenyl bridges.

The possible areas of

substitution are denoted as

1 to 8.


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The carbon atom numbers

1 to 8 have different

substituents attached to

them

Propionate (P)

Methyl (M)

Vinyl (V)


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How to write Metabolic pathways in Exams.

1. Definition

2. Site of Pathway

3. Subcellular Site

4. Actual Pathway/ Cycle (Reactions)

5. Energetics

6. Regulation of Pathway/ Cycle

7. Associated disorders/ Clinical significance


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HEME BIOSYNTHESIS


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DEFINITION:

Synthesis of Heme by Succinyl CoA and Glycine is calledas Heme Biosynthesis.

SITE OF BIOSYNTHESIS:

Major site: Liver & Bone Marrow (Erythryoidroducin cells

Minor Site: synthesized by other tissues also to someextent

Heme is synthesized in Normoblasts, but not in thematured erythrocytes.

SUBCELLULAR SITE:

Partly in Mitochondria, partly in cytoplasm


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STAGES OF BIOSYNTHESIS

Synthesis of porphyrins can be divided into three stages:

STAGE I :

Synthesis of -Amino Levulinic Acid (ALA) which occurs

in Mitochondria.

STAGE II :

Synthesis of Coproporphyrinogen III and

Coproporphyrinogen I which occurs in Cytosol.

STAGE III :

Synthesis of Protoporphyrin III which occurs in

Mitochondria again.


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Overview ofOverview of HemeHeme SynthesisSynthesis

Succinyl CoA + Glycine

-aminolevulinic acid

Coproporphyrinogen III

Protoporphyrinogen IX

Protoporphyrin IX

Heme

ALA synthase

Uroporphyrinogen I Coproporphyrinogen I

Heme synthesis occurs in all cells due to the requirement for heme as a prosthetic group onenzymes and electron transport chain. By weight, the major locations of heme synthesis are the

liver and the erythroid progenitor cells of the bone marrow.

-aminolevulinic acid

Porphobilinogen Uroporphyrinogen III Coproporphyrinogen III

cytoplasm

mitochondrial matrix


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STEPS IN HEME BIOSYNTHESIS

1. Synthesis of -amino levulinic acid (ALA)

2. Formation of Porphobilinogen (PBG)

3. Formation of Uroporphyrinogen III (UPG-III)

4. Formation of Co ro or h rino en III (CPG-III)

5. Formation of Protoporphyrinogen III or IX

6. Formation of Protoporphyrin III or IX

7. Formation of HEME


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STEP 1: Synthesis of -amino levulinic acid

Synthesis of porphyrins

begins with the condensation

of Succinyl CoA, an

intermediate of citric acid

cycle, with glycine (non

This reaction is catalyzed by

-amino levulinic acidsynthetase (ALA synthetase)


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STEP 2: Formation of Porphobilinogen

Two molecules of ALA condense to form the first pyrrole

compound, porphobilinogen (PBG)

This reaction is catalyzed by ALA Dehydratase (cytoplasmic)

which is a Zn containing enzyme.


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STEP 3 TO STEP 5


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STEP 3: Formation of Uroporphyrinogen III

Four porphobilinogen molecules react in the presence of

uroporphyrinogen I synthetase/PBG deaminase/HMB

synthase to form Hydroxymethylbilane (HMB)

,

or

it can be enzymatically cyclised to uroporphyrinogen III

by uroporphyrinogen I synthetase and uroporphyrinogen

III cosynthetase


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STEP 4: Formation of Coproporphyrinogen III

Uroporphyrinogen IIIis decarboxylated to

Coproporphyrinogen

III by

Uroporphyrinogen

decarboxylase

(cytoplasmic)

In this reaction all 4

acetate groups aredecarboxylated to

methyl groups


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STEP 5: Formation of Protoporphyrinogen III or IX

Coproporphyrinogen III is oxidised to Protoporphyrinogen III bycoproporphyrinogen oxidase (mitochondrial)

This enzyme acts specifically on type III series and not on type I

series.

2 Propionate groups (ring I and II) are oxidatively decarboxylated

to vinyl groups.


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STEP 6: Formation of Protoporphyrin III

or IX Protoporphyrinogen III is oxidised to Protoporphyrin III

by protoporphyrinogen oxidase (mitochondrial)

Methylene bridges (-CH2 -) oxidized to methenyl bridges

(CH=)


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STEP 7: Formation of HEME

Protoporphyrin III is the most abundant and mostimportant porphyrin.

Heme is synthesized from attachment of ferrous iron to

protoporphyrin III in the presence of heme synthetase /

ferrochelatase (mitochondrial enzyme)

haemoglobin and other hemoproteins


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Regulation of heme biosynthesis

ALA synthase is a key regulatory enzymeit is an allosteric enzyme that is inhibited by end product heme

(feedback inhibition)

Regulation occurs by Repression-Derepression.

Repression is the mechanism by which the presence of excess

product shuts off the key enzyme of that pathwaye regu a or s eme se

When heme is not being utilized, its concentration increasesand it combines with an Apo-repressor to form the Holo-Repressor

The Repressor acts on the ALA synthetase gene (operator) andstops the transcription (synthesis of ALA synthetase)

When heme begins to be utilized, its concentration decreases,and the synthesis of ALA synthetase is De-Repressed


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ALA synthase is also allosterically inhibited by hematin.

When there is excess of free heme, Fe++ is oxidized toFe+++, thus forming hematin

Porphobilinogen synthase is inhibited by lead ions Pb2+ in

case of lead poisoning.

Ferrochelatase (heme synthase) can be also inhibited by2+

.

Compartmentalization of biosynthetic enzymes

High cellular glucose concentration prevents induction of

ALA synthase


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Disorders of Heme Synthesis

Acquired: Lead poisoning

Con enital: Por h rias


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PORPHYRIAS

A group of rare disorders caused by deficiencies of enzymes

of the heme biosynthetic pathway

Can be hereditary or acquired

PorphyriaPorphyriameans purple colour caused by pigment-like

or h rins in urine of atients.

An inherited defect in an enzyme of heme synthesis results

in accumulation of one or more of porphyrin precursors

depending on location of block of the heme synthesispathway.

These precursors increase in blood & appear in urine of

patients which are toxic at high concentrations


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Their overproduction causes the characteristic

neurovisceral and/or photosensitizing symptoms.

Attacks of the disease are triggered by certain drugs,

chemicals, and foods, and also by exposure to sun

Treatment involves administration o hemin, which

provides negative feedback for the heme biosynthetic

pathway, and therefore, prevents accumulation of

heme precursors.


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PORPHYRIAS

GLYCINE + SuccinylCoA

-aminolevulinic acid(ALA)

Porphobilinogen(PBG)

hydroxymethylbilane

ALA synthase

ALA dehydratase

PBG deaminase

Uroporphyrinogen III

ALAD porphyria

Acute intermittentporphyria

Congenital erythropoietic

Mitochondria

uroporphyrinogen III

coprophyrinogene III

Protoporphyrinogene IX

protoporphyrin IX

Heme

cosyn ase

Uroporphyrinogendecarboxylase

Coproporphyrinogen

oxidase

Protoporphyrinogenoxidase

Ferrochelatase

Prophyriacutanea tarda

Herediatarycoproporphyria

Variegateporphyria

Erythropoieticprotoporphyria


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CLASSIFICATION

1. ACQUIRED PORPHYRIA

2. HEREDITARY PORPHYRIAS

(i) Erythropoietic Porphyrias

(ii) Hepatic Porphyrias


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In hereditary types, though the defective gene is present in

all the tissues, the expression is usually confined to a

particular tissue

Depending upon the site of expression of the genetic defect,

porphyrias may be divided into hepatic porphyrias and

erythropoietic porphyrias


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Hepatic Porphyrias include :

Acute Intermittent Porphyria(AIP) Porphyria Cutanea Tarda (PCT)

Hereditary Coproporphyria(HCP)

Variegate Porphyria(VP)

Erythropoietic Porphyrias include :

Congenital Erythropoetic Porphyria(CEP)

Erythropoetic Protoporphyria(EPP)


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HEPATIC PORPHYRIAS

Acute attacks of abdominal pain, nausea and vomiting Overactivity of the sympathetic nervous system

produces tachycardia, tremors and hypertension

Psychiatric symptoms, e.g. anxiety, insomnia,

disorientation and depression Seizures can also occur

The biochemical basis of neuro-visceral abnormalities is

not known

In addition to the neuro-visceral signs and symptoms,cutaneous photosensitivity of varying degrees is also

present in hereditary coproporphyria, variegate

porphyria and porphyria cutanea tarda


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The hepatic porphyrias primarily affect thenervous s stem resultin in.

What made King Geoge III so very mad?

The British ruler in 1776 was diagnosed with Porphyria a geneticdisorder that causes psychiatric disturbances, among other things.

Many of his obstinate decisions , including the ones which led to warof American independence, were made when he had acute attacks ofintermittent porphyria

Abdominal

pain

vomiting acute

neuropathy

seizures hallucinations depression

anxiety cardiac

arrhythmias

fast heart rate

(tachycardia)

Chronic and

acute pain

paranoia Constipation

and/or

diarrhea


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The neuro-visceral manifestations are precipitated by :

Steroids

Alcohol

. . , ,

mephenytoin, sulphonamides, griseofulvin etc.


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Erythropoietic porphyrias

Severe cutaneous photosensitivity from a very early ageis the hallmark of Erythropoietic Porphyrias.

Porphyrin precursors are present in skin, and damage the

skin on exposure to sunlight.

Multiple vesicles erupt on the skin. Skin is pigmented and ragile

Denuded areas on skin are prone to infections

Bones and teeth may be pigmented due to deposition of

porphyrin precursor


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Excessiveconcentrations of

porphyrins exposed today-light generate free

radicals, leading to cellmembrane damage

and cell death.


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The type of cellular damagedepends on the solubility

and tissue distribution ofthe porphyrins.

Two main patterns of skindamage are seen in theporphyries:

. soluble uro- andcoproporphyrins leadsto blistering.

2. accumulation of thelipophilicprotoporphyrins leadsto burning sensationsin the exposed skin.


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TYPE ENZ. DEFECT INHERI-TANCE

EXCRETION INURINE

OTHER FEATURES

ACUTE

INTERMITTENTPORPHYRIA

UPG-I SYNTHASE/

PBG DEAMINASE

AD PRECURSORS,

ALA, PBGCOLORLESSURINE

ABDOMINAL &

NEUROLOGICALMANIFESTATIONS.NO PHOTOSENSITIVITY

CONGENITALERYTHROPOIETICPORPHYRIA

UPG-IIICOSYNTHASE

AR UP & CP,PORTWINE ORRED

APPEARANCE

PHOTOSENSITIVITY,HEMOLYSIS, DEFECTIVEERYTHROPOIESIS

PORPHYRIA

UPG AD UP. URINE PHOTOSENSITIVITYCUTANEA TARDA DECARBOXYLASE COLORED

HEREDITARYCOPROPORPHYRIA

CPG-III OXIDASE AD UP & CPEXCRETED INURINE AND

FECES

ABDOMINAL &NEUROLOGICALMANIFESTATIONS.

MILD

PHOTOSENSITIVITY

HEREDITARYPROTOPORPHYRIA

HEME SYNTHSE AD NEITHERPORPHYRINSNOR

PRECURSORS

PROTOPORPHYRININ PLASMA, RBC &FECES, FLUORESCENT

RBCs


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Hepatic, autosomal dominant

Affects women more than men, with a ratio of 2:1

Caused by a deficiency in porphobilinogen deaminase, which is involvedin the conversion of porphobilinogen (PBG) to uroporphyrinogen III

Porphyrias leading to accumulation of ALA and porphobilinogen cause

abdominal pain and neuropsychiatric disturbances, ranging from

anxiety to delirium.

Acute intermittent Porphyria

Patients have neuropsychiatric symptoms and abdominal pain

Symptoms of the acute hepatic porphyrias are often precipitated by

administration of drugs such as barbiturates and ethanol.

Course of the neurological manifestations is highly variable.

Acute attacks of porphyria may resolve quite rapidly.

Sudden death may occur, presumably due to cardiac arrhythmia.


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Porphyria Cutanea Tarda

Most common porphyria

Hepatic, AD

Disease is caused by a deficiency in uroporphyrinogendecarboxylase, which is involved in the conversion ofuroporphyrinogen III to Coproporphyrinogen III

Patients are photosensitive (cutaneous photosensitivity)Accumulation of porphyrinogens results in their conversion toporphyrins by light

Photosensitivity is caused by presence of uroporphyrin +

partially decarboxylated porphyrins in the skin Not associated with neurologic symptoms, psychiatric

disturbance, or abdominal pain


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ACQUIRED PORPHYRIA (LEAD TOXICITY)

Porphyria can result from lead poisoning.

Most of the paints contain lead more than the permittedlevels.

Children suck painted toys; and they get the poison.


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ACQUIRED PORPHYRIA (LEAD TOXICITY)

Pathophysiology:

Inhibits multiple enzyme reactions including those involved in hemebiosynthesis (PBG synthase & ferrochelatase)

Toxic effect is due to inhibition of ferrochelatase. So there isdecreased level of heme with consequent increase in ALA synthaseactivity.

S m toms

Irritability Poor appetite Lethargy Abdominal pain (with orSleeplessness without vomiting) Headaches Constipation

Table 21.5 Vasudevan DM


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ALA Synthase deficiency

ALA Synthase (ALAS) have both Erythroid (ALAS2) andnon-Erythroid (Hepatic) (ALAS1) form.

ALAS2 is not induced by drugs that affects hepatic

form(ALAS1)

ALAS2 is not subjected to feedback inhibition by heme.

Defective gene of ALAS1 leads to Pyridoxine Responsive

Sideroblastic Anemia.

Drugs like barbiturates depletes heme, so feedbackinhibition on ALAS is removed.

Leads to excessive production of heme intermediaries

causing neurological porphyria.


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