PORPHYRINSHEME SYNTHESIS AND DEGRADATION

Porphyrins

• Cyclic compounds that bind metal ions

• Chlorphyll (Mg2+)

• Central to solar energy utilization

• Heme (Fe2+)

• Most prevalent metalloporphyrin in humans

• Central to oxygen sensing and utilization

• Cobalamin (Cobalt)

PORPHYRINS

HC

HC

NH

CH

CH

Pyrrole ring

•Porphyrin: Cyclic molecule

formed by linkage of four

pyrrole rings through

methenyl bridges

Porphyrin Side Chains

• M = Methyl (-CH3)

• V = Vinyl (-CH=CH2)

• P = Propionyl (-CH2-CH2-COO-)

• A = Acetyl (-CH2-COO-)

Heme

•One ferrous (Fe2+) atom in the center of the tetrapyrrole ring of Protoporphyrin IX

•Prosthetic group for

• Hemoglobin and Myoglobin

• The Cytochromes

• Catalase and Tryptophan pyrrolase

• Nitric Oxide Synthase

•Turnover of Hemeproteins (Hemoglobin, etc) is coordinated with synthesis and degradation of porphyrins

•Bound iron is recycled

Heme

Protoporphyrin III

prefix or suffix r ing substituents between rings

uro- acetate, propionate - -

copro- methyl, propionate - -

proto- methyl, propionate, vinyl

-porphyrinog e n - - methylene

-porphyrin - - methene

Biosynthesis of Heme

•Synthesized in every human cell

•Liver (15%):

•Bone Marrow (80%)

COOH

CH2

CH2

COSCoA

CH2 NH2

COOH

SUCCINYL CoA

GLYCINE

All Carbon and Nitrogen atoms

provided by 2 building blocks:

COOH

CH2

CH2

COSCoA

CH2 NH2

COOH

SUCCINYL CoA

GLYCINE is

Decarboxylated

IN MITOCHONDRIA

AMINOLEVULINIC ACID SYNTHASE

- CO2

COOH

CH2

CH2

C=O

CH2

NH2

Condense to form:

AMINOLEVULINIC ACID (ALA)

MOVES OUT OF THE MITOCHONDRION

COOH

CH2

CH2

C=O

CH2

NH2

COOH

CH2

CH2

C=O

CH2

NH2

2 Molecules dehydrated by

ALA DEHYDRATASE

-2 H2O

COOH

CH2

CH2

C

C

NH

COOH

CH2

C

C

CH2

NH2

To form Porphobilinogen (PBG)

COOH

CH2

CH2

N

H

COOH

CH2

CH2

NH2

Porphobilinogen (PBG)

Acetate

CH2COO-Propionate

CH2CH2COO-

Porphobilinogen (PBG)

N

HCH2

NH2

A P

HYDROXYMETHYLBILANE SYNTHASE

& UROPORPHYRINOGEN III SYNTHASE

•Four PBG molecules condense

•Ring closure

• Isomerization

NH

NH HN

HN

A B

CD

A

A

P

A

P

P

P

A

Uroporphyrinogen III

NH

NH HN

HN

HOOC-H2C-

HOOC-H2C-

-CH2-CH2-COOH

-CH2-COOH

CH2

CH2

COOH

CH2

CH2

COOH

COOH

CH2

CH2

COOH

CH2

Uroporphyrinogen III

SERIES OF

DECARBOXYLATIONS & OXIDATIONS

• Porphyrinogens:

• Chemically reduced

• Colorless intermediates• Porphyrins:

• Intensely colored

• Fluorescent

• Uroporphyrinogen III

• Coproporphyrinogen III

Moves back into Mitochondrion• Protoporphyrinogen IX

• Protoporphyrin IX

NH

N HN

N

H3C-

H3C-

-CH=CH2

-CH3

CH2

CH2

COOH

CH2

CH2

COOH

CH3

Protoporphyrin IX

CH=CH2

HEME

Fe2+ chelated by Protoporphyrin IX

Assisted by Ferrochelatase

CH3-

REACTIONS FOR

PROTOPORPHYRI

N IX

Regulation of Heme Synthesis

AMINOLEVULINIC ACID SYNTHASE

• Two tissue-specific isozymes

• Coded on separate genes

• In Liver, heme represses synthesis and activity of ALAS

• Heme can be used for treatment of acute porphyric attack

• In RBC heme synthesis regulation is more complex

• Coordinated with globin synthesis

COOH

CH2

CH2

COSCoA

CH2 NH2

COOH

SUCCINYL CoA

GLYCINE

IN MITOCHONDRIA

AMINOLEVULINIC ACID SYNTHASE

RATE-CONTROLLING STEP IN

HEPATIC HEME SYNTHESIS

COOH

CH2

CH2

C=O

CH2

NH2

ALA

Bonkovsky

ASH Education Book

December 2005

Disorders of Heme Synthesis

• Acquired: Lead poisoning

• Congenital: Porphyrias

• Deficiency of heme has far-reaching effects (hemoglobin, cytochromes, etc.)

LEAD TOXICITY

Symptoms Irritibility Poor appetite

Lethargy Abdominal pain (with or

Sleeplessness without vomiting)

Headaches Constipation

Pathophysoiology

Binds to any compound with a sulfhydryl group

Inhibits multiple enzyme reactions including those

involved in heme biosynthesis (ALA dehydratase &

ferrochelatase)

COOH

CH2

CH2

C=O

CH2

NH2

COOH

CH2

CH2

C=O

CH2

NH2

-2 H2O

ALA moves out of the mitochondrion

ALA DEHYDRATASE

Inhibited by Heavy Metal: LEAD POISONING

PBG

N

HCH2

NH2

A P

Lead

Poisoning

Lead Poisoning

ALAD and Ferrochelatase

Are particularly sensitive

to Lead inhibition

Lead

Poisoning

Fe + PPIX

Ferrochelatase

Heme

Disorders of Heme synthesis • Porphyrias are group of inborn errors of metabolism associated with

the biosynthesis of heme.(Greek ‘porphyria’ means purple).

• These are characterised by increased production and production

and excretion of porphyrins and/or their precursors (ALA + PBG).

• Many of the porphyrias are inherited as autosomal dominant traits.

• Porphyrias may be broadly grouped into 3 types:

a. Hepatic porphyrias

b. Erythropoietic porphyrias

c. porphyrias with both erythropoietic and hepatic abnormalities.

PORPHYRIAS A group of rare disorders caused by deficiencies of enzymes of the

heme biosynthetic pathway

The majority of the porphyrias are inherited in a autosomal dominant

fashion - thus, affected individuals have 50% normal levels of the

enzymes, and can still synthesize some heme

Affected individuals have an accumulation of heme precursors

(porphyrins), which are toxic at high concentrations

Attacks of the disease are triggered by certain drugs, chemicals, and

foods, and also by exposure to sun

Treatment involves administration of hemin, which provides

negative feedback for the heme biosynthetic pathway, and therefore,

prevents accumulation of heme precursors

ACUTE INTERMITTENT PORPHYRIA

Hepatic, autosomal dominant

Caused by a deficiency in porphobilinogen

deaminase, which is involved in the conversion of

porphobilinogen (PBG) to uroporphyrinogen III

PBG, uroprophryin, and 5-ALA accumulate in the

plasma and the urine

Patients have neuropyschiatric symptoms and

abdominal pain (neurovisceral)

PORPHYRIA CUTANEA TARDA

Most common porphyria

Hepatic, autosomal dominant

Disease is caused by a deficiency in uroporphyrinogen

decarboxylase, which is involved in the conversion of

uroporphyrinogen III to coproporphyrinogen III

Uroporphyrinogen accumulates in urine

Patients are photosensitive (cutaneous photosensitivity)

Accumulation of porphyrinogens results in their

conversion to porphyrins by light

Porphyrins react with molecular oxygen to form

oxygen radicals

Oxygen radicals can cause severe damage to the

skin

BLOOD CELLS

LIVER

Bilirubin diglucuronide(water-soluble)

2 UDP-glucuronic acid

via bile duct to intestines

Urobilinogenformed by bacteria KIDNEY

CO

Biliverdin

Heme oxygenase

O2

Bilirubin (water-insoluble)

NADP+

NADPH

Biliverdinreductase

Heme

Globin

Hemoglobin

reabsorbedinto blood

Bilirubin (water-insoluble)

via blood to the liver

INTESTINE

Disorder of Hb catabolism•Disease or conditions that interfere with bilirubin

metabolism may cause a rise in its serum concentration of bilirubin

Total bilirubin

0.1 to 1mg/dlConjugated

0.1 to 0.4mg/dlUnconjugated

0.2 to 0.7mg/dl

NORMALLEVELS

Hyperbilirubinaemia• When serum bilirubin exceeds 1mg/dl – HYPERBILIRUBINAEMIA

• >2.2 to 5mg/dl - JAUNDICE

Yellowish discoloration of skin and sclera due to deposition of bilirubin in the tissues. The

condition is called jaundice or icterus

INCREASED HAEMOLYSIS

↑breakdownof Hb

JAUNDICE

LIVER DAMAGE

↓Excretion of bilirubin

JAUNDICE

BILE DUCT OBSTRUCTION

↓Excretion of bilirubin

JAUNDICE

CAUSES OF JAUNDICE

JAUNDICES

NORMAL METABOLISM OF BILE

PIGMENTS

CELLS OF RES

Indirect

bilirubin

NADP+

NADPH2

Biliverdin

reductase

Biliverdin

Iron

Globin

Verdoglobin

NADP+

Hemoxi-

genase

NADPH2

Hemoglobin ERYTHROCYTES

K

I

D

N

E

Y

S

Stercobilinogen

URINE

Stercobilin

Indirect

bilirubin 1,7-

20,5 mkmol/l

albumin

albumin

Indirect bilirubin

UDP-glucoronil-

transferase

Direct

bilirubin 0.8-

4.3 mkmol/l

B

L

O

O

D

L

I

V

E

R

Bilirubin mono-

glucoronid, 20 %

Bilirubin di-glucoronid,

80 %

Dipyrols

-glucoro-

nidase

Glucoronic

acid

Direct

bilirubin

B

I

L

E

I

N

T

E

S

T

I

N

E

Mesobilirubin

Mesobilirubin

(urobilinogen)

Stercobilinogen

Stercobilin

STOOL

METABOLISM OF BILE PIGMENTS IN HEMOLYTIC

JAUNDICE

CELLS OF RES

Indirect

bilirubinIndirect

bilirubin

albumin

albumin

Indirect

bilirubin

Biliverdin

reductase

UDP-glucoronil-

transferase

Direct

bilirubin

NADP+

NADPH2

Biliverdin

Iron

Globin

Verdoglobin

NADP+

NADPH2

Hemoglobin

Hemoxi-

genase

B

L

O

O

D

L

I

V

E

R

Bilirubin mono-

glucoronid, 20 %

Bilirubin diglucoronid,

80 %

-glucoro-

nidase

Glucoronic

acid

Direct

bilirubin

B

I

L

E

ERYTHROCYTES

K

I

D

N

E

Y

S

I

N

T

E

S

T

I

N

E

STOOL

Stool hypercholic

URINEUrine dark

Mesobilirubin

Mesobilinogen

(urobilinogen)

Stercobilinogen

Stercobilin

Stercobilinogen

StercobilinUrobilin

METABOLISM OF BILE PIGMENTS IN HEPATIC

JAUNDICE

CELLS OF RES

Indirect

bilirubin

Indirect

bilirubin

albumin

albumin

Indirect

bilirubin

Biliverdin

reductase

UDP-glucoronil-

transferase

Direct

bilirubin

NADP+

NADPH2

Biliverdin

Iron

Globin

Verdoglobin

NADP+

NADPH2

Hemoglobin

Hemoxi-

genase

B

L

O

O

D

L

I

V

E

R

Bilirubin mono-

glucoronid, 20 %

Bilirubin diglucoronid,

80 %

-glucoro-

nidase

Glucoronic

acid

Direct

bilirubin

B

I

L

E

ERYTHROCYTES

K

I

D

N

E

Y

S

Urobilinogen I

N

T

E

S

T

I

N

E

STOOL

Stool hypocholic

URINEUrine dark

Stercobi-

linogen

StercobilinBilirubinUrobilin

Mesobilirubin

Mesobilinogen

(urobilinogen)

Stercobilinogen

Stercobilin

METABOLISM OF BILE PIGMENTS IN OBSTRUCTIVE

JAUNDICE

CELLS OF RESIndirect

bilirubinIndirect

bilirubin

albumin

albumin

Indirect

bilirubin

Biliverdin

reductase

UDP-glucoronil-

transferase

Direct

bilirubin

NADP+

NADPH2

Biliverdin

Iron

Globin

Verdoglobin

NADP+

NADPH2

Hemoglobin

Hemoxi-

genase

B

L

O

O

D

L

I

V

E

R

Bilirubin mono-

glucoronid, 20 %

Bilirubin diglucoronid,

80 %

Bile

acids

-glucoro-

nidase

Glucoronic

acid

Direct

bilirubin

B

I

L

E

Direct

bilirubin

ERYTHROCYTES

K

I

D

N

E

Y

S

Direct bilirubinI

N

T

E

S

T

I

N

E

STOOL

Stool acholic, steatorhea

URINE Direct bilirubin Bile acids

Urine dark, foaming