Heme metabolism

1. HEME METABOLISM
Dr Parijat Gogoi
Professor

2. ○ Heme is a Metalloporphyrin
Metal  Fe+2
Porphyrin  Protoporphyrin IX
○ It links with many proteins to form
conjugated proteins
○ It is the prosthetic group of proteins
like hemoglobin, myoglobin, &
cytochromes and so on.

4. heme
pyrrole rings
CHARACTERISTICS OF PORPHYRINS
○ Porphyrins are cyclic compounds formed
by fusion of 4 pyrrole rings linked by
methenyl (=HC--) bridges

5. CHARACTERISTICS OF PORPHYRINS

7. CHARACTERISTICS OF PORPHYRINS
○ Porphyrins form complex with metal
ions which are bound to the nitrogen
atoms of the pyrrole rings
Eg: Fe porphyrins -- Heme
Mg porphyrin -- Chlorophyll

8. CHARACTERISTICS OF PORPHYRINS
○ The double bonds absorb light in both
the visible and UV regions of the
spectrum
○ Red colour of Hb is due to this property
○ They show a characteristic sharp
absorbtion band near 400nm and this
band is called Soret Band

9. CHARACTERISTICS OF PORPHYRINS
○ Porphyrins when dissolved in strong
mineral acids or organic solvents and
illuminated by UV light, emit a strong
red fluorescence.
○ It is so characteristic that it is often
used to detect even small amounts of
free porphyrins
○ These photodynamic properties are
applied in cancer phototherapy

10. BIOMEDICAL IMPORTANCE
PHYSIOLOGICAL ROLE
○ Hb  Transport of Oxygen
○ Myoglobin  Storage of Oxygen in muscle
○ Cyt c  Involvement in ETC
○ Cyt P450 Hydroxylation of Xenobiotics
○ Catalase  Degradation of Hydrogen Peroxide
○ Tryptophan Pyrrolase  Oxidation of Trypto

11. BIOMEDICAL IMPORTANCE
DISEASES
o Metabolic abnormalities of heme leads to
at least
two well known group of diseases –
 Porphyrias (biosynthesis)
Jaundice (degradation)
o Study of heme metabolism is necessary to
understand their role in health as well as
in diseases resulting from them

12. BIOSYNTHESIS OF HEME

13. URO
COPRO
PROTO

14. HEME SYNTHESIS
Step 1
Synthesis of d-amino levulinic acid
•Mitochondrial location
•Rate limiting
•Pyridoxal phosphate

15. HEME SYNTHESIS
○ Lack of Vit B6 will decrease the
synthesis of ALA causing anaemia
○ Drugs like INH (iso-nicotinic acid
hydrazide) that decrease the
availability of pyridoxal phosphate may
also affect heme synthesis

16. Pyridoxal phosphate
(PLP) serves as
coenzyme for d-
Aminolevulinate
Synthase
Condensation with
succinyl-CoA takes
place while the amino
group of glycine is in
Schiff base linkage to
the PLP aldehyde.

17. Step 2
Synthesis of
porphobilinogen
•Also called porphobilinogen
synthase
•Zinc-dependent
•Site of lead toxicity

18. 4 PBG units are condensed to form a linear
hexapyrrole Hydroxymethylbilane

19. Note the distribution of acetyl & propionyl side
chains, as flipping over of one pyrrole yields an
asymmetric tetrapyrrole.

20. Fe++ is added to protoporphyrin IX via Ferrochelatase, a
homodimeric enzyme containing 2 iron-sulfur clusters.
A conserved active site His, along with a chain of anionic
residues, may conduct released protons away, as Fe++ binds
from the other side of the porphyrin ring, to yield heme.

21. Protoporphyrin III

22. HEME SYNTHESIS

23. REGULATION OF HEME SYNTHESIS
○ Heme, if not incorporated into proteins
immediately are catabolised
○ Pathway is tightly regulated to prevent
excessive synthesis and catabolism
○ 85% of heme is synthesised by the
erythroblasts for Hb synthesis
○ Its synthesis needs to be co-ordinated
with globin synthesis

24. REGULATION OF HEME SYNTHESIS
○ Heme stimulates the synthesis of
proteins –the enzymes of heme
biosynthesis and also globin synthesis
○ In erythroid cells Ferrochelatase and
Uroporphyrinogen I synthase may act
as the rate limiting enzymes.
○ Availability of Iron also increases heme
synthesis by enhancing Ferrochelatase
activity

25. REGULATION OF HEME SYNTHESIS
○ In the liver ALA Synthase-1 activity is
regulated by free heme concentration
○ Mechanisms :
Repression
Increased Heme concentration
activates a repressor protein
(aporepressor) that turns off ALA
synthase biosynthesis at the
translation level

26. REGULATION OF HEME SYNTHESIS
○ Mechanisms :
It inhibits the transfer of ALAS-1 from
cytosol into mitochondria
It also decreases the activity of ALAS-
1 by feedback allosteric inhibition

27. REGULATION OF HEME SYNTHESIS
○ Lead inhibit ALA Dehydratase and
Ferrochelatase activities
○ Various Xenobiotics, natural steriods
and therapeutic drugs like
barbiturates, phenytoin, griseofulvin,
etc. induces Cyt P450 synthesis
decreasing the concentration of free
heme , thereby decreasing the effect of
the repressor protein and ultimately
leading to increased heme production

28. PORPHYRIA
○ Group of diseases caused by
abnormalities in the pathway of
synthesis of the various porphyrins
○ Not very prevalent but important in
the differential diagnosis of
abdominal pain and of a variety of
neuropsychiatric findings

29. CLASSIFICATION OF THE PORPHYRIAS
○ Multiple ways to categorize
porphyrias:
● Heriditary or Acquired
● Hepatic or Erythropoietic or
mixed: Organ in which
accumulation of porphyrins and
their precursors appears
● Cutaneous or Non- cutaneous
● Acute and non-acute forms

30. CLASSIFICATION OF THE PORPHYRIAS
○ Acute:
● Aminolevulinate dehydratase
deficiency porphyria (ALA-D)
● Acute intermittent porphyria (AIP)
● Hereditary coproporphyria (HCP)
● Variegate porphyria (VP)
○ Chronic:
● Porphyria cutanea tarda (PCT)
● Erythropoietic protoporphyria (EPP)
● Congenital erythropoietic porphyria
(CEP)
● Hepatoerythropoietic porphyria (HEP)

31. CLASSIFICATION OF THE PORPHYRIAS
Depending on the enzyme deficiency they
are classified into six types
1. Acute Intermittent Porphyria
2. Congenital Erythropoietic Porphyria
3. Porhyria Cutenea tarda
4. Heriditary Coproporphyria
5. Porphyria Varigeta
6. Protoporphyria

32. ENZYMATIC DEFICIENCIES

33. PORPHYRIA
○ Accumulation of "porphyrins" or
"porphyrin precursors" in the body
○ All these disorders are inherited as
autosomal dominant trait except
CEP which is genetically recessive

35. CLINICAL FEATURES OF PORPHYRIA
Symptoms vary depending
○ deficient enzyme
○ the severity of the deficiency
○ whether heme synthesis is
affected primarily in liver or in
developing erythrocytes.

36. Clinical features of porphyria

37. CLINICAL FEATURES OF PORPHYRIA
○ Clinical features depends on the level of block
Early Block
Before synthesis of first tetra pyrrole
ALA and PBG accumulates
Acts on autonomic nervous system
Autonomic neuropathy

38. CLINICAL FEATURES OF PORPHYRIA
AUTONOMIC NEUROPATHY
○ ALA inhibits ATPase enzyme and
interferes in nerve and muscle
conduction
○ Alteration of GIT motility  pain
abdomen, diarrhoea,nausea vomiting
etc.
○ Sensory and motor involvement  pain
in limbs,chest,neck and/or weakness of
muscles
○ Restlessness ,anxiety,excessive
sweating ,tremors and hypertension
○ Neuropsychiatric symptoms like
insomnia, depression, hallucinations etc.

39. CLINICAL FEATURES OF PORPHYRIA
LATER BLOCK
Cutaneous manifestations are predominant
Porhyrins accumulated in blood
Visible light (400nm)
Emit intense red light
Generates superoxide radicals and other
ORFs
Damage membranes of lysosomes
Skin damage and scarring

40. PORPHYRIN
POLARITY AND ROUTE OF EXCRETION
○ Uroporphyrin (8 carboxyl gp)
entirely in urine
○ Protoporphyrin (2 carboxyl gp) 
exclusively in faeces
○ Coproporphyrin (4 carboxyl gp) 
Both routes

41. DIAGNOSIS
○ Clinical and family history
○ Physical examination
○ Laboratory tests
Measurement of metabolites
Assay of relevant enzymes
Molecular diagnostic (more
definitive)
● Detection of mutations for AIP provides 95%
sensitivity and around 100% specificity
● Possible to screen asymptomatic gene carriers.

42. DIAGNOSIS
PBG in urine is
oxidized to
porphobilin upon
standing, which
gives a dark-brown
color to urine, and
often referred to as
‘port-wine reddish
urine’.

43. PORPHYRIA
Treatment
○ I.V. Hematin (hydroxide of heme)
○ Avoid drugs causing induction of
Cyt P450
○ Ingestion of glucose
○ β carotene (decrease production of
ORF)
○ Sunscreens that filter visible light
○ Correction at the gene level

44. ACUTE INTERMITTENT PORPHYRIA
Mary Queen of Scots
King George III (mad king
george)

45. ACUTE INTERMITTENT PORPHYRIA
○ AIP is a typical hepatic porphyria
○ Commonest variety (1:10,000 to
1:1,00,000)
○ inherited in an autosomal dominant
fashion.
○ Affects women more than men, with a
ratio of 2:1.
○ Most patients become symptomatic at
age 18-40 years.
○ Most patients are completely free of
symptoms between attacks.

46. ACUTE INTERMITTENT PORPHYRIA
○ Attacks involve neuro-visceral symptoms but no
skin manifestations:
● The sequence of events in attacks usually is (1)
abdominal pain, (2) psychiatric symptoms, such as
hysteria, and (3) peripheral neuropathies, mainly
motor neuropathies.
○ Gastroenterological Symptoms most common:
● Constipation, colicky abdominal pain , vomiting,
diarrhea
○ Patients may have CNS signs consisting of seizures,
mental status changes, cortical blindness, and coma.
○ Patients often experience peripheral
neuropathies
○ Patients may develop fever, hypertension and
tachycardia

47. CASE DISCUSSION
An 18 yr woman presents in moderate distress with a five-
day history of severe abdominal pain and self-reported
discoloration of urine. Patient was menstruating at time of
admission. She had previous episodes of abdominal pain
usually starting two to three days prior to menstrual cycle
which lasted the length of the cycle. History of six months
of intermittent, localized, severe, sharp epigastric and
periumbilical pain and two previous admissions for
hyponatremia, irregular menstruation, and abdominal
pain. Abdomen was tender on deep palpation, no
hepatosplenomegaly and auscultation of abdomen
revealed reduced bowel sounds .No visible ulcers or
lesion. Blood count and liver function tests revealed no
abnormality. Ultrasonography of abdomen unremarkable
○ Urine porphobilinogen: 95 µmol/L (normal, 0 - 8.8
µmol/L)
○ ALA (urine): 724 µmol/L (normal, 0 - 35 µmol/L)
Urinary and serologic tests confirmed diagnosis of AIP.

48. CASE DISCUSSION
○ A 35-year-old women
○ uncooperative behaviour and slurred
speech- Hysteria
○ Admitted 5 days ago in a semiconscious
state after repeated attacks of generalized
tonic–clonic seizures
○ No family history
○ On phenytoin sodium,which worsened her
condition
○ Peripheral neuropathy and Delirium.
○ Her high coloured urine
○ No pain abdomen.
○ Urine showed porphobilinogen in 1:80 titre.

49. PORPHYRIA CUTANEA TARDA
○ One of the commonest porphyria
○ Hepatic, autosomal dominant
○ Disease is caused by a deficiency in
uroporphyrinogen decarboxylase
○ Uroporphyrins levels increase in
urine,RBCs, liver and plasma

50. PORPHYRIA CUTANEA TARDA
○ Patients suffer from photosensitivity –
fluid filled vesicles and bullae on
exposed body parts
○ Treatment include removal of the
precipitating factor
reducing hepatic iron overload by
phlebotomy
low doses of chloroquin as it forms a
complex with porphyrins and enhance its
excretion

51. Photosensitivity

52. CASE DISCUSSION
○ A 3-year-old male child, born of consanguineous
marriage, presented with blisters on exposed
areas since the age of 6 months. The blisters
used to heal with scars. Since early infancy the
mother had noticed reddish colored urine. The
child's mental and physical development had
been normal. There was no family history of a
similar problem. There was no history of acute
attacks. On examination, the child's face was
badly scarred. There was hypertrichosis on the
shoulders, arms, and face. The teeth were of
coppery-red color . There were a few intact
blisters and crusted lesions on the hands and
feet . Atrophic scars were also present on the
extremities.

54. CONGENITAL ERYTHROPOIETIC PORPHYRIA
○ CIP is due to the severely depressed
activity of Uroporphyrinogen III
cosynthase activity
○ Hydroxymethylbilane  Cop I
○ Uro I and Cop I  oxidised to
porphyrins  severe cutaneous
photosensitivity
○ Their accumulation gives a pink to
dark red colour to teeth
(erythrodontia),bones and urine

55. CONGENITAL ERYTHROPOIETIC PORPHYRIA
○ symptoms include:
- skin rashes and blisters early in
childhood
- later scarring ,
hyperpigmentation and
hypertrichosis (monkey facies)
- symptoms suggestive of bone
marrow involvement like anaemia ,
hemolysis, splenomegaly etc.

56. CONGENITAL ERYTHROPOIETIC PORPHYRIA

57. 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”

58. 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

59. Type Enzyme Involved Major Symptoms Laboratory tests
Acute intermittent
porphyria
Uroporphyrinogen
synthase
Abdominal
pain Neuropsych
iatric
Urinary ALA,
porphobilinogen 
Congenital
erythropoietic
porphyria
Uroporphyrinogen
cosynthase
Photosensitivity Urinary,fecal and RBC
uroporphyrin-I 
Porphyria cutanea
tarda
Uroporphyrinogen
Decarboxylase
Photosensitivity urinary uroporphyrin

Variegate porphyria Protoporphyrinoge
n Oxidase
Photosensitivity
Abdominal
pain Neuropsych
iatric
Urinary ALA, PBG and
coproporphyrin 
fecal protoporphyrin

Erythropoietic
protoporphyria
Ferrochelatase Photosensitivity fecal protoporphyrin 
red cell protoporphyrin 
Diagnosis

61. THANK YOU