SMC, JSMU, MBBS, Blood module

1. ENZYMES
A protein with catalytic properties due to its
power of specific activation
© 2007 Paul Billiet ODWS

2. ObjectivesObjectives
• Understand the structure of HemeUnderstand the structure of Heme
• Identify the rate limiting stepIdentify the rate limiting step
• Describe the site of effect of certain drugs on hemeDescribe the site of effect of certain drugs on heme
biosynthesis and its clinical importancebiosynthesis and its clinical importance
• Identify how blocking in one of the enzyme involved inIdentify how blocking in one of the enzyme involved in
heme biosynthesis will affect the mode of presentation ofheme biosynthesis will affect the mode of presentation of
the diseasethe disease
• Identify the most common type of porphyrias & its causeIdentify the most common type of porphyrias & its cause

3. Abnormalities in synthesis heme
(porphyrias)

4. Objectives
 By the end of this lecture the student should
be able to:
 Understand the porphyrias
 Describe the site of effect of certain drugs on heme
biosynthesis and its clinical importance
 Identify how blocking in one of the enzyme involved in
heme biosynthesis will affect the mode of presentation
of the disease.
 Identify the most common type of porphyrias & its
cause

5. HEMOGLOBIN SYNTHESIS

6. Structure of heme prosthetic group
Protoporphyrin ring w/ iron =Protoporphyrin ring w/ iron =
hemeheme
Four Pyrrole groups [A to D]Four Pyrrole groups [A to D]
linked by methane bridgeslinked by methane bridges
FeFe+2+2
coordinated by prophyrin Ncoordinated by prophyrin N
atoms and a N from Histidineatoms and a N from Histidine
(blue)(blue)
A molecule of OA molecule of O22 acts as 6acts as 6thth
ligandligand

7. Heme structure
Heme is a metaloporphyrine
(cyclic tetrapyrrole)
Heme contains:
 conjugated system of
double bonds → red colour
 4 nitrogen (N) atoms
 1 iron cation (Fe2+
)
→ bound in the middle of
tetrapyrrole skelet by
coordination covalent
bonds
methine bridge pyrrole ring

8. Properties of iron in heme
• Coordination number of
iron in heme = 6
6 bonds:
• 4x pyrrole ring (A,B,C,D)
• 1x link to a protein
• 1x link to an oxygen

9. Heme biosynthesis - repetition
• in bone marrow (85% of Hb) and liver (cytochromes)
• cell location: mitochondria / cytoplasm / mitochondria
• substrates: succinyl-CoA + glycine
• important intermediates:
 δ-aminolevulinic acid (= 5-aminolevulinic acid, ALA)
 porphobilinogen (PBG = pyrrole derivate)
 uroporphyrinogen III (= porphyrinogen – heme
precursor)
 protoporphyrin IX (= direct heme precursor)
● key regulatory enzyme: ALA synthase

10. Regulation of heme biosynthesis

ALA synthase is a key regulatory enzyme
● it is an allosteric enzyme that is inhibited by an end product - heme
(feedback inhibition)
● requires pyridoxal phosphate as a coenzyme
● certain drugs and steroid hormones can increase heme synthesis

Porphobilinogen synthase is inhibited by lead ions Pb2+
in case of lead
poisoning.

Ferrochelatase (heme synthase) can be also inhibited by Pb2+
. Its
activity is influenced by availability of Fe2+
and ascorbic acid.

11. Porphyrias - disturbances of heme synthesis
• are hereditary or acquired disturbances of heme
synthesis
• in all cases there is an identifiable abnormality of
the enzymes which synthesize heme
• this leads to accumulation of intermediates of the
pathway and a deficiency of heme → excretion of
heme precursors in feces or urine, giving them a dark
red color
● accumulation of porphyrinogens in the skin can lead to
photosensitivity
• the neurological symptoms

12. Disorders of Heme Synthesis
 Acquired: Lead poisoning
 Congenital: Porphyrias

13. LEAD TOXICITY Mechanism
• Binds to any compound with a sulfhydryl group
• Inhibits multiple enzyme reactions including
those involved in heme biosynthesis (ALA
synthase & ferrochelatase)
• One symptom of lead toxicity is increases in 5-
ALA without concomitant increases in PBG

15. Porphyria Cutanea TardaPorphyria Cutanea Tarda
 Chronic hepatic porphyria
 The most common type of porphyria
 a deficiency in uroporphyrinogen decarboxylaseuroporphyrinogen decarboxylase
 Clinical expression of the enzyme deficiency is influenced by
various factors, such as exposure to sunlight, the presence of
hepatitis B or C
 Clinical onset is during the fourth or fifth decade of life.
 Porphyrin accumulation leads to cutaneous symptomscutaneous symptoms and urineurine
that is red to brown in natural light and pink to red in fluorescent
light

16. (1) Porphyria cutanea tarda:

17. Symptoms of Cutaneous Forms
Occur most commonly with
exposure to sunlight
Mainly skin symptoms that occur
Due to excess poryphorins that
accumulate in surface of skin
Symptoms:
 Fluid filled blisters
 Changes in pigmentation
 Breakdown (necrosis) of the skin
when exposed to sunlight
 Overall skin can become scarred,
brown, blotchy and fragile

18. Treatment for Cutaneous Forms
 Avoiding sunlight
 Attention to skin care
 Beta-carotene
supplements
 Function to neutralize the
effects of reactive
protoporphyrins

19. Acute Hepatic PorphyriasAcute Hepatic Porphyrias
e.g. Acute Intermittent Porphyria
 Porphyrias leading to accumulation of ALA and
porphobilinogen cause abdominal pain and
neuropsychiatric disturbances, ranging from anxiety to
delirium.
 Symptoms of the acute hepatic porphyrias are often
precipitated by administration of drugs such as barbiturates
and ethanol.

20. Porphyrias
(A) Acute intermittent Porphyria:
 An acute disease caused by a deficiency in
hydroxymethylbilane synthase.
 Porphobilinogen and δ-aminolevulinic acid
accumulate in the urine.
 Urine darkens on expoure to light and air.
 Patients are not photosenstive

21. Porphyrias :
Acute hepatic Porphyrias
(2) Hereditary coproPorphyria:
 An acute disease caused by a deficiency in
coproPorphyrinogen oxidase
 CoproPorphyrinogen lll and other
intermediates prior to the block
accumulate in the urine.
 Patients are photosenstive.

22. Porphyrias :
Acute hepatic Porphyrias
(3) Varigate Porphyria:
 An acute disease caused by a deficiency in
protoporphyrinogen oxidase .
 ProtoPorphyrinogen lX and other
intermediates prior to the block
accumulate in the urine.
 Patients are photo-senstive.

23. Porphyrias
Erythropoietic Porphyrias

24. (1) Erythropoietic Porphyrias:
 The disease caused by a deficiency in
ferrochelates.
 ProtoPorphyrin accumulate in the
erythrocytes, bone marrow and plasma
 Patients are photosenstive.

25. (2) Congenital erythropoietic Porphyrias:
 The disease caused by a deficiency in
uroporphyrinogen lll synthase.
 Uroporphyrinogen l & coproPorphyrinogen l
accumulate in urine
 Patients are photosenstive.

26. Overall Pathway

27. Overall pathway

28. ALA Synthetase
Most important rate
limiting enzyme
Deficiency may cause
 Sideroblastic
anemia
Bone marrow
produces ringed
sideroblast?
Respond to pyridoxine
treatment

29. Overall pathway

30. ALA dehydratase deficiency
 Autosomal recessive
 Very rare
Aminolevulinic
Acid
Porphobilinogen
ALA dehydratase

31. Overall pathway

32. Acute intermittent porphyria (AIP)
 2nd
most common form of porphyria
 Caused by deficiency of PGB deaminase
 Metabolite porphobilinogen accumulates in cytoplasm
 raised concentration of urinary porphyrins
Porphobilinogen (PGB)
PGB deaminase
Hydroxymethylbilane

33. Overall pathway

34. Congenital erythropoietic porphyria (CEP)
 Deficiency of Uroporphyrinogen III synthase
 Severe photosensitivity
Hydroxymethylbilane Uroporphyrinogen III
Uroporphyrinogen III synthase

35. Overall pathway

36. Porphyria cutanae tarda (PCT)
Most common porphyria
Classified as such when Uroporphyrinogen
decarboxylase activity <20%
Inherited or obtained through Hepatitis C, alcohol,
Uroporphyrinogen III
Uroporphyrinogen
decarboxylase
Coproporphyrinogen
III

37. Overall pathway

38. Hereditary coproporphyria:
 Deficiency of Coproporphyrinogen III Oxidase
 Autosomal dominant
 No cure exists

39. Overall pathway

40. Variegate porphyria
 Deficiency in protoporphyrinogen IX-oxidase
 Autosomal dominant

41. Overall pathway

42. Erthropoietic Protoporhyria
Caused by deficiency of Ferrochelatase
Autosomal dominant
Photosensitivity- can be managed by limiting exposure

44. Thank You!