ENZYMESA protein with catalytic properties due to its power of specific activation

© 2007 Paul Billiet ODWS

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 heme Describe 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 in Identify how blocking in one of the enzyme involved in

heme biosynthesis will affect the mode of presentation of heme biosynthesis will affect the mode of presentation of the diseasethe disease

• Identify the most common type of porphyrias & its cause Identify the most common type of porphyrias & its cause

Abnormalities in synthesis heme (porphyrias)

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

HEMOGLOBIN SYNTHESIS

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 coordinated by prophyrin N atoms and a N from Histidine N atoms and a N from Histidine (blue)(blue)

A molecule of OA molecule of O22 acts as 6 acts as 6thth ligandligand

Heme structureHeme 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

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

Heme biosynthesis - repetition• in bone marrow (85% of Hb) and liver (cytochromes)• cell location: mitochondria / cytoplasm / mitochondria• substrates: succinyl-CoA + glycine• important intermediates:

-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

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.

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

Disorders of Heme Synthesis

Acquired: Lead poisoning

Congenital: Porphyrias

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

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

(1) Porphyria cutanea tarda:

Symptoms of Cutaneous Forms Occur most commonly with

exposure to sunlight Mainly skin symptoms that occurDue to excess poryphorins that

accumulate in surface of skinSymptoms:

Fluid filled blisters Changes in pigmentation Breakdown (necrosis) of the skin when exposed to sunlight Overall skin can become scarred, brown, blotchy and fragile

Treatment for Cutaneous Forms

Avoiding sunlight Attention to skin care Beta-carotene

supplements Function to neutralize the

effects of reactive protoporphyrins

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.

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

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.

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.

Porphyrias Erythropoietic Porphyrias

(1) Erythropoietic Porphyrias: The disease caused by a deficiency in

ferrochelates.

ProtoPorphyrin accumulate in the erythrocytes, bone marrow and plasma

Patients are photosenstive.

(2) Congenital erythropoietic Porphyrias:

The disease caused by a deficiency in uroporphyrinogen lll synthase.

Uroporphyrinogen l & coproPorphyrinogen l accumulate in urine

Patients are photosenstive.

Overall Pathway

Overall pathway

ALA Synthetase Most important rate

limiting enzymeDeficiency may cause Sideroblastic anemia Bone marrow

produces ringed sideroblast?

Respond to pyridoxine treatment

Overall pathway

ALA dehydratase deficiency

Autosomal recessive Very rare

Aminolevulinic Acid

Porphobilinogen

ALA dehydratase

Overall pathway

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

Overall pathway

Congenital erythropoietic porphyria (CEP)

Deficiency of Uroporphyrinogen III synthase Severe photosensitivity

Hydroxymethylbilane Uroporphyrinogen III

Uroporphyrinogen III synthase

Overall pathway

Porphyria cutanae tarda (PCT)

Most common porphyriaClassified as such when Uroporphyrinogen

decarboxylase activity <20%Inherited or obtained through Hepatitis C, alcohol,

Uroporphyrinogen III

Uroporphyrinogen decarboxylase

Coproporphyrinogen III

Overall pathway

Hereditary coproporphyria:

Deficiency of Coproporphyrinogen III Oxidase Autosomal dominant No cure exists

Overall pathway

Variegate porphyria

Deficiency in protoporphyrinogen IX-oxidase Autosomal dominant

Overall pathway

Erthropoietic Protoporhyria

Caused by deficiency of FerrochelataseAutosomal dominant Photosensitivity- can be managed by limiting exposure

Thank You!