Heme Synthesis and Degradation

Question 1

Structure of Heme

Answer 1

Ferrous protoporphyrin IX

4 Pyrrole rings linked via mehenyl bridges,
Side chains of Heme b: methyl, vinyl, methyl, vinyl,
methyl, propionyl, asymmetric ring D: propionyl, methyl

Question 2

Location of Heme Synthesis

Answer 2

85% in bone

15% in liver for cytochrome P450

Question 3

General Overview of Heme Synthesis

Answer 3

Succinyl CoA and Glycine (In mitochondria)
To
Delta-aminolevulinic Acid (ALA)

Two ALA combine to form Porphobilinogen (In Cytosol)

4 Porphobilinogen are alligned to form Hydroxymethylbilane

Ring closure forms Porphyrin Ring III

Modification of side chains to eventually form
Protoporphyrin IX (in mitochondria)

Insertion of Ferrous Iron to form Heme

Question 4

ALA Synthase

Answer 4

Converts Succinyl CoA annd Glycine to d-aminolevulinicacid (ALA) in the mitochondria. Requires PLP

First step in Heme Synthesis

Question 5

ALA Dehydratase

Answer 5

AKA - Porphobilinogen Synthase

Joins two ALA in the cytosol to form Porphoilinogen. Requires Zn+ as a cofactor

Second step in Heme Synthesis

Question 6

HMB Synthase

Answer 6

AKA - porphobilinogen deaminase

Aligns 4 porphobilinogens to form the linear hydroxymethylbilane

Third step in Heme Synthesis

Question 7

Uroporphyrinogen III Synthase

Answer 7

Forms ring structure Urophorphyrinogen III from the linear hydroxymethylbilane

Forth step in Heme Synthesis

Question 8

Uroporphyrinogende carboxylase

Answer 8

In cytosol decarboxylates all acetyl chains of the pyrrol rings to methyl groups. Forms Coporphyrinogen III which enters the mitochondria

Fifth step in Heme Synthesis

Question 9

Fate of Coporphyrinogen III

Answer 9

Enters the mitochondria and undergoes further modification of side chains (decarboxylatedto vinyl groups)

Ultimately forms Protoporphyrin IX

Question 10

Ferrochelatase

Answer 10

Inserts Ferrous Iron into porphoryn ring to form Heme. Requires Zn+ as a cofactor.

Question 11

Lead Poisoning

Answer 11

Interacts with the Zinc cofactors of ALA dehydrotase and Ferrochelatase

ALA and Protoporphyrin accumulate in the urine

Question 12

Acute Intermittent Porphyria

Answer 12

AIP

Deficiency of hydroxymethylbilane synthase (porphobilinogen deaminase)

Genetic defect autosomal dominant trait

Accumulation of ALA and porphobilinogen

Change of normal urine color to dark purple color of urine after 24 hrs exposure to light and air.

Characterized by:

• very severe abdominal pain, abdominal colic
• highly agitated state, tachycardia, respiratory problems, nausea
• confusion, mental disturbance
• weakness of lower extremities

Question 13

Congenital Erythropoietic Porphyria

Answer 13

CEP

Deficiency of uroporphyrinogen III synthase
Genetic defect autosomalrecessive trait

HMB is spontaneously converted to Uroporphyrin I and coproporphyrinI that accumulates in tissues and blood and urine (red):

Extremely severe photosensitivity!

Characterized by:

• severe damage to skin beginning in childhood
• blisters, poor wound healing
• ulcers
• infections
• hypertrichosisis often severe
• can include reddish-brown teeth
• “werewolf” features, hairy front and arms

Treatment: bone marrow transplantation

( hemin infusion would not help, erythroid cells contain ALAS2 which is not inhibited by hemin)

Question 14

Porphyria Cutanea Tarda

Answer 14

PCT

Deficiency of uroporphyrinogen III decarboxylase

Type I (sporadic, 80%)
Type II (familial, autosomaldominant trait, 20%)

most common porphyriaand often due to a chronic disease of the liver

Characterized by:
Erosions and Bullous lesions in sun-exposed areas

Treatment: avoidance of sun-light, alcohol and iron

Question 15

Heme Degradation

Answer 15

RBCs life span of ~120 Days

Senile RBCs are sequestered by reticuloendothelial system (RES) (Spleen, Kupffer cells of liver, macrophages)

Hemoglobin broken down to Heme which is further broken down to bilirubin, and globin which forms AA for the AA pool.

Question 16

Heme Oxygenase

Answer 16

Opens heme ring to form biliverdin. Fe++ is converted to Fe+++ and CO is formed.

Takes place in macrophages

(biliverdin has a green pigment)

Question 17

Biliverdin Reductase

Answer 17

Converts Biliverdin to Bilirubin. requires NADPH

Takes place in macrophages

(Bilirubin has a orange-yellow pigment)

Question 18

Transport of bilirubin in blood to liver

Answer 18

Bilirubin binds to Albumin for blood transport - unconjugated bilirubin.

Binding to albumin prevents the unconjugated bilirubin from crossing the blood-brain barrier

Many drugs can displace bilirubin from albumin eg: salicylates, sulfonamides

Question 19

Ligandin

Answer 19

Intracellular protein in hepatocytes that bind bilirubin after transport into cell from blood

Question 20

UDP-glucuronyl transferase

Answer 20

Conjugates bilirubin in hepatocytes by the addition of 2 molecules of glucuronic acid (from UDP-glucuronic acid)

Conjugated bilirubin = Bilirubin Diglucuronide

Question 21

Secretion of bilirubin into bile.

Answer 21

Conjugated bilirubin is actively transported from the hepatocytes into the bile canaliculus AGAINST a concentration gradient by a specific ABC TRANSPORTER

Question 22

Formation of urobilinogen

Answer 22

Conjugated bilirubin in large intestine undergoes deconjugation and is converted to urobilinogen (colorless) by bacterial flora

Question 23

Formation of stercobilin

Answer 23

Bacterial action on urobilinogen in the large intestine forms stercobilin (brown color) that is lost in the feces

Question 24

Urobilin

Answer 24

~10% of urobilinogen from the large intestine is absorbed and enters the liver.

Urobilinogen is converted to Urobilin (yellow color) and is excreted in the urin

Question 25

Jaundice

Answer 25

Characterized by binding of bilirubin to connective tissue

Occurs with elevated serum bilirubin levels >2mg/dL (normal is <1mg/dL)

Question 26

Van Den Bergh Reaction

Answer 26

Used to detect the type of serum bilirubin

Bilirubin reacts with diazo reagent (diazotized sulfanilic acid) to form a red colored complex

Conugated bilirubin acts quickly and is referred to as direct bilirubin

Unconjugated bilirubin reacts in the presence of methanol (slower) and is referred to as indirect bilirubin

Total bilirubin –Direct (conjugated) bilirubin = Indirect (unconjugated) bilirubin

Question 27

Prehepatic Jaundice

Answer 27

Hemolytic Jaundice

Increased breakdown of RBCs. Increased serum unconjugated (indirect) bilirubin

Urine urobilinogen present

Question 28

Hepatic Jaundice

Answer 28

Decreased capacity of conjugation in the liver and decreased excretion if bilirubin from hepatocytes.
Elevated Conjugated and unconjugated serum bilirubin

hepatocellular damage results in lower uptake of unconjugated bilirubin into liver

Conjugated bilirubin regurgitates into plasma (as it cannot be excreted into bile)

Urine bilirubin present

Question 29

Posthepatic Jaundice

Answer 29

Obstructive Jaundice

Decreased excretion of bilirubin via bile. Increased conjugated serum bilirubin

Obstruction due to gall stones or cancer

Urine bilirubin present

Question 30

Kernicterus

Answer 30

High levels of unconjugated bilirubin crosses the blood-brain barrier and deposits in basal ganglia of the brain

characterized in newborns by lethargy, altered muscle tone, high pitched cry

•Patients surviving kernicterus have severe permanent neurologic symptoms (choreoathetosis, spasticity, muscular rigidity, ataxia, mental retardation)

Question 31

Crigler-Najjar syndrome I

Answer 31

Severe Deficiency in UDP-glucuronyl transferase

Serum bilirubinlevels can reach up to 50 mg/dL (unconjugated hyperbilirubinemia) and result in kernicterus and mental retardation

Question 32

Arias syndrome

Answer 32

Crigler-Najjar syndrome II

Characterized by lower activity of bilirubin glucuronyl transferase (10-20% of normal)

–Characterized by jaundice, but less severe than type I (serum bilirubinlevels usually 6-22mg/dL)

–Children respond to phenobarbital(induces the enzyme)

–Regular phototherapy is used in patients with persisting high bilirubinlevels

Question 33

Gilbert’s syndrome

Answer 33

Characterized by occurrence of mild jaundice (2-5 mg/dL) usually following an infection or stress or starvation

–UDP-glucuronyltransferase activity is about 50% of normal

–Characterized by a mild increase in unconjugated bilirubin

Present in 3-7% of population

Question 34

Dubin-Johnson syndrome

Answer 34

Inherited deficiency of the ABC transporter that transports conjugated bilirubin from the hepatocyte into the biliary canaliculus

–Characterized by elevated levels of conjugated (direct) bilirubin in circulation