Heme synthesis

Question 1

What is the structure of the heme prosthetic group? Which two precursors are used to make a heme group?

Which elements is bound by a heme group?

Answer 1

Porphyrin ring

synthesized from glycine and succinate

binds Fe2+ (iron)

Question 2

Describe the Heme synthesis pathway, include the diseases caused the respsective defective enzyme.

If you do not feel confident, draw the pathway.

Answer 2

1. Succinyl CoA and glycine are combined by delta-aminolevulinic acid synthase to produce delta-aminolevulinic acid (delta-ALA)
   
   1. reaction inhibited by hi heme levels
2. delta-ALA is converted to porphobilinogen by delta-ALA dehydrogenase
   
   1. defective delta-ALA dehydrogenase = delta-ALA dehydrogenase porphyria
3. porphobilinogen is converted to hydroxymethylbilane by porphobilinogen deaminase
   
   1. defective porphobilinogen deaminase = acute intermittent porphyria
4. hydroxymethylbilane is converted to uroporphyrinogen III by uroporphyrinogen III cosynthase
   
   1. defective uroporphyrinogen III cosynthase = congenital erythropoietic porphyria
5. uroporphyrinogen III is comverted to coproporphyrinogen III by uroporphyrinogen decarboxylase
   
   1. defective uroporphyrinogen decarboxylase = porphyria cutanea tarda (most common)
6. coproporphyrinogen III is converted to protoporphyrinogen IX by coproporphyrinogen oxidase
   
   1. defective coproporphyrinogen oxidase heriditary coproporyria
7. protoporphyrinogen IX is converted to protophyrin IX by protoporphyrinogen odixase
   
   1. defective protoporphyrinogen odixase = variegate porphyria
8. protophyrin IX is converted to heme by ferrochelatase with the addition of Fe
   
   1. defectie ferrochelatase = erythropoietic protoporphyria

Question 3

explain the simple 5 steps to make heme (don’t include enzymes)

Answer 3

1. glycine and succinate form dela-aminolevulinate
2. two delta-aminolevulinate form porphobilinogen
3. four porphobilinogen form hydroxymethylbilane
4. linear hydroxymethylbilane to cyclic uroporphyrinogen III
   
   1. spontaneoulsy forms uroporphyrinogen I
   2. final ring is inverted by enzyme
5. side chains converted, iron added to make heme

Question 4

Explain why there is a build-up of heme synthesis precursors in people with porphyria

Answer 4

The main regualtory step for heme synthesis is the production of delta-ALA from glycine and succinate. The step (catalyzed by delta-ALA synthase) is inhibited by high levels of heme, so if one of the subsequent steps is not working properly, and heme is never produce, the pathway continues to run, producing more and more intemediates

Question 5

What is the lifespan of an erythrocyte?

What happens after it reaches this point?

Answer 5

120 days (~4 months)

hemoglobin is degraded to aa and heme

heme is converted to bilirubin

bilirubin is picked up by albumin and transported to the liver

Question 6

What is bilirubin?

Answer 6

toxic pigment derived from heme

two enzymatic steps covnert heme to bilirubin

bind to albumin, which is insoluble in water

taken up by hepatocytes to filter from plams

Question 7

Where does the conversion from heme to bilirubin take place?

Answer 7

• mostly in macrophages in the reticuloendothelial system in th spleen
  
  • taking aged RBC and getting rid of them
• also happens in hematomas
• responsible for color change of a bruise

Question 8

Explain steps in converteing heme to builirubin

Answer 8

• heme is acted on by heme oxygenase, releasing CO2 and a heme to produce biliverdin
• biliverdin is acted on by biliverdin reductase with the help of NADPH to produce NADP and bilirubin

Question 9

What is the cause of jaundice? What are the 2 general causes?

Answer 9

high serum bilirubin

• excess bilirubin production
  
  • hemolytic anemia
• decreased bilirubin excretion
  
  • liver disease (many forms), transport defects, glucuronidation defects

Question 10

How is bilirubin coverted for excretion?

Answer 10

enzyme is converted to mono- and di- glucuronide forms (UGT1A1)

conjugated bilirubin is transported to canaliculus (MRP2)

it is excreted in bile

Question 11

What information is provided by the direct bilirubin plasma level? Indirect?

What is the normal pecent direct?

What test could you run for suspected hyperbilirubinemia?

Answer 11

• Direct:
  
  • conjugated
• Indirect:
  
  • unconjugated
• Direct should be less than 15%
• urine dipstick (less than 5 cents)

Question 12

If not all bilirubin is excreted in bile, where does it go?

Answer 12

conjugated bilirubin can cross the glomerulus, unlike albumin-bound unconjugated bilirubin

Question 13

What is special about UGT1A1?

Answer 13

It the only bilirubin conjugating enzyme, which adds glucuronic acid to compounds to improve their solubility.

Question 14

Which editiing process is responsible for producing the different types of UGT enzymes?

Answer 14

RNA splicing

Question 15

Which syndrome affecting UGT1A1 has two types?

What is the difference between these two types? (frequency, UGT1A1 activity, bilirubin levels)

What mutation is responsible?

Answer 15

Crigler-Najjar Syndrome

nonsense, missense, frameshift splice site, mutations and deletions identified in UGT1A1

If exons 2-5 are involved, other substrates also are not conjugated

• Type I
  
  • essentailly no UGT1A1 activity
  • very rare
  • 18-45 (usually >20), unconjugated
• Type II
  
  • UGT1A1 <10% normal activity
  • uncommon
  • 6-25 (usually <20), unconjugated

Question 16

Which syndrome caused by a UGT1A1 mutation is only noticible during fasting conditions or hemolysys?

What type of mutation? Typical serum bilirubin levels?

Answer 16

Gilbert’s Syndrome

prevalence in some populations is greater than 10%

mainly promoter mutations, sometimes structural mutations

typically <4 in absence of fasting or hemolysis, mostly unconjugated

Question 17

What is the normal TATA box for UGT1A1 promoter? What is the Gilbert’s Syndrome promoter?

Answer 17

• Normal: A(TA)6TAA (TA6)
• Gilbert’s: A(TA)7TAA (TA7)
  
  • TA5 form has also been found

Question 18

What is an example of a drug that inhibits UGT1A1 activity? Why is this a concern for people with Gilbert’s?

Answer 18

• HIV protease inhibitors,
• Certain neoplastic-agents
• If activity is already lowered due to Gilberts, this may cauase jaundice

Question 19

What is the name of the disorder characterized by conjugated hyperbilirubimia with a black, coarse, dark centrilobular liver?

Frequency?

Mutation?

Serum bilirubin levels?

Unique diagnostic criteria?

Answer 19

Dubin-Johnson

• recessive, uncommon
• cMOAT/MRP2
  
  • defect in transport of conjugated bilirubin from hepatocyte to bile canaliculus
• Typically 2-5, less often <25, about 60% direct-reacting
• diagnostic urinary coproporphyrin pattern
  
  • total range is normal
  • Isomer 1 increased to > 80% of total
• Avoid estrogens

Question 20

What is the name of the disorder characterized by moderately high bilirubinimia and 2.5 x increase in urinary coproporphyrins excretion, but isomer 1 is < 80% of the total?

Frequency?

What type of disease is this?

Serum bilirubin levels?

Answer 20

Rotor’s Syndrome

• rare recessive disorder
• genetic defect, organic ion transporter that recovers bilirubin glucuronide that leaked to intracellular space
• usually 3-7, occassionally <20, about 60% direct reacting