Final: Heme + Iron Metabolism (Ben)

Question 1

What is heme composed of?

(2 main “ingredients”)

Where/how does it function?

Answer 1

Protoporphyrin IX and Fe²⁺

• present as a prosthetic group in hemoglobin, myoglobin, cytochrome catalase, peroxidase and NO synthase
• functions mostly in transport of diatomic gases and electron transfer for redox reactions

Question 2

Describe the structure of protoporphyrin IX.

Answer 2

• 4 pyrrol rings (5 atoms = 4 C + 1 N )
• Methyl (-CH3) Vinyl (-CH=CH2) and Propionate (-CH2-CH2-COOH) groups attached to the pyrrols
  
  • ​clockwise order from top left = MVMVMPPM

Question 3

Describe the structures of myoglobin and hemoglobin.

What can change the O2 affinity of hemoglobin?

Answer 3

• Myoglobin
  
  • a monomer with a single hem__e at its center
• Hemoglobin
  
  • a tetramer of 2 alpha and 2 beta subunits each with own Heme B
  • 2,3-BPG binds to heme to decrease O₂ affinity of Hb in tissues where O₂ is required
  • there is a “positive cooperativity” of O₂ binding, where affinity increases with each O₂ bound

Question 4

What is the first and rate-limiting/committed/regulatory step in synthesis of protoporphyrin IX for heme?

Where does it take place?

Answer 4

ALA Synthase

• Succinyl-CoA + Glycine –> δ-aminolevulinate + CoA + CO₂
• has a 6C intermediate (alpha-amino-beta-keto-adipate)
• is mitochondrial

Question 5

How is the rate-limiting step in Heme synthesis regulated?

Answer 5

ALA Synthase has two isoforms

1. ALAS-L (liver)
   
   • Heme inhibits transcription, mRNA export, mitochondrial uptake AND enzyme activity (allosterically)
2. ALAS-E (marrow)
   
   • Erythropoietin stimulates transcription
   • heme does not inhibit ALAS-E but does inhibit iron release from ferritin + stimulates globin synthesis

Question 6

How many aminolevulinate substrate molecules go into the 2nd reaction of heme synthesis?

Whats the enzyme and its co-factor?

Cellular location + product?

What important functional groups are added?

Inhibition?

Answer 6

ALA Dehydratase

• requires 2 ALA** molecules plus **Zn²⁺
• located in cytosol
• makes porphobilinogen
  
  • ​has added acetate and propionate groups which become methyl and vinyl groups later
• inhibited by Pb²⁺

Question 7

How many porphobilinogen molecules go into the 3rd reaction of heme synthesis?

What is the enzyme and product?

What two things can happen to the product?

Answer 7

Porphobilinogen Deaminase

• requires 4 porphobilinogens to make hydroxymethylbilane
• OH-methylbilane is an unstable linear intermediate which can:
  • Spontaneously close - which is slower and results in false endproduct uroporphyrinogen I excreted in urine
  • Continue on to become uroporphyrinogen III

Question 8

After the tetrapyrrole ring structure is formed via uroporphyrinogen III synthase…

What are the next 3 steps in heme synthesis?

Answer 8

1. Uroporphyrinogen Decarboxylase​
   
   • makes coproporphyrinogen III
   • decarb. acetyl -> methyl
2. Coproporphyrinogen III Oxidase
   
   • oxidizes 2 propionyl grps -> vinyl
   • makes protoporphyrinogen IX
3. Protoporphyrinogen Oxidase
   
   • oxidizes interpyrrole bridges + 2 nitrogens
4. Ferrochelatase
   
   • adds Fe²⁺ ….. (inhibited by Pb²⁺)

Question 9

What molecule transports iron in blood?

In what form?

How does it enter cells?

And how is the form changed for use?

Answer 9

• Transferrin transports two Fe^{<strong>3+</strong>} per molecule (note it’s 3+ !!!)
  
  • ​this “diferric” transferrin binds to transferrin receptors and is absorbed into endosomes in which the lower pH triggers iron release
• Fe³⁺ binds to Ferritin** and is reduced to usable Fe²⁺ by **Ferritin Reductase
• empty “apotransferrin” leaves cell

Question 10

Where and how is heme degradation started?

2 steps

Answer 10

In macrophages…

1. Heme Oxygenase
   
   • ​​Heme B + 3 NADPH + 3 H⁺ + 3 O² –> Biliverdin + etc. + H₂O + Fe²⁺ + CO
   • destabilizes methinyl bridges via hydroxylation to linearize rings
2. Biliverdin Reductase
   
   • ​**​Biliverdin + NADPH + H⁺ –> **Bilirubin
   • converts central methinyl bridge from =CH- to -CH₂-

Question 11

What is special about bilirubin that makes some disorders of heme metabolism benefical to health?

Answer 11

Bilirubin can act as an antioxidant…

• Bilirubin + H₂O₂ –> Biliverdin + 2 H₂O

Question 12

What is the next two-step process in heme degradation after bilirubin is formed?

Where does it take place?

Answer 12

Conjugation in the liver…

• UDP-glucose DH
  
  • ​UDP-Glucose + 2 NAD⁺ + H2O –> UDP-Glucuronic Acid + 2 NADH + 2 H⁺
• UDP Glucuronosyl Transferase
  
  • ​Bilirubin + 2 UDP-glucuronic acid –> Bilirubin Diglucuronide + 2 UDP
  • 2 steps… first monogluc then digluc
• Makes the bilirubin more polar and thus water-soluble

Question 13

What results from issues with pumping heme degradation products out of the liver?

What 2 mechanisms can cause this?

Answer 13

Post-hepatic Jaundice / Direct Hyperbilirubinemia

• means conjugation is working, but liver secretion is not
• Mechanisms:
  
  1. Obstructred bile duct
  2. Dysfunctional MRP2 transporter (Dubin-Johnson/Rotor Syndrome)
• conjugated bilirubin increases in urine/plasma

Question 14

What results from abnormal increases in the amount of heme which must be degraded?

Answer 14

Prehepatic Jaundice

• caused by anything which increases hemolysi__s and thus heme breakdown
  
  • sickle cell anemia, Pyr Kinase deficiency, G6P DH deficiency
• a form of indirect (unconjugated) hyperbilirubinemia
  
  • _​_increased heme degradation overwhelms bilirubin conjugation capacity of liver
• increased unconj. bilirubin in blood

Question 15

What results from issues with bilirubin conjugation in the liver?

What general condition and what two specific diseases?

Answer 15

Hepatic Jaundice

• a form of Indirect (Unconjugated) Hyperbilirubinemia

1. Crigler-Najjar Syndrome (Type I)
   
   • congenital non-hemolytic jaundice
   • total UDP-glucuronosyl transferase deficiency
   • Phenobarbitol induces conjugation in partial deficiency (type II) but useless in type I
2. Gilbert’s Disease​
   
   • ​slightly reduced transferase activity, 5% of people, anti-oxidant capacity of increased bilirubin can be beneficial