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Flashcards in Heme Synthesis & Hemoglobin Deck (49)
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What are the functions of heme?

  1. Transport of oxygen (hemoglobin, myoglobin)
  2. Electron transport (respiratory cytochromes)
  3. Oxidation-reduction reactions (cytochrome P450 enzymes)


  • Where are the major sites of heme synthesis?
  • Where else is heme synthesized?
  • What cannot synthesize heme?

  • Major Sites:
    • bone marrow ⇒ hemoglobin (6-7g hemoglobin are synthesized each day)
    • liver ⇒ cytochrome P450 enzymes (drug detoxificaton)
  • However, heme is also required for other important cellular proteins and is synthesized in virtually all cells,
  • mature erythrocytes do not synthesize heme (lack mitochondria)


  • What are porphyrins?
  • What is the structure of heme?

  • Porphyrins: cyclic tetrapyrroles capable of chelating to various metals to form essential prosthetic groups for various biological molecules
  • Heme is predominantly a planar molecule
    • porphyrin derrivative + a single ferrous ion (Fe2+ = reduced form of iron)


Heme = ?

  • What is heme oxidized to?

Heme = Ferroprotoporphyrin IX

  • Ferroprotoporphyrin IX (heme) is rapidly autooxidized to ferriprotoporphyrin IX ("hemin"; contains ferric Fe3+ iron)


7 major steps of heme biosynthesis:

  • The 1st and last 3 steps occur in the ....
  • The intermediate steps occur in the ....

  • The 1st and last 3 steps occur in the mitochondrion
  • The intermediate steps occur in the cytosol


What is the committed step of heme synthesis?

Step 1: condensation of glycine and succinyl-CoA with decarboxylation, to yield 5-aminolevulinate (ALA) 


Step 1 of heme synthesis:

  • Reaction: 
  • Enzyme: 
  • Location: 
  • Cofactor: 

  • Reaction: condensation of glycine and succinyl-CoA with decarboxylation, to yield 5-aminolevulinate (ALA)
  • Enzyme: 5-aminolevulinate synthase (ALAS)
  • Location: ALAS is localized to the inner mitochondrial membrane
    • encoded by a nuclear gene family
    • must be imported into the mitochondrion
  • Cofactor: pyridoxal phosphate (PLP) dependent enzyme (vitamin B6)
    • Condensation with succinyl-CoA takes place while the amino group of glycine is in Schiff base linkage to the PLP aldehyde


What are the isoforms of ALAS?

Two isoforms of ALAS:

  1. ALAS1 is the liver isoform
  2. ALAS2 is the erythroid/reticulocyte isoform


Describe the regulation of ALAS1:

  • Feedback inhibition by heme or hemin regulates heme biosynthesis in the liver
  • Heme (hemin) exerts multiple regulatory effects on hepatic heme biosynthesis by inhibiting ALAS1 synthesis at both transcriptional and translational levels, as well as its mitochondrial import
  • drugs or metabolites can increase ALAS1 activity
    • increase the synthesis of cytochrome P450 enzymes ⇒ increasing the demand for heme


Describe the regulation of ALAS2:

  • Heme biosynthesis in erythroid cells is NOT regulated by feedback repression of ALAS2 by heme
  • In reticulocytes (immature RBCs), heme stimulates synthesis of globin and ensures that heme & globin are synthesized in the correct ratio for assembly into hemoglobin
  • Drugs that cause a marked elevation in ALAS1 activity, such as phenobarbital, do not affect ALAS2


Step 2 of heme biosynthesis:

  • Reaction:
  • Enzyme: 
  • Cofactor: 
  • Location: 
  • Complication:

  • Reaction: condensation of two molecules of ALA to form one molecule of porphobilinogen (PBG)
    • first pathway intermediate that includes a pyrrole ring
  • Enzyme: ALA dehydratase (ALAD)
  • Cofactor: Zn2+  
    • lead and other heavy metals can displace the Zn2+ and eliminate catalytic activity
  • Location: cytosol
  • Complication: lead poisoning
    • increase ALA in urine
    • clinical manifestations that mimic acute porphyrias


Effects of lead poisoning: 

  • Heme synthesis:
  • Neurologic symptoms:

  • Inhibition of ALA dehydratase (aka, porphobilinogen synthase) by lead (Pb2+) results in elevated blood ALA 
    • as impaired heme synthesis leads to de-repression of transcription of the ALAS gene
  • ALA is toxic to the brain, perhaps due to:
    • Similar ALA & neurotransmitter GABA (γ-aminobutyric acid) structures
    • ALA autoxidation generates reactive oxygen species (ROS)


Step 3 of heme synthesis:

  • Reaction:
  • Enzyme:
  • Coenzyme:
  • Location: 

  • Reaction:   
    • Step 1: head-to-tail condensation of 4 porphobilinogen molecules to form hydroxymethylbilane (linear tetrapyrrole)
      • Each condensation ⇒ liberation of one ammonium ion
    • Step 2: hydroxymethylbilane ⇒ uroporphyrinogen III 
  • Enzyme: porphobilinogen deaminase (PBGD) or uroporphyrinogen I synthase
  • Coenzyme: uroporphyrinogen III cosynthase
  • Location: cytosol


What is the role of uroporphrinogen III cosynthase?

  • The tetrapyrrole can spontaneously cyclize to form uroporphyrinogen I (nonenzymatic) which IS NOT in the normal pathway for heme biosynthesis
  • However, PBGD is tightly associated with a second enzyme uroporphyrinogen III cosynthase (UROS) 
    • no enzymatic activity alone
    • serves to direct the stereochemistry of the condensation reaction to yield the uroporphyrin ogen III isomer which IS on the pathway for heme biosynthesis


Step 4 of heme synthesis:

  • Reaction:
  • Enzyme: 
  • Location:

  • Reaction: uroporphyrinogen III coprophorphyrinogen III
    • decarboxylation of acetate side chains to methyl groups 
  • Enzyme: Uroporphyrinogen decarboxylase (UROD) 
  • Location: cytosol


Step 5 of heme synthesis:

  • Reaction:
  • Enzyme:
    • What is being converted?
  • Location:
    • What does this imply?

  • Reaction: Coproporphyrinogen III ⇒ protoporphyrinogen IX
    • transported into the intermembrane space
  • Enzyme: coproporphyrinogen III oxidase (CPO)
    • converts specific propionic acid side chains to vinyl groups
  • Location: intermembrane space of the mitochondrion 
    • implying that its product or protoporphyrin IX must cross the inner mitochondrial membrane because heme is formed within the inner membrane


Step 6 of heme synthesis:

  • Reaction: 
  • Enzyme: 
  • Location: 

  • Reaction: protoporphyrinogen IX ⇒ protoporphyrin IX (moving double bonds)
  • Enzyme: protoporphyrinogen IX oxidase (PPO)
  • Location: mitochondrion


Step 7 of heme synthesis:

  • Reaction: 
  • Enzyme: 
  • Location: 

  • Reaction: Insertion of Fe2+ into protoporphyrin IX to generate HEME
  • ​​Enzyme: ferrochelatase
  • Location: mitochondrion 


  • What can inhibit Step 7 of heme synthesis?  
  • What will result in a brillant flourescent complex?

  • Ferrochelatase is inhibited by lead (lead poisoning; increase protoporphyrin in urine) and is also inhibited during iron deficiency (anemia)
  • In the absence of Fe2+:
    • ferrochetalase can insert Zn2+ into the protoporphyrin ring to yield a brilliantly fluorescent complex


What are porphyrias?

inherited genetic or acquired (rarely) disorders resulting from deficiency in specific enzymes of the porphyrin/heme biosynthetic pathway



  • How are porphyrias classified?  
  • What is the pattern of inheritance?

 Either hepatic or erythroid

  • reflect the principal sites of heme biosynthesis 
  • depend on the site of expression of the enzyme defect
  • Inheritance: autosomal dominant
    • Except congenital erythropoietic porphyria (autosomal recessive)


  • What causes symptoms seen in porphyrias?
  • What is the difference in location of the defect (early vs. late)?

  • Accumulation of intermediates upstream from the enzyme defect results in the clinical symptoms associated with the various porphyrias
  • Defects early in the biosynthetic pathway (accumulation of ALA, prophobilinogen) result in neurologic dysfunction
  • Defects later in the pathway (accumulation of cyclic tetrapyrroles, but not prophobilinogen) result in sunlight-induced cutaneous lesions:
    • in the presence of molecular oxygen, UV irradiation of cyclic tetrapyrroles generates reactive oxygen species that can produce cellular damage


What are the acute porphyrias?

  • Definiton:
  • Symptoms: 
  • Examples:

  • Periodic acute attacks
  • Symptoms: abdominal pain, neurologic deficits, psychiatric symptoms, and reddish-colored urine.
  • Examples:
    1. Doss porphyria (ALA dehydratase deficiency)
    2. Acute intermittent porphyria
    3. Hereditary coproporyphyria
    4. Variegate porphyria


What are chronic porphyrias?


  • Dermatologic diseases that may or may not include the liver and nervous system
  • Examples:
    1. Congenital erythropoietic porphyria (Gunther's disease)
    2. Erythropoietic porphyria/protoporphyria
    3. Porphyria cutanea tarda


  • What enzymes are particularly sensitive to lead poisoning?  
  • What will be seen in the urine during lead poisoning?

  • Ferrochelatase and ALA dehydratase are particularly sensitive to lead poisoning
  • Protoporphyrin and ALA accumulate in the urine


What is the function of hemoglobin?

  • Hemoglobin is a specialized protein designed to transport oxygen (O2) from the lungs, a region of high Oconcentration, to peripheral tissues, where oxygen is low
  • Metabolism in the peripheral tissues generates CO2 and H+ that are transported back to the lungs, in part, by hemoglobin.
  • O2 has very low solubility in plasma 
  • As a consequence, >98% of the O2 that reaches tissues is carried in red blood cells (RBCs) bound to Hemoglobin


How is CO2 transport different from O2 transport?

  • RBCs contain carbonic anhydrase which catalyzes the rapid reversible hydration of CO2 to carbonic acid (H2CO3). 
  • H2CO3 then rapidly and spontaneously dissociates to bicarbonate (HCO3-) and a H+ 
  • CO2 and  HCO3- are soluble in plasma and RBC cytosol
    • most of the CO2 made in tissues returns to the lungs as those species
    • about 14% of the CO2 made is carried bound to Hb


  • What is the structure of hemoglobin?
  • What is hemoglobin related to?
  • Which form of Fe can bind O2
  • Which form of iron cannot bind O2? What is it called?

  • Hemoglobin is a heterotetrameric protein (αβ)2
  • Both subunits are evolutionarily related to myoglobin 
    • a monomeric protein abundant in muscle that is designed to store O2
    • myoglobin: 1 heme groups
    • hemoglobin: 4 heme groups
  • Fe2+ is the ferrous form of iron that is capable of binding O2 
  • Fe3+ is the ferric form of iron that CANNOT bind O2 and is present in an INACTIVE form
    • methemoglobin (metHb)


Describe the cooperative binding curve for oxygen:

  • Myoglobin gives a normal binding curve which is hyperbolic in shape
  • Hemoglobin shows sigmoidal cooperative binding of oxygen
    • direct result of its more complex subunit structure
  • P50partial pressure of oxygen yielding 50% saturation of binding
    • analogous to Km for the binding of substrates to enzymes


What kind of cooperativity does hemoglobin exhibit for oxygen?

Sequential cooperativity for oxygen binding:

  • binding of oxygen to one subunit induces a conformational change that is partially transmitted to adjacent subunits 
  • transmission of the partial conformational change induces an increased affinity for oxygen by these adjacent subunits
    • R=relaxed=high affinity; T=taut=low affinity