Heme Synthesis & Hemoglobin Flashcards Preview

 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)

• 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

• 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

• 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

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

• Hemoglobin is a specialized protein designed to transport oxygen (O₂) from the lungs, a region of high O₂ concentration, 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

• 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

• 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)

• 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
• P₅₀: partial pressure of oxygen yielding 50% saturation of binding
  • analogous to Km for the binding of substrates to enzymes

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

• Carbon monoxide (CO) has ~250-fold higher affinity for Hb than does O₂ 
• When bound to the heme group of one subunit, it causes all four subunits to "lock" in the R conformation
  • ​limiting oxygen release in peripheral tissues

Without O2 bound:

• heme Fe²⁺ is pulled away from the plane of the porphyrin ring by a His residue of the Hb polypeptide chain
  • a His ring N is bound to the Fe²⁺ 

When O2 binds:

• it pulls the Fe²⁺ back into the plane of the ring 
  • moves the His residue and its whole section of the polypeptide chain
• That in turn causes the Hb subunits to shift relative to one another to an arrangement that favors the R-conformation

• Reduces affinity: 
  • ​H⁺, CO₂, and 2,3-diphosphoglycerate (DPG) ALL can bind to Hb
  • leftward curve shift
• Increases affinity:
  • high O₂ 
  • causes H⁺, DPG and CO₂ to dissociate from Hb
  • rightward curve shift

H⁺ and CO₂ are heterotropic negative allosteric effectors that decrease the affinity of Hb for O2

During catabolism

• In the blood of tissues, CO₂ and H⁺ are high
• HbO₂ will release its O₂ load ⇒catabolism can continue

Oxygen is a positive homotropic allosteric effector of O₂ binding

Reciprocal relationship between O₂ and H⁺ binding 

• Oxygen is a negative allosteric effector of H⁺ and CO₂ binding 
• Changes in H⁺ binding result from a shift in the pKa of specific residues (mostly histidines) due to microenvironment effects triggered by conformational changes in the hemoglobin molecule

2,3-diphosphoglycerate is a negative allosteric effector of O₂ binding

• 2,3-DPG binds to a specific site in a central cavity​ between the β subunits 
  • Binding is by ionic interactions
• Special regulatory mechanisms exist in RBCs to control the concentration of 2,3-DPG in order to fine tune the affinity of hemoglobin for O₂ in response to changes in metabolism and environment

1. Without any 2,3-DPG
   • ​​Hb would be much more like myoglobin  
   • 2,3-DPG stabilizes the T-state, making it easier for Hb to release O₂
2. 2,3-DPG levels increase at high altitudes 
   • Because there is less O₂ at high altitudes, tissues tend to become somewhat hypoxic 
   • By increasing the concentration of 2,3-DPG ⇒ RBC adapt to hypoxia ⇒ easier for O₂ to dissociate from Hb
     • anemia and smoking also cause an increase in 2,3- DPG
   • Changes in 2,3-DPG occur over hours and days
     • takes most people a few days to adapt to high altitude
     • exercise or strenuous activity will be difficult until then

↑ temp = ↓O₂ affinity

↓ temp = ↑ O₂ affinity

• Each chromosome 16 has two α-globin genes
  • a person has 4 total, each is active and codes ~1/4 of expressed α-globin subunit
• Each chromosome 11 has a single β-globin gene

• ~95% HbA (α₂β₂)
• ~3% HbA₂ (α₂δ₂)
• ~2% HbF (α₂γ₂)

Note: all forms have an α-globin

• most common hemoglobinopathy ⇒ sickle cell anemia
• pattern of inheritance ⇒ autosomal recessive

• Caused by a point mutation in the adult β-globin gene that causes substitution of valine (Val) for glutamic acid (Glu) at amino acid 6 
• Patient's RBCs containing mainly hemoglobin S (HbS)
  • ​two normal adult α-globin subunits and two sickle adult β-globin subunits.
• Valine is hydrophobic and its presence creates a sticky patch on deoxyHb
  • leads to polymerization of Hb tetramers into long chains
  • intracellular fibers cause the sickle cell shape and reduced deformability of the RBCs
  • leads to problems in their passage through the microcirculation

1. degree of deoxygenation
   • which can be affected by subtle changes in pH, ionic strength and temperature
   • Deoxygenated HbS forms insoluble polymers
2. intracellular hemoglobin concentration
3. relative amount of HbF present
   • HbF inhibits polymerization owing to a GLU residue at position 87 of the gamma chain, which prevents a critical lateral contact in the sickle cell fiber
   • HbF decreases with post-partum age but varies from 1-30% of total Hb in sickle cell individuals

a heterogeneous group of disorders caused by inherited mutations that decrease the synthesis of adult hemoglobin HbA (α₂β₂)

1. β-Thalassemias
   • Caused by mutations that diminish the synthesis of β-globin chains
2. α-Thalassemias
   • Caused by mutations that result in reduced or absent synthesis of α-globin chains