Erythrocytes: Hemoglobin

Chapter 6: Erythrocytes: Hemoglobin

Characteristics and Biosynthesis of Hemoglobin

  • Genetic inheritance of hemoglobin:
    • Normal adult hemoglobin, referred to as hemoglobin A, is inherited in a simple Mendelian fashion.
    • The genotype for normal hemoglobin is expressed as A/A.
    • There exist approximately 350 variant hemoglobin types.
    • Hemoglobin defects primarily arise due to:
    • Amino acid substitutions, known as hemoglobinopathies.
    • Diminished production of polypeptide chains, categorized as thalassemias.

Chemical Composition and Configuration of Hemoglobin

  • Hemoglobin is a complex protein with the following structural features:
    • Comprised of four polypeptide chains, usually arranged as two alpha (α) and two beta (β) chains (α2β2 configuration).
    • Each hemoglobin molecule contains an iron (Fe) atom that is critical for binding oxygen.

Hemoglobin Function

Function #1: Role of 2,3-Diphosphoglycerate (2,3-DPG)
  • The oxygen affinity of hemoglobin is influenced by molecular rearrangement, regulated primarily by the concentration of phosphates, particularly 2,3-DPG.
  • 2,3-DPG binds to the beta chains of deoxyhemoglobin (reduced hemoglobin), thereby diminishing hemoglobin’s affinity for oxygen.
Function #2: Oxygen Dissociation and Affinity
  • The relationship between oxygen pressure (PO₂) and hemoglobin saturation is illustrated in the oxygen dissociation curve.
    • P50 values indicate the partial pressure of oxygen at which hemoglobin is 50% saturated.
  • The curve demonstrates:
    • The impact of factors such as pH and carbon dioxide levels on the oxygen affinity of hemoglobin.

Carbon Dioxide Transport

  • The transport of carbon dioxide (CO₂) in erythrocytes includes:
    • The indirect mechanism, where approximately 75% of CO₂ is enzymatically converted in erythrocytes.
    • CO₂ diffuses into erythrocytes, catalyzed by the enzyme carbonic anhydrase, producing carbonic acid (H2CO3):
      H2O + CO2
      ightarrow H2CO3
    • Carbonic acid dissociates into hydrogen ions and bicarbonate:
      H2CO3
      ightarrow H^+ + HCO_3^-
    • The hydrogen ion is accepted by the alkaline deoxyhemoglobin, while bicarbonate diffuses into the plasma.

Globin Chains and Hemoglobin Composition

  • Comparative composition of various hemoglobin types is indicated in Table 6.2:
    • Embryonic Hemoglobins:
    • Gower-1 (2 zeta + 2 epsilon)
    • Gower-2 (2 alpha + 2 epsilon)
    • Portland (2 zeta + 2 gamma)
    • Fetal Hemoglobin (HbF):
    • Composed of 2 alpha and 2 gamma chains, appearing by the 5th week of gestation and present until several months after birth.
    • Adult Hemoglobin (HbA):
    • Primarily composed of 2 alpha and 2 beta chains.
    • Hemoglobin A2 (HbA2):
    • Composed of 2 alpha and 2 delta chains.

Disorders of Heme (Porphyrin) Synthesis

  • Porphyrias are characterized by disrupted porphyrin synthesis.
    • Classification may be based on:
    • Clinical presentation: acute vs. chronic.
    • Source and site of enzyme deficiency in the heme biosynthetic pathway.

Disorders of Iron Metabolism

Iron Deficiency Anemia
  • Iron is compartmentalized into three categories:
    • Storage (as ferritin)
    • Transport (via serum transferrin)
    • Functional roles (hemoglobin, myoglobin, cytochromes)
  • Pathogenesis includes:
    • Normal iron status dwindles with excessive loss leading to iron store depletion.
Sideroblastic Anemia
  • Causes include:
    • Congenital defects (hereditary sex-linked, autosomal).
    • Acquired defects (myelodysplastic syndromes, drugs, toxins).
Hereditary Hemochromatosis
  • Involves mutations in the HFE gene (type 1), with other types defined by different genetic mutations:
    • Juvenile hemochromatosis (type 2)
    • Midlife onset (type 3)
    • Ferroportin-related defects (type 4)

Disorders of Globin Synthesis

  • Impaired globin synthesis leads to reduced protoporphyrin synthesis and can result in accumulation of iron.
  • Thalassemias are a common manifestation of globin synthesis disorders.

Ontogeny of Hemoglobin

Embryonic Hemoglobins
  • These primitive forms include Gower-1, Gower-2, and Portland, persisting until approximately 12 weeks gestation.
Fetal Hemoglobin
  • Predominantly present in the fetus, consisting of two alpha and two gamma chains.
  • Levels decrease after birth but remain present at low concentrations in adulthood.
Adult Hemoglobin
  • Primarily hemoglobin A (95%-97%) composed of two alpha and two beta chains, produced in higher concentrations around 3 to 6 months post-birth.
  • Hemoglobin A2 is present in smaller amounts (2%-3%).

Glycosylated Hemoglobin (Hemoglobin A1)

  • Hemoglobin A1, a subfraction of hemoglobin A, is glycosylated and includes fractions A1a, A1b, and A1c.
  • It serves as a meaningful marker for long-term glucose control in diabetes, with a normal concentration of 3%-6% and 6%-12% in diabetic patients.

Variant Forms of Normal Hemoglobin

  • Carboxyhemoglobin:
    • High affinity for carbon monoxide (200×), resulting in oxygen deprivation.
  • Sulfhemoglobin:
    • Results from hydrogen sulfide binding, causing denaturation and Heinz body formation.
  • Methemoglobin:
    • Hemoglobin in the ferric state, leading to impaired oxygen delivery and cyanosis at levels > 10%.

Abnormal Hemoglobin Molecules

  • Conditions like sickle cell anemia arise from mutant, codominant genes.
    • Mutant genes can be homozygous (SS in sickle cell disease) or heterozygous (SA in sickle cell trait).

Catabolism of Erythrocytes

  • As erythrocytes age, they experience:
    • Decreased membrane flexibility.
    • Increased hemoglobin concentration.
    • Decreased enzyme activity, particularly glycolysis.

Extravascular Catabolism of Erythrocytes

  • Process of Catabolism:
    • Phagocytosis by macrophages results in hemoglobin decomposition into:
    • Iron
    • Protoporphyrin
    • Globin
    • Iron is transported by transferrin for recycling, while globin undergoes catabolism to amino acids.

Intravascular Catabolism of Erythrocytes

Intravascular Breakdown
  • This pathway accounts for <10% of erythrocyte destruction.
  • Hemoglobin released into the bloodstream forms dimers bound by haptoglobin to avoid urinary excretion.
  • If haptoglobin is depleted, free hemoglobin appears in urine, correlating with hemoglobinuria.
  • Renal processing can transform hemoglobin into hemosiderin or methemoglobin.
  • Haptoglobin-hemoglobin complex is subsequently processed by the liver.

These comprehensive notes detail the structure, function, disorders, and metabolism of hemoglobin, incorporating knowledge necessary for deep understanding in the field of hematology.