Study Notes on Heme, Hemoglobin, and Related Biochemical Metabolism

Heme and Hemoglobin Structure

  • Heme is also known as Haem.
  • Composed of Iron (Fe) in a ferrous state: Fe+2Fe^{+2}.
  • Heme is linked by 6 bonds: 4 with porphyrin and 2 with amino acids in proteins.

Porphyrin Structure

  • Porphyrins are cyclic compounds composed of:
    • 4 pyrrole rings
    • Joined by 4 methenyl bridges (-CH=)
Types of Porphyrins

  1. Type 1 Isomer:

    • Substituted groups are symmetrically arranged.
    • Structure: AP-AP-AP-AP.

  2. Type 2 Isomer:

    • Arranged in reverse order.
    • Structure: AP-AP-AP-PA.
Coordination in Heme
  • The nitrogen of the imidazole ring from proximal histidine forms a coordination bond with iron in heme.
  • Hemoglobin (Hb) is composed of:
    • 4 heme groups
    • 4 globin chains (2 alpha, 2 beta)

Functionality of Hemoglobin

  • The binding of O₂ leads to the formation of oxyhemoglobin (Oxy Hb) and the release of deoxyhemoglobin (Deoxy Hb).
  • Hemoglobin protects iron from irreversible oxidation, allowing for the reversible binding of O₂.
  • One molecule of Hb can carry 4 O₂ molecules, while myoglobin can carry 1 O₂ molecule.

Types of Hemoglobin

Normal Types

  1. Hb A (Adult Hemoglobin):

    • Major adult hemoglobin, constitutes 95-97% of total Hb.
    • Glycosylated Hb: HbA1C represents carbohydrate attachment at the NH₂ terminal end.
    • Structure: 2 alpha + 2 beta chains.

  2. Hb F (Fetal Hemoglobin):

    • Present in newborns during early fetal life, constitutes 90% at 7 months gestation.
    • Structure: 2 alpha + 2 gamma chains.
    • Higher affinity for oxygen compared to adult hemoglobin.
    • Provides crucial oxygen supply from mother to fetus.

  3. Hb A2:

    • Constitutes 2-4% of total Hb, composed of 2 alpha + 2 delta chains.
    • Elevated levels in conditions like pre-diabetes.
Abnormal Types of Hemoglobin

  1. Sickle Cell Disease (Hb S):

    • Mutation in the beta chain where glutamic acid is replaced by valine, causing a change in structural properties.
    • Leads to the formation of sticky patches and polymerization of deoxy Hb, resulting in sickle-shaped red blood cells (RBCs).

  2. Thalassemia:

    • Hereditary hemolytic disease, characterized by abnormal hemoglobin synthesis.
    • Can manifest as alpha or beta thalassemia, leading to anemia due to decreased oxygen transport.

  3. Methemoglobinemia:

    • Condition wherein the iron in hemoglobin is oxidized to the ferric state (Fe+3Fe^{+3}), leading to a reduced oxygen-carrying capacity.
    • Treatment includes Hemin injection and avoiding triggers such as certain drugs.

Heme Biosynthesis

  • Heme is synthesized predominantly in the bone marrow (85%), with minor synthesis in the liver (15%).
  • Key precursor: Glycine and Succinyl-CoA triggers synthesis through ALA synthesis (Aminolevulinic acid).
  • Key steps include:
    1. Formation of ALA
    2. Production of porphobilinogen (PBG)
    3. Formation of heme through a series of enzymatic reactions regulated by heme concentrations and other factors.
Regulation of Heme Biosynthesis
  1. ALAS (ALA Synthase):
    • Rate-limiting enzyme highly regulated by heme levels.
    • Vitamin B6 is essential for enzyme function.

Hemoglobin Catabolism

  • Old RBCs are destroyed in the spleen by macrophages releasing hemoglobin.
  • Heme is converted into bilirubin via a sequence of reactions in the liver.
  • Bilirubin is processed and excreted in the bile, contributing to the orange-yellow color of bile.

Folic Acid

  • Folic acid serves as a coenzyme in amino acid synthesis and nucleic acid formation.
  • It is crucial for the synthesis of tetrahydrofolate (THF), which is necessary for nucleotide synthesis and DNA replication.
  • Folic acid deficiency can lead to megaloblastic anemia.

Vitamin B12

  • Known as cobalamin, essential for DNA synthesis and red blood cell production.
  • Deficiency leads to pernicious anemia, characterized by macrocytic cells and neurological disorders.
  • Requires intrinsic factor for absorption, which is synthesized in the stomach.

Glycolysis and Gluconeogenesis

  • Glycolysis is the first step in glucose metabolism, occurring in cytosol, yielding pyruvate and ATP.
  • Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (like lactate) occurring mainly in the liver.
  • Enzymes such as pyruvate carboxylase and phosphoenolpyruvate carboxykinase play vital roles.

Glycogen Metabolism

  1. Glycogenesis:

    • Formation of glycogen from glucose, primarily occurring in the liver and muscles.
    • Key enzyme: Glycogen Synthase.

  2. Glycogenolysis:

    • Breakdown of glycogen into glucose, yielding glucose-1-phosphate and free glucose for energy production in muscles.
    • Key enzyme: Glycogen Phosphorylase.

Regulation of Blood Glucose

  • Insulin and glucagon maintain blood sugar levels.
  • Insulin promotes glucose uptake and glycogen synthesis, while glucagon stimulates gluconeogenesis and glycogenolysis.
  • Imbalance can lead to hypoglycemia or hyperglycemia, necessitating careful monitoring in diabetic patients.