Blood Constituents — Haemoglobin

STRUCTURE

Structure

• Globular proteins which contain a haem moiety which binds O₂.
• Haem is an protoporphryin ring derivative with a central Fe2+ molecule that binds O₂
• Haemoglobin: MW ≈ 65000 daltons
  • Hb contains 65-70 % of total body iron
  • Globular molecule made up of four subunits, each containing a haem moiety conjugated to a polypeptide.
  • Polypeptides collectively = globin → two pairs 2α + 2β → 4 haem moieties (Tetramer)
  • Can bind a total of 4 O₂ and also exhibits cooperative affinity (each subsequent O₂ binding takes less energy → sigmoid shaped OHDC

Location

• Hb is located in large concentrations (≈15g/L) in red blood cells that circulate throughout the blood stream.

Synthesis

• Haeme is synthesised in mitochondria and cytosol of immature RBCs, Globin is synthesized by ribosomes in cytosol.
• Production continues till RBCs lose their RNA after entering vasculature

Degradation

• RBCs at the end of their life cycle get phagocytosed by macrophages in liver or spleen, or get hemolysed in circulation.
• Haemoglobin is then broken up
  • Haeme gets degraded into bilirubin
  • iron gets recycled

FUNCTION

• O2 carrier:
  • O2 loading exhibits positive cooperativity:
    • α1ß1 & α2ß2 contacts stabilise Hb molecule as O2 reacts with it
    • reaction of O2 with each subunit occurs sequentially with each facilitating the next
    • ∴ ↑ing affinity as O2 loads ⇒ sigmoid OHDC
• O2 unloading – vice versa:
  • ß chains pulled apart
  • 2,3-DPG enters molecule ⇒ ↓affinity of Hb for O2
• Protein buffer in RBC:
  • Haemoglobin exists as a weak acid (HHb) as well as its potassium salt (KHb)
  • In acidosis:
    • Additional H⁺ ions are bound to Hb molecules
    • HCO₃^– diffuses down its concentration gradient into plasma
      • Electroneutrality is maintained through the inwards movement of Cl^–.
    • Dissolved CO₂ will also form carbamino compounds by binding to the terminal amino groups
• Ligand Binding:
  • Competitive inhibitors such as carbon monoxide (CO) and allosteric ligands such as carbon dioxide (CO2) and nitric oxide (NO).
  • The carbon dioxide is bound to amino groups of the globin proteins to form carbaminohemoglobin; this mechanism is thought to account for about 10% of carbon dioxide transport in mammals.
  • Nitric oxide can also be transported by haemoglobin; it is bound to specific thiol groups in the globin protein to form an S-nitrosothiol, which dissociates into free nitric oxide and thiol again, as the hemoglobin releases oxygen from its heme site. This nitric oxide transport to peripheral tissues is hypothesized to assist oxygen transport in tissues, by releasing vasodilatory nitric oxide to tissues in which oxygen levels are low.