F8vii: Physiology of Haemoglobin

Definition

  • A molecule of 4 protein chains, each carrying a heme group: MV 65,000 Daltons GG
  • Normal levels \( 
    \begin{cases}
    \text{M 13 – 18g/dL}\\
    \text{F 11 – 16g/dL}
    \end{cases}
    \)
  • Each RBC consists of 200 – 300 million molecules of Hb

Synthesis

  • 2 parts: heme + globin
  • Each heme moiety consists of PROTOPORPHYRIN RING & central iron ion in ferrous state (Fe2+)
  • Heme synthesis occurs in RBC cytosol & mitochondria
  • Fe2+ forms 6 bonds with heme moiety; 5 bind Fe2+ firmly, 1 binds O2
  • 4 polypeptide chains determine type of Hb
    • HbA = 2α + 2β globin chains
    • HbF = 2α + 2γ globin chains
    • HbA2 = 2α + 2δ globin chains
  • Altered polypeptide chains create variants important to know because alter body’s capacity to transport O­2

NB: 98% of normal adult Hb is HbA

Function

  1. Transport O2 from lungs → tissues
  2. Transport CO2 from tissues → lungs as CARBAMINO Hb
  3. Buffer H+ formed in RBC
  4. No metabolism

O2 Transport

  • O2 binds reversibly to heme
  • 4 hemes → each Hb can carry 4 x O2
  • Hb is an allosteric protein
  • The binding of O2 to one heme group ↑affinity of remaining heme groups for O2
  • This +ve cooperativity means that DeoxyHb & oxyHb have very different structures
  • This shape is maintained by electrostatic bonds b/w α-acid sequences
  • DeoxyHb: electrostatic bonds are strong, hiding heme deep in crevice, making it difficult for O2 to access heme this quaternary structure → KA TENSE FORM 
  • OxyHb = binding of first O2 to a heme changes the electrostatic bonds. Relaxes the crevice holding heme and enlarges the access → transmits this to other globin chains by distracting their electrostatic bonds & the quaternary structure is altered
    • ↑ other globin affinity for O2
    • KA RELAXED FORM when 4O2 bound to 1Hb
Transport
  • The conformational state of Hb (R/T) is also altered by other factors, which influence strength of electrostatic bond:
    • CO2
    • pH
    • Temperature
  • Huffner Constant
    • O2 binding capacity of Hb = amount of O2 in mL each Hb can carry
    • KA HUFFNER’S CONSTANT = 1.3mL/g of Hb 
  • The Oxyhaemoglobin Dissociation Curve
    • O2 binds reversible with ferrous iron of Hb to form oxyhaemoglobin

Hb + O2 HbO2

    • Forward reaction occurs in lungs due to ↑PO2
    • Backward reaction occurs in tissues due to ↓PO­2
    • Curve is SIGMOID SHAPE due to COOPERATIVITY
    • Affinity for O2 is lowest when first molecule binds to ‘TENSE’ deoxygenated Hb
    • As each subsequent molecule binds, the gradient of the curve increases
    • As PO2 increases, all the Hb – O2 binding sites become occupied & the curve levels off
    • 3 important points on ODC:
      1. Arterial PO2 where Hb 100% saturated (PO2 100 = SpO2 97%)
      2. Venous saturation (PO2 40 = SpO2 75%)
      3. P50 (P50 = 27mmHg PO2 = SpO2 50%)
  1.  
ODC
  • P50 = the partial pressure of oxygen in blood where Hb is 50% saturated
  • P50 is a measure of oxygen affinity
  • ∴It is useful to compare changes in the position of the ODC
  • P50 for any single form of Hb is variable → the hydrogen bonds & ionic interactions within Hb result in altered affinity of Hb for O2

R) Shift

↑2,3 DPG

↑Temp

↓pH

↑CO2

L) Shift

↓2,3 DPG

↓Temp

↑pH

↓CO2

pH

  • An ↑[H+] will ↓Hb affinity for O2
  • DeoxyHb binds more actively with H+ ions
  • ∴as pH ↓so does Hb affinity for O­2

KA BOHR EFFECT: ↓ O1 affinity at low pH & ↑ O2 affinity at high pH

  • The Bohr Effect is important because it offloads O2 to metabolically challenged areas
  • CO2 has a similar effect because results in ↑[H+]

CO2 + H2O H2CO3 H + + HCO3

2, 3 DPG

  • 2,3 DPG is a highly anionic organic phosphate
  • Produced by Rapport – Luebering Shunt
Luebering Shunt
  • This shunt comes off the glycolysis pathway
  • Level of 2,3 DPG determined by balance of synthesis/degradation
  • Activity of DPG Mutase ↑ with ↑H+
  • DPG binds b/w two β globin chains & stabilises the Hb in the TENSE configuration
  • ∴↓Hb affinity for oxygen & displacing ODC to RIGHT
  • 2,3 DPG is ↑in ANAEMIA & HIGH ALTITUDE
  • ∴tissue hypoxia of anaemia partially corrected by ↑P50
  • The ↑RESP ALKALOSIS at altitude has much more pronounced effect than the ↑2,3 DPG & at high altitude ODC shifts LEFT
  • Blood storage alters 2,3 DPG levels
  • 26°C glycolysis ↓<5% normal rates
  • ∴so does the production of 2,3 DPG
  • After 2 weeks, 2,3 DPG levels are zero
  • Once transfused, RBC become warmed & provided with metabolites for glycolysis
  • The limiting factor to restore 2,3 DPG levels is the activity of 2,3 DPG MUTASE
  • DPG levels of transfused blood = 50% normal in 7hrs
  • Normal levels are reached in 48hrs

Hb Destruction

In ILEUM/COLON converted to UROBILINOGEN by bacterial by removing Glucuronic Acid

UROBILINOGEN

  • Lipid soluble
  • 10% reabsorbed & transported by albumin → enterohepatic circulation → URINE
  • 90% further oxidised to STEROCOBILIN
  • Excreted in faeces