Haemoglobin

[Ref: WG21:p536-539; KB:p211]

Haemoglobin structure

• Protein with MW of 64450
• 4 subunits
  * Each subunit contains a heme conjugated to a polypeptide (globin)
  * 2 pairs of globin in each haemoglobin
  * Heme = Iron-containing porphyrin derivative
  * Iron = ferrous ion (Fe2+)
• Adult haemoglobin (HbA) has
  * 2 alpha chains and 2 beta chains
  --> alpha2beta2
• About 2.5% of hemoglobin is HbA2
  * 2 alpha chains and 2 delta chains

Other species

• HbA1c
  * Small amounts of HbA has glucose attached to the terminal valine in each beta chain
  * Used in monitoring diabetes mellitus
• Carboxyhaemoglobin (COHb)
  * Hb bound with carbon monoxide
• Sickle cell (HbS)
  * Polymerise at low O2 tension --> Haemolysis
  * Confers resistance to one type of malaria
  * 40% of population in some parts of Africa
• Myoglobin
  * Found in red muscles
• Neuroglobin
  * Found in brain
• Haem also present in cytochrome c

Haemoglobin synthesis

• 0.3g of Hb is destoryed and synthesized each hour
  = 1 x 10^10 RBC per hour
• [KB2:p211] Daily breakdown of Hb = 6g

Steps in Hb synthesis

Condensation of glycine and succinyl-CoA
--> Protoporphyrin
* Catalyzed by delta-aminolaevulinic acid (ALA) synthase
* Co-enzyme: pyridoxal phosphate (vitamin B6)
* This first step is also the rate-limiting step

--> Protoporphyrin combines with iron (Fe2+) to form haem (or heme)
* Catalyzed by ferrochelatase

ALA synthase

• Short halflife (1hr)
• Activity and synthesis of the enzyme is reduced by haem
  --> Negative feedback mechanism

In foetal life

Foetal haemoglobin (HbF)

• 2 alpha and 2 gama chains
• HbF
  = 80% of Hb at birth
  = <5% of Hb 6 months after birth
• Also see Oxygen dissociation curve

Beta globin only appears after 6 months in foetal life

RBC breakdown

[KB2:p211]

• Most breakdown is by macrophages in reticuloendothelial system, especially spleen
  * Some occurs in liver and bone marrow
• 10% of daily red cell breakdown occurs within the blood
  --> Hb dissociates into alpha-beta dimers
  --> Dimers are bound by haptoglobin
• Haemopexin binds to free haem when haptoglobin is saturated
• Binding is important in preventing renal loss of haem and dimers (thus iron loss)

Haemoglobin breakdown

## Haematology_-_Haemoglobin_breakdown_20050402-02.png

• Haemoglobin
  --> Globin + Haem
• Globin --> Amino acids
• Haem --> Iron + protoporphyrin
  * By haem oxygenase
• Protoporphyrin --> Carbon monoxide + Biliverdin
• Biliverdin
  --> Bilirubin (by biliverdin reductase in macrophages)
  --> Carried by albumin in blood
  --> Taken up by hepatocytes by facilitated diffusion
  --> Conjugated with UPD-glucuronic acid (more soluble)
  --> Secreted into bile
  --> Enters into GIT with bile
  --> Conversion to stercobilinogen and then sterobilin
  --> Excreted in faeces
• Some stercobilin is reabsorbed and excreted in kidney as urobilinogen

NB:

• Bilirubin is converted to lumirubin by whitelight
  --> Shorter halflife
  --> Used to treat jaundice in neonates
• Unconjugated bilirubin = indirect
• Conjugated bilirubin = Direct bilirubin
  * Reacts directly in van den Burgh reaction