ICU Primary Snippet 20- Structure and Functions of Adult Haemoglobin

Structure and Function of adult haemoglobin

Written by Dr Madhuri Anupindi

Structure:

• Haemoglobin is a tetramer consisting of four polypeptide chains, each with its own haem group
• Adult haemoglobin (HbA) consists of 2 alpha and 2 beta polypeptide chains, which are similar in size and structure
  • Alpha polypeptide consists of 141 amino acid residues, while the beta polypeptide has 146 amino acid residues
• The two alpha-beta subunits are arranged tightly together in a tetrahedral form that is held together by hydrophobic bonds
• The alpha and beta chains are each comprised of 7 or 8 helices named A-H that are joined by non-helical segments  the E and F helices form a pocket for haem to bind via histidine residues
• Haem is comprised of a ferrous ion held in the middle of a porphyrin ring called protoporphyrin IX
• Haem is the site of reversible oxygen attachment – therefore, each haemoglobin molecule can carry up to four oxygen molecules

Function

• Oxygen carriage to the tissues
  • The main role is oxygen transport  increases the oxygen-carrying capacity of blood by greater than 50 times.
  • The binding of oxygen to one haem group causes the Fe2+ ion to be pulled closer towards the protoporphyrin ring, which slightly changes its shape  this alters the quaternary structure of the haemoglobin molecule and increases the oxygen binding affinity of the other globin chains  positive co-operativity
  • When haemoglobin is deoxygenated, it exists in the T or tense state, which is not favourable for oxygen binding  as oxygen binds, it makes it easier for further oxygen molecules to bind and haemoglobin changes to its R or relaxed state, which has a higher affinity for oxygen binding. Therefore, in the lungs where the oxygen pressure is high, it allows for haemoglobin to be fully saturated with oxygen more easily and for haemoglobin to easily release oxygen at the tissues where oxygen pressure is low.
• Carbon dioxide carriage
  • Haemoglobin carries carbon dioxide as carbaminohaemoglobin
  • Haemoglobin has a higher affinity for CO2 when its deoxygenated and a lower affinity when it is oxygenated (releases Co2)
• Hb as a buffer:
  • A buffer is a solution containing a weak acid and its conjugate base. It resists pH change when exposed to a stronger acid or base. In the body, the buffer binds H+ irons reversibly to minimise the change in pH
  • Haemoglobin has multiple histidine residues with imidazole side chains  imidazole has a pKa of 6.8 and binds H+
  • Haemoglobin accounts for 50 – 60% of the buffering capacity of blood
  • Hb is a weak acid: buffers H+ and forms HCO3 within FBC  intracellular HC03 increases leading to diffusion down concentration gradient out of cell
• Transport of nitric oxide
  • Nitric oxide binds with high affinity to the ferrous ion of haem
    • If haemoglobin is de-oxygenated, this forms nitrosyl-haemoglobin. Affinity for nitric oxide is greater than for either oxygen or carbon monoxide.
    • If haemoglobin is oxygenated, the nitric oxide can displace the oxygen  oxidises Fe2+ to Fe3+ and forms methaemoglobin and a nitrate ion. This reaction is very rapid and can quickly sequester nitric oxide (which is a potent vasodilator). This reaction is limited by the hydrophobic RBC membrane.
  • Nitric oxide can also bind to cysteine residues in the B-globin chains  form S-nitroshaemoglobin (SNO-Hb) – this reaction occurs faster with oxygenated haemoglobin.
    • This helps regulate local blood flow as the release of SNO is increased in low pO2 environments and causes vasodilatation.