Haemoglobin (Hb)

Haemoglobin is a tetrameric protein that consists of two alpha and two beta chains. Each chain noncovalently binds a haem group (Figure 2) which allows oxygen binding via a central iron atom and in association with a distal and proximal histidine residue. Oxygen binding shows positive cooperativity resulting from the movement of the proximal histidine in combination with the iron in the haem group which induces a conformational shift from tense (T) to relaxed (R) state and therefore a higher affinity for oxygen (Voet and Voet, 2011). When being released into the blood plasma the iron atoms can be involved in a reaction with H₂O₂. The iron in its Fe³⁺ state it is oxidised and binds to oxygen with a double bond. In this reaction water is released and a free radical is formed on either the haem group or an adjacent amino acid (Svistunenko, 2005). As shown in knock-out mice experiments without haptoglobin the damaged haemoglobin proteins would precipitate in renal tubular cells and lead to death or failure to repair severe damage to renal tissue (Lim et al., 1998).

Figure 2: Tetramer of haemoglobin with two α-chains (yellow), two β-chains (green) and haem groups (blue with red iron atoms). The molecule displays the T-state without bound oxygen. 

When being released into the blood plasma the iron atoms can be involved in a reaction with H₂O₂. The iron in its Fe³⁺ state it is oxidised and binds to oxygen with a double bond. In this reaction water is released and a free radical is formed on either the haem group or an adjacent amino acid (Svistunenko, 2005). As shown in knock-out mice experiments without haptoglobin the damaged haemoglobin proteins would precipitate in renal tubular cells and lead to death or failure to repair severe damage to renal tissue (Lim et al., 1998).