Structure and Function of Haemoglobin
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Protein Assignment
Unit 3 – Biology,
Structure and Function of Haemoglobin
Haemoglobin is a protein that is carries via the red blood cells. It picks up oxygen in the lungs and delivers it to the peripheral tissue to maintain that viability of cells. It is significantly composed of globin and heme that gives the red blood cells their characteristic colour and function.
Structure:-
A hemoglobin molecule consists of four polypeptide chains: two alpha chains, each with 141 amino acids and two beta chains, each with 146 amino acids. The protein portion of each of these chains is called “globin”. The and b globin chains are very similar in structure. In this case, a and b refer to the two types of globin.
Figure 2 is a close up view of one of the heme groups of the human a chain from dexoyhemoglobin. In this view, the iron is coordinated by a histidine side chain from amino acid 87 (shown in green.)
Each a or b globin chain folds into 8 a helical segments (A-H) which, in turn, fold to form globular tertiary structures that look roughly like sub-microscopic kidney beans. The folded helices form a pocket that holds the working part of each chain, the heme.
A heme group is a flat ring molecule containing carbon, nitrogen and hydrogen atoms, with a single Fe2+ ion at the center. Without the iron, the ring is called a porphyrin. In a heme molecule, the iron is held within the flat plane by four nitrogen ligands from the porphyrin ring. The iron ion makes a fifth bond to a histidine side chain from one of the helices that form the heme pocket. This fifth coordination bond is to histidine 87 in the human a chain and histidine 92 in the human b chain. Both histidine residues are part of the F helix in each globin chain.
Function:-
Hemoglobin is the protein that carries oxygen from the lungs to the tissues and carries carbon dioxide from the tissues back to the lungs. In order to function most efficiently, hemoglobin needs to bind to oxygen tightly in the oxygen-rich atmosphere of the lungs and be able to release oxygen rapidly in the relatively oxygen-poor environment of the tissues. It does this in a most elegant and intricately coordinated way. The story of hemoglobin is the prototype