In: Biology
**Biochemistry**
Clearly explain how the structure of Hemoglobin is intricately connected to its function, using the principle listed below as the outline.
Answer :
Haemoglobin is a red blood pigment exclusively found in RBC's. It is a conjugated protein containing globin- the apoprotein part and heme - the non protein part or prosthetic group. Haemoglobin is tetrameric allosteric protein. It has a quaternary structure.
Globin consists of 4 polypeptide chains to each of primary monomeric units. The adult haemoglobin HbA1 has 2 alpha chains and 2 beta chains. The 4 subunits are held together by non-covalent interactions like hydrophobic, ionic and hydrogen bonds. Each subunit contains one heme group
Heme group contains a porphyrin molecule with iron at its center. Protoporphyrin IX consists of 4 pyrrole rings to which 4 methyl, 2 propionyl and 2 vinyl groups are attached.The red color of haemoglobin and of blood is due to this heme group.
The main function of haemoglobin is in transport of oxygen to tissues and of CO2 to lungs. The structure of haemoglobin is unique and adapted to this function. One molecule of haemoglobin with 4 heme groups can bind to 4 molecules of O2. When a single haem group in haemoglobin gets oxygenated, it alters the whole molecule of haemoglobin in such a way that other 3 heme groups also pick up one molecule of O2 each. Carrying of 4 molecules of oxygen by each molecule of haemoglobin makes it a very efficient oxygen carrier.
Haemoglobins' "cooperative binding" property wherein attachment of one oxygen molecule alters the whole structure is explained here briefly. When the iron atom moves in the porphyrin plane during oxygenation, the histidine residue to which iron is attached is drawn closer to heme group. This movement of histidine residue causes a shift in relative positions of nearby amino acids causing alteration in the structure of the interfaces between the 4 subunits. This alteration causes the whole protein to change its shape making it easier for the iron of the remaining 3 heme groups to pick up one oxygen molecule each
In the lungs, the concentration of oxygen is high , haemoglobin gets fully saturated with oxygen and carries it off to all the body organs and tissues. At tissue level, the concentration of CO2 is high, causing haemoglobin to offload its oxygen and pick up Carbon dioxide. To carry out this work, the affinity of haemoglobin to CO2 is higher that affinity for oxygen. The oxygen it gives off to the tissue is picked up and stored by myoglobin which has higher affinity for oxygen.