The coordination chemistry of iron in the active site of hemoglobin plays a crucial role in the transport of oxygen in the bloodstream. Hemoglobin is a metalloprotein that contains heme groups, which are responsible for binding and releasing oxygen. Each heme group consists of an iron Fe atom coordinated to a porphyrin ring. The iron atom can form up to six coordination bonds, four of which are occupied by the nitrogen atoms of the porphyrin ring, one by a histidine residue from the protein, and the last one is available for binding to oxygen.In the deoxygenated state also known as the T-state or tense state , the iron atom is in the Fe II oxidation state and is slightly out of the plane of the porphyrin ring. In this state, the iron atom has a lower affinity for oxygen. When an oxygen molecule approaches the heme group, it binds to the available coordination site on the iron atom, forming a bond with the Fe II ion. This binding causes the iron atom to move into the plane of the porphyrin ring, which in turn triggers a conformational change in the protein structure.This conformational change is transmitted through the protein, causing a shift from the T-state to the R-state relaxed state , which has a higher affinity for oxygen. This cooperative binding mechanism allows hemoglobin to pick up oxygen more efficiently in the oxygen-rich environment of the lungs and release it more readily in the oxygen-poor environment of the tissues.The coordination chemistry of iron in hemoglobin also plays a role in preventing the oxidation of the Fe II ion to Fe III , which would render the heme group unable to bind oxygen. The histidine residue coordinated to the iron atom helps stabilize the Fe II state and prevent its oxidation.In summary, the coordination chemistry of iron in the active site of hemoglobin allows for the efficient transport of oxygen in the bloodstream by facilitating the reversible binding of oxygen to the heme group and enabling cooperative binding through conformational changes in the protein structure.