The coordination chemistry of iron in hemoglobin plays a crucial role in oxygen transport and binding in the human body. Hemoglobin is a protein found in red blood cells, and it contains four heme groups, each with an iron Fe atom at its center. The iron atom is coordinated to a nitrogen atom from a histidine residue in the protein and a nitrogen atom from a porphyrin ring in the heme group. This coordination allows the iron atom to bind to an oxygen molecule reversibly.When hemoglobin is in the deoxygenated state also known as T-state or tense state , the iron atom is in a high spin state and slightly out of the plane of the porphyrin ring. In this state, the iron atom cannot bind to oxygen effectively. However, when an oxygen molecule approaches the iron atom, the iron atom changes its spin state to a low spin state, which allows it to bind to the oxygen molecule. This binding causes the iron atom to move into the plane of the porphyrin ring, leading to a conformational change in the hemoglobin protein.This conformational change shifts the hemoglobin from the T-state to the R-state relaxed state , which has a higher affinity for oxygen. The binding of one oxygen molecule to a heme group increases the affinity of the other heme groups for oxygen, a phenomenon known as cooperative binding. This allows hemoglobin to pick up oxygen more efficiently in the oxygen-rich environment of the lungs.As hemoglobin travels through the body and reaches oxygen-poor tissues, the concentration of oxygen decreases, causing the hemoglobin to release its bound oxygen molecules. This release is facilitated by the presence of carbon dioxide and protons H+ , which bind to specific sites on the hemoglobin protein, stabilizing the T-state and promoting the release of oxygen. This process is known as the Bohr effect.In summary, the coordination chemistry of iron in hemoglobin enables the reversible binding of oxygen, allowing efficient oxygen transport and delivery to tissues throughout the human body. The iron atom's ability to change its spin state and the conformational changes in the hemoglobin protein upon oxygen binding and release are essential for this process.