The metal ion in the active site of hemoglobin is iron Fe , which is part of the heme group. The coordination number of the iron ion in hemoglobin is 6, and its geometry is octahedral.In the heme group, the iron ion is coordinated to four nitrogen atoms from the porphyrin ring, which forms a planar square. The fifth coordination site is occupied by a nitrogen atom from a histidine residue of the protein proximal histidine . The sixth coordination site is available for binding to external ligands, such as oxygen O2 or carbon monoxide CO .The coordination of the heme group with the iron ion plays a crucial role in the protein's function, which is to transport oxygen from the lungs to the tissues and return carbon dioxide from the tissues to the lungs. When an oxygen molecule binds to the sixth coordination site of the iron ion, it causes a conformational change in the protein. This change increases the affinity of the other heme groups in the hemoglobin molecule for oxygen, a phenomenon known as cooperativity. As a result, hemoglobin can efficiently pick up oxygen in the oxygen-rich environment of the lungs and release it in the oxygen-poor environment of the tissues.The coordination of the heme group with the iron ion also helps protect the iron from being oxidized, which would render it unable to bind oxygen. Additionally, the coordination of the iron ion with the histidine residue prevents the binding of carbon monoxide CO to the iron ion in a linear fashion, which would inhibit the protein's ability to transport oxygen. Instead, CO binds at an angle, allowing for some oxygen transport, although it still has a higher affinity for the iron ion than oxygen, making CO poisoning dangerous.