Hemoglobin is a metalloprotein that contains a central metal ion, iron Fe , in its heme group. The coordination number of the central metal ion in hemoglobin is 6.In the heme group, the iron ion is coordinated to four nitrogen atoms from the porphyrin ring, forming a planar square structure. This accounts for four of the six coordination sites. The fifth coordination site is occupied by a nitrogen atom from an imidazole group of a histidine residue in the protein called the proximal histidine . This histidine residue is part of the globin protein chain and helps to anchor the heme group within the protein structure.The sixth coordination site is the one that plays a crucial role in the biological function of hemoglobin. This site is available for binding to external ligands, such as oxygen O2 , carbon monoxide CO , or other small molecules. When an oxygen molecule binds to the iron ion at this sixth site, the hemoglobin is in its oxygenated form oxyhemoglobin , which is the form that transports oxygen from the lungs to the tissues. When the oxygen molecule is released, the hemoglobin returns to its deoxygenated form deoxyhemoglobin , which is the form that transports carbon dioxide from the tissues back to the lungs.The coordination of the central metal ion in hemoglobin is essential for its biological function as an oxygen carrier. The reversible binding of oxygen at the sixth coordination site allows hemoglobin to pick up oxygen in the lungs and release it in the tissues, thus facilitating oxygen transport throughout the body.