The metal ion in the active site of the metalloprotein myoglobin is iron Fe . The coordination environment of the iron ion is a porphyrin ring, which is a large organic molecule with a planar structure. The iron ion is coordinated to four nitrogen atoms from the porphyrin ring in a square planar geometry. Additionally, the iron ion is coordinated to a histidine residue from the protein chain, making the overall coordination number of the iron ion 5. The oxidation state of the iron in myoglobin is +2 Fe2+ .This coordination environment allows myoglobin to efficiently bind and transport oxygen in the body because the Fe2+ ion can form a reversible coordinate covalent bond with an oxygen molecule O2 . When an oxygen molecule approaches the iron ion, it binds to the sixth coordination site, forming a bent geometry. This binding causes the iron ion to move slightly into the plane of the porphyrin ring, which in turn triggers a conformational change in the protein structure. This change in structure stabilizes the binding of oxygen and allows myoglobin to effectively store and transport oxygen in muscle tissues.The reversible nature of the coordinate covalent bond between the iron ion and the oxygen molecule allows myoglobin to easily release the bound oxygen when needed, such as during periods of increased oxygen demand in the muscles. This efficient binding and release mechanism is crucial for the proper functioning of myoglobin in the body.