The coordination environment of the metal ion in the active site of the metalloenzyme myoglobin involves a heme group, which contains an iron Fe atom at its center. Myoglobin is a monomeric protein found in muscle tissues that plays a crucial role in oxygen binding and storage.The iron atom in the heme group has a coordination number of six, which means it can form six bonds with surrounding ligands. Five of these coordination sites are occupied by nitrogen atoms from the porphyrin ring system of the heme group, forming a planar structure. The sixth coordination site is where the oxygen binding occurs.In the resting state deoxy-myoglobin , the sixth coordination site is occupied by a histidine residue called the proximal histidine from the protein chain. This histidine residue is connected to the iron atom via a coordination bond, keeping the iron in a slightly out-of-plane position high-spin state .When oxygen binds to myoglobin, it forms a coordination bond with the iron atom at the sixth coordination site. This binding causes the iron atom to move into the plane of the porphyrin ring system low-spin state , which in turn leads to a conformational change in the protein structure. This change weakens the bond between the iron and the proximal histidine, allowing for more efficient oxygen binding.The coordination environment of the iron atom in myoglobin is crucial for its function in oxygen binding and storage. The heme group provides a suitable environment for reversible oxygen binding, while the histidine residue ensures that the iron atom remains in the appropriate oxidation state Fe2+ for oxygen binding. The conformational change upon oxygen binding allows for efficient oxygen uptake and release, making myoglobin an effective oxygen storage protein in muscle tissues.