The metal center in the active site of the metalloenzyme myoglobin is an iron Fe atom, which is coordinated by a heme group. The specific coordination geometry of the iron atom in myoglobin is a distorted octahedral geometry. In this geometry, the iron atom is coordinated by six ligands. Four of these ligands are nitrogen atoms from the porphyrin ring of the heme group, which form a square planar arrangement around the iron atom. The fifth ligand is a histidine residue imidazole nitrogen from the protein chain, which coordinates the iron from the proximal side. The sixth coordination site is on the distal side of the heme and is available for binding to small molecules such as oxygen O2 , carbon monoxide CO , or nitric oxide NO .The significance of this coordination geometry in the enzyme's function is that it allows for the reversible binding of oxygen to the iron atom. The distorted octahedral geometry enables the iron atom to switch between different oxidation states Fe2+ and Fe3+ and accommodate the binding and release of oxygen molecules. This is crucial for myoglobin's role as an oxygen storage and transport protein in muscle tissues. The geometry also helps to discriminate between O2 and other small molecules like CO, ensuring that myoglobin preferentially binds to O2 and fulfills its biological function.