What part of myoglobin is involved in binding iron's fifth bond?

Myoglobin is a globular protein that plays a crucial role in oxygen storage in muscle cells. It contains a heme group, which is responsible for binding oxygen. This heme complex is where the iron atom is located, and the coordination of the iron atom is essential for its functionality.

The iron atom in the heme can form six coordination bonds. Four of these bonds are occupied by the nitrogen atoms of the porphyrin ring of the heme. The fifth bond is formed with the proximal histidine, which is a specific imidazole side chain from a histidine residue located at F8 position (the eighth residue of the helical F segment). This histidine is directly bonded to the iron atom, helping to stabilize its position in the heme structure.

The sixth bond is available for the binding of oxygen, allowing myoglobin to pick up oxygen in the lungs and release it in tissues. The role of the proximal histidine is essential because it influences the electronic properties of the iron and affects how tightly myoglobin binds to oxygen.

Thus, the proximal histidine is integral to the functionality of myoglobin, specifically in the formation of the iron's fifth bond, which is why this choice is correct.