Which histidine on myoglobin helps decrease its affinity for carbon monoxide?

The correct choice indicates that the distal histidine plays a crucial role in decreasing myoglobin's affinity for carbon monoxide. In myoglobin, the iron atom in the heme group can bind to oxygen and carbon monoxide, but carbon monoxide has a much higher affinity for hemoglobin and myoglobin than oxygen does. The interaction with carbon monoxide can be influenced by the presence of surrounding amino acid residues.

The distal histidine, which is located on the opposite side of the heme iron from the proximal histidine (the one that directly coordinates with the iron), plays a protective role. It is positioned to interact with the bound ligand. Notably, the distal histidine can form hydrogen bonds with carbon monoxide, stabilizing the molecule in a way that reduces the binding affinity compared to oxygen. This stabilization encourages the release of carbon monoxide, allowing for a preferential binding of oxygen, which is crucial for the protein's main function of oxygen transport and storage in muscle tissues.

This understanding relates to the overall biochemical properties of myoglobin and its interaction with gases like oxygen and carbon monoxide.