Complex sentence: A "proximal histidine" group is attach directly to iron, and a "distal histidine" group on the opposite face, not bonded to the iron interacts with the substrate O 2.
Split into simpler sentences: A "proximal histidine" group is attach directly to iron, and a "distal histidine" group hovers near the opposite face. The distal imidazole is not bonded to the iron but is available to interacts with the substrate O 2.