You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "where is oxygen attached on a hemoglobin molecule?" whose answer is The iron ion.
Each document should contain about 30 words, and these documents should contain useful information.

Query: structure of hemoglobin and oxygen binding site
Useful Output:

Doc 1: Hemoglobin contains a five-coordinate Fe(II) centre in a heme or porphyrin ligand. In the picture, only the coordination complex is shown,
Doc 2: The quaternary structure of hemoglobin accounts for the different behavior with respect to another oxygen-binding protein, myoglobin, a
Doc 3: Reversible Binding of Oxygen. The iron atom in heme binds to the 4 nitrogen atoms in the centre of the porphyrin ring, but this leaves two free bonding sites
Doc 4: Each hemoglobin molecule is made up of four heme groups surrounding a globin group, forming a tetrahedral structure. Heme, which accounts for
Doc 5: Here we report the cryo-electron microscopy structure of Lumbricus terrestris Hb in its native, oxygenated state at 9.1 Å resolution.