When O₂ binds to a heme group, the two bonds of Fe²⁺ that are not planar with the heme are occupied by:

• A. One O₂ molecule and one Ser amino acid atom.
• B. One O₂ molecule and one His amino acid atom.
• C. One O₂ molecule and a nitrogen atom of the heme.
• D. Two O₂ molecules.

Answer: (B)

Verified step by step guidance

1. Understand the structure of the heme group: The heme group consists of a porphyrin ring with an iron (Fe2+) atom at its center. The iron atom can form six coordination bonds.
2. Identify the planar bonds: Four of these bonds are planar and are formed with the nitrogen atoms of the porphyrin ring.
3. Determine the non-planar bonds: The remaining two bonds are perpendicular to the plane of the heme. These are the axial positions.
4. Recognize the role of oxygen: One of these axial positions is typically occupied by an oxygen (O2) molecule when the heme is in its oxygenated form.
5. Identify the amino acid interaction: The other axial position is usually occupied by a histidine (His) residue from the protein structure, specifically the proximal histidine, which helps stabilize the iron-oxygen complex.