Which atom is most likely involved in the coordination of calcium ions found in HRP?

• A. Hydrogen
• B. Carbon
• C. Nitrogen
• D. Oxygen

Answer: (D)

Calcium ions are typically coordinated by electronegative atoms that can donate lone pairs of electrons to form coordinate bonds. Among the choices, oxygen is most likely to be involved in the coordination of calcium ions. This is because oxygen possesses two lone pairs of electrons and has a relatively high electronegativity, allowing it to effectively interact with and stabilize positively charged calcium ions.

In many biological systems, including proteins like horseradish peroxidase (HRP), carboxylate groups and hydroxyl groups that contain oxygen can form stable complexes with metal ions. Oxygen can act as a Lewis base, donating electron pairs to the Lewis acid, which in this case is the calcium ion. This interaction is essential for the function of various enzymes and proteins, facilitating roles such as structural support and catalysis.

The other atoms listed—hydrogen, carbon, and nitrogen—either lack the necessary electronegativity or the proper geometry for effective coordination with calcium ions. Thus, oxygen is the atom that plays the most vital role in the coordination of calcium ions in the context of HRP.