Deoxyhemoglobin coordination: which ligands bind Fe(II) in the heme pocket of deoxy Hb?

Introduction:
Hemoglobin’s function depends on how the Fe(II) center is coordinated in the heme pocket. In deoxyhemoglobin, Fe(II) is high-spin and five-coordinate; in oxyhemoglobin, binding O2 gives a sixth ligand and low-spin character.

Given Data / Assumptions:

• We are considering the deoxygenated state (deoxy Hb).
• Heme contributes four in-plane nitrogen ligands.
• One histidine (proximal His F8) binds axially to Fe(II).

Concept / Approach:

Count coordination number and identify axial ligands. In deoxy Hb, there is no bound O2 or water at the sixth site; the distal His does not coordinate directly but stabilizes ligands via H-bonding when present.

Step-by-Step Solution:

Verification / Alternative check:

Spectral and structural data show a five-coordinate, high-spin Fe(II) in deoxy Hb and six-coordinate, low-spin Fe(II) in oxy Hb.

Why Other Options Are Wrong:

Options with water or multiple histidines misrepresent deoxy coordination. A water ligand occurs in some met or aquo states, not in deoxy Hb.

Common Pitfalls:

Confusing distal His (non-bonding in deoxy) with a coordinating ligand; assuming O2/water present in the deoxygenated state.

Final Answer:

Four nitrogens of heme and the proximal histidine (five-coordinate)