Deoxyhemoglobin coordination: which ligands bind Fe(II) in the heme pocket of deoxy Hb?
-
AFour nitrogens of heme, the proximal histidine, and a water molecule
-
BFour nitrogens of heme and a water molecule (no histidine)
-
CTwo nitrogens of heme and three histidines
-
DTwo nitrogens of heme and three water molecules
-
EFour nitrogens of heme and the proximal histidine (five-coordinate)
Answer
Correct Answer: Four nitrogens of heme and the proximal histidine (five-coordinate)
Explanation
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:
1) Porphyrin macrocycle provides 4 N donors.2) Proximal His provides the 5th coordination to Fe(II).3) Sixth site is vacant in deoxy Hb; upon oxygenation, O2 occupies it as the sixth ligand.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)