How is the heme group held in the globin pocket? Identify the key bond that anchors the porphyrin to the protein.
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AFe2+ to cysteine (thiolate) coordination
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BFe3+ to histidine coordination
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CFe3+ to cysteine coordination
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DFe2+ to histidine (proximal His) coordination
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ECovalent thioether linkages to porphyrin (as in c-type cytochromes)
Answer
Correct Answer: Fe2+ to histidine (proximal His) coordination
Explanation
Introduction:Heme proteins position the porphyrin cofactor precisely to enable reversible ligand binding. In globins, a specific axial coordination anchors heme within the pocket.
Given Data / Assumptions:
- Globin family (myoglobin/hemoglobin), not c-type cytochromes.
- Iron is Fe2+ in the physiologically active state.
Concept / Approach:
The proximal histidine (His F8) coordinates Fe2+ axially, anchoring heme and transmitting conformational changes upon ligand binding. The distal histidine modulates ligand binding through hydrogen bonding, not direct coordination in deoxy state.
Step-by-Step Solution:
1) Identify axial ligands in globins: proximal His binds Fe2+ directly.2) Distinguish from cytochrome c, where covalent thioether bonds to cysteines anchor heme.3) Conclude Fe2+–His as the anchoring bond.Verification / Alternative check:
Structural data show Fe–N(His) bond length consistent with axial coordination in globins.
Why Other Options Are Wrong:
Fe3+ states reflect met forms; cysteine ligation and thioether attachments are characteristic of other heme proteins, not globins.
Common Pitfalls:
Confusing globins with c-type cytochromes or P450 enzymes.
Final Answer:
Fe2+ to histidine (proximal His) coordination