Biochemistry—Heme Protein Oxygen Binding In hemoglobin and myoglobin, oxygen binds directly to which site?
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AThe iron atom of the heme group (Fe2+).
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BThe nitrogen atoms on the porphyrin ring instead of iron.
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CA histidine side chain in the globin protein.
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DA lysine side chain in the globin protein.
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EA calcium ion coordinated near the heme pocket.
Answer
Correct Answer: The iron atom of the heme group (Fe2+).
Explanation
Introduction / Context:Hemoglobin and myoglobin bind oxygen via a heme prosthetic group. Understanding exactly where O2 coordinates is essential for grasping oxygen transport and storage mechanisms.
Given Data / Assumptions:
- Heme consists of protoporphyrin IX with a central Fe2+ ion.
- Globin provides a pocket with proximal and distal histidines that position and stabilize the heme and bound O2.
- Oxygen binding involves direct coordination to iron.
Concept / Approach:Identify the direct ligand of O2. Although histidines are crucial (proximal His binds Fe2+; distal His stabilizes O2 via H-bonding), O2 coordinates the iron atom itself, not the nitrogen atoms of the porphyrin or amino acid side chains.
Step-by-Step Solution:
Step 1: Recall heme chemistry: Fe2+ has coordination sites within the porphyrin.Step 2: Recognize that O2 binding occurs at the sixth coordination position on Fe2+.Step 3: Therefore the correct binding partner for O2 is the iron atom.Verification / Alternative check:Spectroscopic and structural studies confirm reversible O2 binding to Fe2+ with modulation by the distal histidine environment.
Why Other Options Are Wrong:
- Nitrogen atoms on the porphyrin already coordinate Fe2+; O2 does not replace them.
- Histidine or lysine side chains do not directly bind O2 (proximal His binds Fe2+, not O2).
- Calcium is not part of the heme O2-binding chemistry.
Common Pitfalls:Confusing the role of proximal/distal histidines with the direct O2 ligand; thinking O2 binds the porphyrin ring nitrogens.
Final Answer:The iron atom of the heme group (Fe2+).