Biochemistry—Heme Protein Oxygen Binding In hemoglobin and myoglobin, oxygen binds directly to which site?

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:

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+).