How is the heme group held in the globin pocket? Identify the key bond that anchors the porphyrin to the protein.

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:

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