Cofactor identity — Which metal ion is specifically coordinated at the center of vitamin B12 (cobalamin)?

Introduction / Context:
Vitamin B12 (cobalamin) is defined structurally by a corrin ring that coordinates a central metal ion. Identifying this metal explains both its name and its unique redox chemistry in enzymatic reactions.

Given Data / Assumptions:

• The corrin macrocycle binds a single transition metal ion.
• Cobalamin participates in redox and radical reactions.
• Naming often reflects the metal (e.g., cobalamin, hemoglobin's heme iron).

Concept / Approach:
The center of cobalamin contains cobalt in variable oxidation states (Co(III), Co(II), Co(I)). The cobalt–carbon bond in adenosylcobalamin is among the few biologically relevant organometallic bonds, enabling rearrangements and methyl transfers.

Step-by-Step Solution:

Verification / Alternative check:
Spectroscopic and crystallographic data show cobalt at the core with axial ligands (5,6-dimethylbenzimidazole below and variable upper ligands above).

Why Other Options Are Wrong:

• Copper, zinc, iron are key in other metalloproteins but not in the corrin ring of B12.

Common Pitfalls:
Confusing B12 with heme (iron) or zinc fingers; the defining metal of cobalamin is cobalt.

Final Answer:
Cobalt