Coenzyme B12 (adenosylcobalamin) structure — in coenzyme B12, the position occupied by cyanide in vitamin B12 (cyanocobalamin) is bonded directly to which atom/group of adenosine?

Introduction / Context:
Cobalamins have a cobalt center with two axial ligands. In cyanocobalamin (vitamin B12), the upper ligand is cyanide. In coenzyme B12 (adenosylcobalamin), this position is replaced by a unique covalent Co–C bond to a 5′-deoxyadenosyl group, a hallmark of radical B12 enzymes.

Given Data / Assumptions:

• Lower axial ligand is typically 5,6-dimethylbenzimidazole (via nitrogen).
• Upper axial ligand varies: CN− (cyanocobalamin), OH− (hydroxycobalamin), CH3− (methylcobalamin), or 5′-deoxyadenosyl (coenzyme B12).

Concept / Approach:
Identify the precise atom on adenosine that binds cobalt: the 5′ carbon of the ribose (C5′) in 5′-deoxyadenosyl forms a direct Co–C sigma bond, enabling homolytic cleavage in enzyme mechanisms.

Step-by-Step Solution:
Note the ligand replacement: CN− (B12) → 5′-deoxyadenosyl (coenzyme B12).Determine the bonding atom: the cobalt–carbon bond is to C5′ of the ribose.Exclude adenine nitrogens and ribose C1′ (glycosidic carbon) as binding sites.Select the option specifying “5′-carbon of ribose.”

Verification / Alternative check:
Structural studies (X-ray, spectroscopy) show the distinctive Co–C bond to the 5′-deoxyadenosyl moiety in AdoCbl.

Why Other Options Are Wrong:
N-7 of adenine and C-1′ are not cobalt ligation sites; the benzimidazole is the lower ligand, not the upper; phosphate is not the binding atom.

Common Pitfalls:
Confusing the lower axial base (5,6-dimethylbenzimidazole) with the upper ligand; forgetting the bond is to C5′ of deoxyadenosyl.

Final Answer:
5′-carbon of the ribose of adenosine (C5′).