Question

Among the following, the biomolecule with a direct metal-carbon bond is

• A. coenzyme $B_{12}$
• B. nitrogenase
• C. chlorophyll
• D. hemoglobin

Answer 1

The Correct Option is A

$$\text{The biomolecule with a direct metal-carbon bond is } \mathbf{\text{Coenzyme B12}}$$

$\text{Explanation}$

$\text{1. Definition of Organometallic}$

A compound is classified as organometallic if it contains at least one direct bond between a metal atom and a carbon atom of an organic group.

$\text{2. Structure of Coenzyme B12}$

Coenzyme $\text{B}_{12}$ (specifically, adenosylcobalamin or $\text{AdoCbl}$) is the biologically active form of Vitamin $\text{B}_{12}$ (cobalamin).

The molecule contains a central cobalt ($\text{Co}$) atom coordinated by a complex macrocyclic ligand called the corrin ring.

The key feature that makes Coenzyme $\text{B}_{12}$ a unique and historically important biomolecule is the bond between the central cobalt atom and an organic group.

$$\text{Coenzyme B12: } \text{Cobalt} \mathbf{-} \text{Carbon Bond}$$

In $\text{AdoCbl}$, the cobalt atom is directly bonded to the $\text{C}5'$ carbon atom of the $5'$-deoxyadenosyl group ($\text{Adenosyl-C}5'$). This is a stable $\sigma$-covalent Cobalt-Carbon ($\text{Co}-\text{C}$) bond.

The low dissociation energy of this $\text{Co}-\text{C}$ bond (which is easily broken in enzymatic reactions) is critical for its biological function in 1,2-rearrangement and methyl transfer reactions.

$\text{3. Other Biomolecules (General Comparison)}$

In contrast to Coenzyme $\text{B}_{12}$, the metal centers in many other common metallo-biomolecules are typically coordinated to non-carbon atoms:

Chlorophyll (containing $\text{Mg}$): $\text{Magnesium}$ ($\text{Mg}$) is coordinated to the nitrogen ($\text{N}$) atoms of the porphyrin-like ring.

Hemoglobin/Myoglobin (containing $\text{Fe}$): $\text{Iron}$ ($\text{Fe}$) is coordinated to the nitrogen ($\text{N}$) atoms of the heme porphyrin ring and a nitrogen atom of a histidine residue.

Nitrogenase (containing $\text{Fe}$ and $\text{Mo}$): The metals are coordinated primarily by sulfur ($\text{S}$) atoms and nitrogen atoms.

Coenzyme $\text{B}_{12}$ was the first naturally occurring biomolecule discovered to possess this direct, stable $\text{metal}-\text{carbon}$ bond, making it the classic example of a bioorganometallic compound.