Which of the following is a common inorganic cofactor for enzymes?

• A. Biotin
• B. Magnesium
• C. NAD+
• D. FAD

Answer: (B)

Cofactors come in two broad types: inorganic metal ions and organic molecules that act as enzymes’ helpers. Magnesium is a standout as a common inorganic cofactor because many enzymes rely on a metal ion to assist catalysis and substrate binding. It frequently forms a complex with ATP (Mg-ATP), which helps stabilize the negative charges on the phosphate groups and properly position them for transfer in energy- and phosphorylation-requiring reactions. That role is widespread across kinases and other ATP-using enzymes, making magnesium a go-to inorganic cofactor.

Biotin is a vitamin-derived coenzyme involved in carboxylation reactions, so it’s organic. NAD+ and FAD are also organic cofactors that shuttle electrons in redox reactions. They’re not inorganic metal ions, which is why they don’t fit the “inorganic cofactor” category.