Which of the following substances can act as a cofactor in enzymatic reactions?

Cofactors are essential for the proper functioning of many enzymes. They can be either organic molecules or inorganic ions that assist in catalyzing biochemical reactions. In this context, metal ions serve as important cofactor examples, as they help stabilize enzyme structures and participate directly in the chemical reactions.

Metal ions, such as zinc, magnesium, iron, and copper, can help enzymes by providing necessary charge or by enabling substrate binding and transformation. They often play critical roles in facilitating various biochemical processes, including electron transfer, stabilization of negative charges, and structural stabilization of enzymes.

The other options, while they are vital components of biological systems, do not act as cofactors. Proteins serve as enzymes themselves or provide structural functions but are not cofactors. Carbohydrates and fats serve primarily as energy sources or structural components in cells, not as cofactors for enzymatic reactions. Therefore, metal ions are clearly identified as cofactor substances crucial for enzyme activity.