Explanation:

- Enzymes are molecules that act as catalysts within living cells.

- ? Proteins: Enzymes are primarily proteins, which are made up of long chains of amino acids. They speed up chemical reactions necessary for life.

- Vitamins: These are organic compounds that are essential for normal growth and nutrition. They do not act as catalysts.

- Carbohydrates: These are sugars and starches used as energy sources. They do not function as enzymes.

- Calcium: An essential mineral crucial for bone health, but not a catalyst.