When a water-soluble vitamin assists in the chemical reaction involved in the metabolism of the macronutrients, this is known as its function.

The function you're referring to is known as a "coenzyme" function. Here's a step-by-step explanation:

1. Water-soluble vitamins are a type of vitamin that can dissolve in water. They are carried to the body's tissues but are not stored in the body, so they need to be taken in daily through diet.

2. These vitamins include vitamin C and the B-vitamins (thiamin, riboflavin, niacin, pantothenic acid, biotin, vitamin B6, vitamin B12, and folate).

3. Many of these vitamins act as coenzymes. A coenzyme is a substance that enhances or is necessary for the action of enzymes. They assist in the chemical reactions in the body.

4. In the context of macronutrient metabolism, these vitamins (as coenzymes) help to break down carbohydrates, fats, and proteins for energy. They are involved in the conversion of these macronutrients into usable forms.

5. Therefore, when a water-soluble vitamin assists in the chemical reaction involved in the metabolism of the macronutrients, this is known as its coenzyme function.