Final answer:
A coenzyme is the nonprotein organic part of an enzyme's structure, often derived from vitamins and necessary for the enzyme's function, forming a holoenzyme when combined with an apoenzyme.
Step-by-step explanation:
The nonprotein organic part of an enzyme structure that often includes a vitamin as a subpart is called a coenzyme. Coenzymes are organic molecules that act as cofactors, which are necessary for the proper functioning of an enzyme. They may be derived from vitamins or directly act as vitamins in their coenzyme form. An enzyme without its cofactor is referred to as an apoenzyme, and once it is combined with its cofactor it becomes a holoenzyme, which is the active form. Some examples of coenzymes include the vitamins B, which serve as coenzymes in many metabolic reactions, and vitamin C, which is a coenzyme for enzymes involved in synthesizing collagen.