What is coenzyme?

A coenzyme is an organic non-protein compound that binds with an enzyme to catalyze a chemical reaction. Coenzymes are not active on their own. They only function when bound to an enzyme. 

Coenzymes play an integral role in the functioning of cells. A coenzyme binds to the active site of an apoenzyme (an inactive protein molecule) to form a complete, active enzyme known as holoenzyme. When the coenzyme binds to the enzyme it serves as an intermediate carrier, transporting electrons during the reaction catalyzed by that enzyme. This may help to trigger the reaction or accelerate the rate of the reaction. Some enzymes require multiple coenzymes and cofactors to function efficiently. 

A coenzyme is an independent molecule and is not part of an enzyme's structure. It can be reused and recycled without any change in reaction rate or effectiveness. Most coenzymes are vitamins or derivatives of vitamins. Example of a vitamin coenzymes - B vitamins serve as coenzymes essential for enzymes to form proteins, carbohydrates, and fats. There are very few non-vitamin coenzymes, e.g. S-adenosylmethionine.