What is coenzyme

Coenzymes are a general term for a large class of organic cofactors, which are essential factors for enzyme catalyzed redox reactions, group transfer, and isomerization reactions. They play a role in transferring electrons, atoms, or functional groups in enzyme catalyzed reactions. Coenzymes can also be considered as secondary substrates, as the chemical changes that occur in coenzymes during catalytic reactions are exactly opposite to those of substrates. 
Coenzymes are a type of organic small molecule that can transfer chemical groups from one enzyme to another, loosely bound to enzymes, and are necessary for the activity of specific enzymes. There are many vitamins and their derivatives, such as thiamine and folate, which belong to coenzymes. These compounds cannot be synthesized by the human body and must be supplemented through diet. Different coenzymes can carry different chemical groups: NAD+or NADP+carries reducing hydrogen, coenzyme A carries acetyl, folate carries formyl, and S-adenosylmethionine can also carry formyl.