An organic molecule that acts as an enzyme *cofactor
. Coenzymes usually function as intermediate carrier molecules, transferring or removing functional groups, atoms, or electrons. The coenzyme may be tightly bound to the enzyme, in which case it is known as a *prosthetic group
, or it may be only loosely bound and act like a second substrate of the enzyme. Examples of important coenzymes are NAD and NADP, coenzyme A, and biotin.