Originally the term coenzyme was used to designate any substance like Ca, Co, Cu, Na, K and Zn ions whose presence in small amount is responsible for the action of a given enzyme.
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These simple substances are not the coenzymes of modern Enzymology but they are metalloenzymes.
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Bersin simply called them “complements”. In some enzymes the metal ions may be tightly bound to the proteins while in others they may be quite easily dissociated.
Many enzymes, but not all, possess in addition to their protein moiety, a non-proteinaceous organic component of relatively smaller molecular weight (about 500) called coenzyme.
It is essential for enzymatic activity. The protein moiety constitutes the apoenzyme. The apoenzyme and coenzyme are collectively known as holoenzyme.
The coenzymes generally act- as acceptor or donors of hydrogen atoms or other functional groups that are removed from or contributed to the substrate.
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Since these organic cofactors are frequently readily dissociable from the enzyme protein, they may properly be regarded as cosubstrates.
A specific coenzyme is associated with each enzyme catalyzed reaction as a nonprotein prosthetic group. Alone, coenzymes cannot perform any catalytic function, nor is the enzyme alone active.
Thus, though the “coenzyme is involved in the actual transfer of a functional group or a hydrogen atom, the protein component of enzyme is required for catalytic activity.
The enzyme determines the proper alignment of all the reactants and thereby orients the coenzyme for specific reaction.
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Coenzymes are present in all cells in minute amounts and like enzymes, they function over and over again. Hydrogen atoms picked up in one reaction are passed on to another molecule in another reaction, generating the coenzyme. Plants are capable to synthesize the required coenzymes while the higher animal cannot.
Analysis of various coenzymes has shown that they all either are vitamins or contain vitamins as component of their structure.