Enzyme Cofactors


Some enzymes consist only of polypeptide chains.

for example: pepsin and trypsin


Other enzymes contain nonprotein portions.

If they are tightly bound, they are prosthetic groups.

If they are only weakly bound and easily separated from the protein,

they are cofactors.


prosthetic group the non-amino acid part of a conjugated protein.


cofactor - a nonprotein molecule or ion required by an enzyme for catalytic


- can be an organic molecule or metal ion,

such as Mg2+, Zn2+, Fe2+, Ca2+


coenzyme - an organic cofactor


apoenzyme - a catalytically inactive protein

(formed by removal of the cofactor from the active enzyme).

- activated by a cofactor


apoenzyme + cofactor(coenzyme or inorganic ion) --> active enzyme


An important group of coenzymes are vitamins.

For example, vitamin B12 acts as coenzyme B12

in the shift of H atoms between adjacent carbon atoms

and in the transfer of methyl groups.

Another example,

niacin acts as nicotinamide adenine dinucleotide (NAD+)

in hydrogen transfer.


Another important group of cofactors are minerals in our diet.

For example, the coenzyme Zn2+ is required

for the enzyme carbonic anhydrase to function.



When a metal ion is a cofactor, it can be bound directly to the protein or the coenzyme, if the enzyme contains one.