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Cofactors are non-protein molecules required by enzymes to catalyze specific biochemical reactions.

Coenzymes are organic molecules that are often loosely bound to the active site of an enzyme, for example, ATP. In contrast, coenzymes that are transiently associated with the enzyme molecules are referred to as cosubstrates, for example, NAD⁺.

However, prosthetic groups are those cofactors that are permanently attached to the enzymes through covalent bonds, for example, the heme prosthetic group in cytochromes.

Cofactors can be broadly classified into metal ions and coenzymes (organic cofactor). Further coenzymes are classified into cosubstrates and prosthetic groups depending upon the type of bond with which they are associated with the enzyme.