Cofactors are important accessories to biochemical processes. Generally present as small organic compounds or metal ions, cofactors empower enzymes to functional at maximal catalytic effectiveness or endurance. A related term, coenzymes, relates to a subgroup of cofactors whose structure in part is derived from water-soluble B vitamins. Historically, cofactors were often inadvertently removed during purification and had to be added back to restore enzyme activity. Today, we regard a cofactor as an obligatory component of the catalytic mechanism. Compounds meeting the criteria are either: (1) small organic molecules that bind directly to the enzyme surface forming an active site for the substrate to bind or interact, or assist in these events indirectly, or (2) inorganic ions that bind to specific groups on an enzyme surface and aid in substrate binding, catalysis, stabilizing the transition state, or contributing to the overall stability of the enzyme's structure. Practically speaking, any substance in an assay medium that promotes the catalytic activity or stability of an enzyme is a candidate for its cofactor.
As will be illustrated in this and was in the last article, cofactors are indispensable adducts of the catalytic machinery of the body and have provided nutritionists with the strongest insights into the essential role of vitamins and trace elements. It is still fashionable to consider coenzymes as vitamin derivatives that bind loosely to enzymes or serve as transient active sites. Cofactors and coenzymes are terms that are used interchangeably. It is important to note, however, that the prefix 'holo' is used to refer to an enzyme and its coenzyme together as a catalytic unit and 'apo' when the coenzyme is missing. Apoenzymes are functionless and are of no benefit to the organism.
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