Cofactors, Coenzymes, and Prosthetic Groups

The terms cofactor, coenzymes, and prosthetic group are used to describe the nonprotein moieties of the enzyme active center. The distinction between these terms is not sharp. Some of the cofactors are derivatives of vitamins that form either covalent or noncovalent linkages at or near the active site of the enzyme, and some are metal ions. If a cofactor (coenzyme) is tightly bound to the protein moiety (the apoenzyme), it is often referred to as a prosthetic group. A coenzyme that is easily removed from the holoenzyme, leaving behind the apoenzyme, is often regarded as a second substrate. 

Many enzymes require metal ion cofactors for their activity. Such enzymes are either metalloenzymes, in which case the metal ion is tightly bound, or metal-activated enzymes, in which case the bound metal ion is retained in an equilibrium with free metal ions. 

Many of the coenzymes are derivatives of vitamins. A coenzyme function is known for each of the water-soluble vitamins except vitamin C. A detailed discussion of the water-soluble vitamins and their conversion to coenzymes is found in Chapter 6. The biochemical functions of fat-soluble vitamins (A, D, E, K) are, with exception, less clearly understood. They are also further considered in Chapter 6. 

Because the intermediary metabolism of various organs is virtually the same, organ-specific enzymes are very rare. One example usually cited is the acid phosphatase of the prostate. However, the enzyme complement of the various organs may differ with respect to relative activities of the enzymes, the time dependence of their appearance in plasma, and the pattern of their isoenzymes (see below). Table 5.2 presents a list of enzymes commonly used for organ- and disease-specific diagnoses. 

Isoenzymes or isozymes are enzymes from a single species that have the same kind of enzymatic activity but differ in chemical structure. In addition, they may differ in quantitative characteristics such as possessing different Km s with the same substrate and may differ in response to temperature and effectors. Isozymes of more than 100 enzymes have been demonstrated in humans. The most important of these for diagnostic purposes are the isozymes of LDH, CK, alkaline phosphatase, leucine aminopeptidase, acid phosphatase, and aldolase. These have been exploited for differential organ diagnosis. 

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Certain classes of enzymes require small, auxiliary, nonprotein molecules called cofactors, coenzymes, and prosthetic groups. Definitions for these three terms are somewhat arbitrary and, in fact, the term cofactor will be used in the following chapters to represent broadly the identity and functional roles of cocatalysts. The roles of cofactors are structural, functional, or both. They provide the enzyme with the chemical or photochemical capabilities lacking in the normal amino acid side chains. An enzyme devoid of a cofactor is called an apoenzyme. Apoenzymes are catalytically inactive. The active complex of the protein and the cofactor is termed a holoenzyme. The cocatalysts can be defined on the basis of the catalytic functions that are mediated (76). 

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