Prosthetic group, definition

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). 

Definition of a cofactor, coenzyme, and pros thetic group Some enzymes require cofactors for activity. These can be metal ions or organic molecules called coenzymes that are often derivatives of vitamins. Tightly bound coen zymes are called prosthetic groups. 

In general, cytochrome P-450s have very characteristic UV/visible spectra due to the presence of their heme prosthetic groups, and thus display characteristic absorptions at 450 nm (the so-called Soret band) upon CO binding when reduced with dithionate. While this spectroscopic property has long been used to estimate P-450 contents, the O2/NADPH dependence and characteristic light-reversible CO inhibition properties of P-450s are also frequently employed to definitively identify enzymes of this class. 

The hydrogen is next transferred to the cytochrome reductase molecule, the prosthetic group of which is alloxazine (121). In this case there is definite evidence that the two hydrogen atoms are added separately, the alloxazine passing through a semiquinone stage (122). 

Coenzyme in the narrow sense, the dissociable, low-molecular-mass active group of an enzyme which transfers chemical groups (see Group transfer) or hydrogen or electrons. C. in this sense couple two otherwise independent reactions, and can thus be regarded as transport metabolites. In a wider sense, a C. can be regarded as any catalytically active, low-molecular-mass component of an enzyme. This definition includes C. that are covalently bound to enzymes as prosthetic groups. A holoenzyme consists of a C. in combination with an apoenzyme (enzyme protein). 

Rare examples of unique topologies in such systems are known. However, it was recently realized that when the analysis includes cofactors and prosthetic groups such as seen in quinoproteins or iron-sulfur cluster proteins, interesting topologies including knots and catenanes are in fact more common than previously realized. As always, in considering stereochemical phenomena, our definition of connectivity is crucial. Earlier studies had counted only the amino acids as contributing to the connectivity of the system. When cofactors are included, more complex connectivities result. 

Flavin Coenzymes, Although many flavin derivatives have been suspected of functioning in oxidizing enzymic systems as prosthetic groups, only two—riboflavin 5-phosphate (flavin mononucleotide, FMN) and flavinade-nine dinucleotide—have been definitely established in enzymic systems. Riboflavin 5 -phosphate (FMN) was identified by Warburg and Christian m ) as a constituent of the old yellow enzyme and its structure elucidated by several workers in different laboratories. Riboflavin, also known as vitamin B2 or lactoflavin, has been synthesized by the following procedure which establishes its structure (147) ... 

Lastly, the complex proteins have been gathered into a separate classification. They are composed of a protein part and an additional, nonprotein, prosthetic group. It is rather difficult to maintain the distinction between proteins which adsorb metals and carbohydrates or incorporate them in small quantities, and proteins with a definite metal or carbohydrate component. The usual division of complex proteins comprises the following (1) metalloproteins, (2) phosphopro-teins, (3) lipoproteins, (4) nucleoproteins, (5) glycoproteins, and (6) chromoproteins. 

The following considerations may lead to a sensible distinction between coenzyme and prosthetic group. Both types of groups participate chemically in the catalyzed process they are thereby altered and their original condition is restored only in a second reaction, usually through the mediation of a second enzyme. In this sense they do not conform to the strict definition of a catalyst, because they do not emerge from the reaction unaltered. This process of restoration differs between the coenzymes and the prosthetic groups. 

Some flavin enzymes may be separated into the apoenzyme and coenzyme components by a shift of pH and dialysis, or protein precipitation procedures. The old yellow enzyme was the flrst example of this, in that the active enzyme protein could be regenerated from coenzyme and apoenzyme (Theorell, 1934). But the bonds often are much tighter, and the flavin groups generally do not dissociate from the protein. According to our definition, they are prosthetic groups. Some flavoproteins also contain tightly bound metal ions which probably participate in catalysis (cf. Chapt. X-4). 

The structural formula of thiamine has two heterocyclic rings, which are not condensed one pyrimidine and one thiazol ring. They are connected at the quaternary nitrogen of the thiazol ring thiamine, therefore, always carries a charge. The coenzyme is the pyrophosphate of thiamine. Aside from the pyridine nucleotides, thiamine pyrophosphate was one of the first recognized coenzymes. In conformity with our definition, however, it must be considered a prosthetic group, since it remains tied to the enzyme protein. 

Independent evidence that olefin oxidation can proceed via a nonconcerted mechanism is provided by the fact that terminal olefins are not only oxidized to epoxides but, in many cases, simultaneously alkylate the P450 prosthetic heme group by covalently binding to one of its pyrrole nitrogen atoms (Fig. 4.29) [180]. It should be noted, however, that this heme alkylation process is relatively infrequent, with ratios of epoxidation to heme alkylation usually greater than 200. Despite the structures of the heme adducts, which nominally could arise by nucleophilic attack of the pyrrole nitrogen on the epoxide, epoxides are not involved in heme alkylation. This was definitely established by the fact that the synthetic epoxides do not react with the heme [181], and... 

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