Coenzymes and prosthetic groups

In the development and application of biosensors based on enzymes several factors required for the catalytic process have to be taken into account, which are either directly involved in catalysis or influence the formation of the enzyme-substrate complex. They are designated coenzymes, prosthetic groups, and effectors. 

Of the types of cofactors known, the most common are coenzymes, prosthetic groups, and metal ions. A coenzyme is some other organic material that is loosely attached to the protein enzyme (apoenzyme). If the organic compound is strongly attached to the apoenzyme, it called a prosthetic group. 

All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. 

All proteins have CD spectra in the ultraviolet region due to the transitions of the peptide backbone and side-chain residues. The CD bands due to the protein itself are called intrinsic Cotton effects. However, many proteins have CD bands at wavelengths that do not overlap with the intrinsic Cotton effects. These bands are due to enzyme-bound chromophores such as coenzymes, prosthetic groups, metal ions, substrates, inhibitors, etc. and are called extrinsic Cotton effects. 

Under appropriate conditions these enzyme electrodes are capable of determining not only substrates, but also co-substrates, effectors, prosthetic groups and coenzymes, and enzyme activities via substrate determination. 

Prosthetic groups and coenzymes are complex organic compounds, many of which are derived from vitamins. These compounds are recycled and are needed only in catalytic amounts to convert a large amount of reactants to products. Coenzymes function as substrates in... 

Many enzymes require a component, other than the protein portion, for their catalytic activity - a co-factor. If the co-factor is removed, the remaining protein (apoenzyme) has no catalytic activity. A co-factor that is hrmly bound to the apoenzyme is termed a prosthetic group and most contain a metal centre. A co-factor that is bound loosely to the apoenzyme and can be readily separated from it is called a co-enzyme. Coenzymes play a critical role in the catalysis. 

From the standpoint of utilizing this effect it is fortunate that many coenzymes, prosthetic groups of enzymes, and substrates contain identifiable absorptions in the near ultraviolet as do the aromatic side chains of amino acids. Thus, if one could titrate a coenzyme or substrate onto an enzyme and note this type of reciprocal relations involving a transition in the enzyme, for example in a tryptophan or tyrosine group, and an absorption band in the group being added, then one would have identified an interaction at the active... 

Porphyrin-a-ZrP intercalates are interesting materials for catalytic applications due to the vast array of transition metal ions that can be bound to these ring systems, and the metal ions can assume various oxidation states appropriate for catalytic applications. Intercalation of other biologically significant cofactors, prosthetic groups, and coenzymes into the a-ZrP matrix are yet to be explored in detail, but this activity is certain to expand the catalytic potential of a-ZrP and its derivatives. 

While some enzymes are composed solely of protein, others require one or more non-protein substances of low molecular weight for their activity. If the non-protein moiety is firmly bound to the protein part it is known as a prosthetic group and the protein is a conjugated protein-, but where the enzyme is only active in the presence of a discrete small organic molecule, separable by dialysis, the non-protein substance is known as a coenzyme. 

The terms coenzyme" and prosthetic group" are frequently used synony-mously. Lately, the tendency has been to call the tightly bound groups that cannot be removed (for example by dialysis) prosthetic groups, and those which dissociate easily, coenzymes. Naturally it is often difficult to decide what is easily dissociable and what is not, for there is a complete range of possibilities. 

Neither coenzymes alone nor proteins alone are active catalysts. Prosthetic groups and coenzymes enable the formation of active centers for enzyme activity. Many enzymes require the presence of metal ions (Co, Fe , Zn, Cu, Mg ) for their activity. Coenzyme A is one of the central molecules in metabolism. It has been noticed that many enzyme-catalyzed acetylations need a heat resistant factor, which is called coenzyme A, where A indicates acetylation. The substance was isolated a few years later and its structure was deteimined. 

Enzymes often need for their activity the presence of a non-protein portion, which may be closely combined with the protein, in which case it is called a prosthetic group, or more loosely associated, in which case it is a coenzyme. Certain metals may be combined with the enzyme such as copper in ascorbic oxidase and selenium in glutathione peroxidase. Often the presence of other metals in solution, such as magnesium, are necessary for the action of particular enzymes. 

FIGURE 25.6 Fatty acids are conjugated both to coenzyme A and to acyl carrier protein through the sulfliydryl of phosphopantetheine prosthetic groups. 

Many enzymes contain small nonprotein molecules and metal ions that participate directly in substrate binding or catalysis. Termed prosthetic groups, cofactors, and coenzymes, these extend the repertoire of catalytic capabilities beyond those afforded by the limited number of functional groups present on the aminoacyl side chains of peptides. 

Organic and inorganic prosthetic groups, cofactors, and coenzymes play important roles in catalysis. Coenzymes, many of which are derivatives of B vitamins, serve as shutdes. ... 

Br Riboflavin Coenzyme in oxidation and reduction reactions prosthetic group of flavoproteins Lesions of corner of mouth, lips, and tongue seborrheic dermatitis... 

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