Nucleotide coenzymes group transfer reactions

With the advent of an adequate nucleotide technology and the development of widespread interest in nucleotide metabolism, many free nucleotides have been found in cells however, no comprehensive survey has appeared since the 1958 report of Henderson and LePage (17), which lists over 100 nucleotides, and the list has probably more than doubled since. A large number of free nucleoside diphosphate conjugates have been isolated from cells and characterized many such compounds serve as coenzymes in group-transfer reactions, for example, CDP-X and UDP-X derivatives participate, respectively, in phospholipid and polysaccharide synthesis (see Chapter 3). As well, many new nucleotides have been recognized as metabolites of nucleoside antibiotics and of synthetic purine and pyrimidine analogues. 

Nucleotide coenzymes involved in group-transfer reactions (see below, Section IV) and similar compounds are frequently formed by reaction of a nucleoside triphosphate with a phosphorylated compound to give pyrophosphate and a product of the type, nucleoside diphospho-X. Pyro-... 

A new, heat-stable, coenzyme concerned with methyl group transfer has been isolated from Methanobacterium. The coenzyme, which is involved in transmethylation reactions prior to methane formation by the organism, contains phosphorus and has a u.v. absorption at 260 nm, suggesting that it may be a nucleotide. 

The initial step of the two-electron-transferring reactions is the removal of a proton from the substrate, followed by the intermediate formation of an adduct between the substrate and prosthetic group at N-5 of the flavin. This undergoes cleavage to yield dUiydroflavin and the oxidized product, which is commonly a carbon-carbon double bond. The reduced flavin is then reoxidized by reaction with an electron-transferring flavoprotein, as discussed above, or in some cases by reaction with nicotinamide nucleotide coenzymes. 

Protein enzymes frequently operate with the help of metal ions and with that of organic cofactors, which either are covalently bound to the enzyme molecule (prosthetic groups) or act in free, soluble form (coenzymes). As is to be expected, these cofactors serve mostly as carriers in transfer reactions. Several are nucleotide derivatives or have a nucleotide-like structure, thus being related to nucleic acid components. Many contain a vitamin in their molecule. 

In some cases a nucleotidyl derivative may be further modified either to form an active group-transfer coenzyme, or to form a different active coenzyme. This applies particularly to the nucleotide sugars, where very significant transformations occur. The types of reaction which occur are given in Table 3-III. 

Cytidine nucleotides apparently are the only nucleotides involved in the activation and transfer of alcohols. Like many other group-transfer coenzymes, they are formed by an ordinary nucleotidyl transfer reaction ... 

Coenzyme A (see also p. 106) is a nucleotide with a complex structure (see p. 80). It serves to activate residues of carboxylic acids (acyl residues). Bonding of the carboxy group of the carboxylic acid with the thiol group of the coenzyme creates a thioester bond (-S-CO-R see p. 10) in which the acyl residue has a high chemical potential. It can therefore be transferred to other molecules in exergonic reactions. This fact plays an important role in lipid metabolism in particular (see pp. 162ff), as well as in two reactions of the tricarboxylic acid cycle (see p. 136). 

ATP and the other nucleoside triphosphate coenzymes not only transfer phosphate residues, but also provide the nucleotide components for this type of activation reaction. On this page, we discuss metabolites or groups that are activated in the metabolism by bonding with nucleosides or nucleotides. Intermediates of this type are mainly found in the metabolism of complex carbohydrates and lipids. 

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